A search for partonic collective effects inside jets produced in proton-proton collisions is performed via correlation measurements of charged constituents using the CMS detector at the CERN LHC. The analysis uses data collected at a center-of-mass energy of $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. Jets are reconstructed with the anti-$k_\mathrm{T}$ algorithm with a distance parameter of 0.8 and are required to have transverse momentum greater than 550 GeV and pseudorapidity $\lvert\eta\rvert$$\lt$ 1.6. Two-particle correlations among the charged constituents within the jets are studied as functions of the particles' azimuthal angle and pseudorapidity separations ($\Delta\phi^*$ and $\Delta\eta^*$) in a jet coordinate basis, where constituents' $\eta^*$, $\phi^*$ are defined relative to the direction of the jet. The correlation functions are studied in classes of in-jet charged-particle multiplicity up to $N_\text{ch}^\mathrm{j}$$\approx$ 100. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for $\lvert\Delta\eta^*\rvert$$\gt$ 2. For low-multiplicity jets, the long-range elliptic anisotropic harmonic, $v^*_2$, is observed to decrease with $N_\text{ch}^\mathrm{j}$. This trend is well described by Monte Carlo event generators. However, a rising trend for $v^*_2$ emerges at $N_\text{ch}^\mathrm{j}$$\gtrsim$ 80, hinting at a possible onset of collective behavior, which is not reproduced by the models tested. This observation yields new insights into the dynamics of parton fragmentation processes in the vacuum.
Examples of two-particle angular correlations projected onto 1D $\Delta\phi^*$ for $\abs{\Delta\eta^*}>2$.
A continuous evolution of extracted two-particle Fourier coefficients $V^*_{N\Delta}$ as a function of $N_{ch}^{j}$.
The single-particle elliptic anisotropies $v^*_2$, as a function of $N_{ch}^{j}$.
In relativistic heavy-ion collisions, a global spin polarization, $P_\mathrm{H}$, of $\Lambda$ and $\bar{\Lambda}$ hyperons along the direction of the system angular momentum was discovered and measured across a broad range of collision energies and demonstrated a trend of increasing $P_\mathrm{H}$ with decreasing $\sqrt{s_{NN}}$. A splitting between $\Lambda$ and $\bar{\Lambda}$ polarization may be possible due to their different magnetic moments in a late-stage magnetic field sustained by the quark-gluon plasma which is formed in the collision. The results presented in this study find no significant splitting at the collision energies of $\sqrt{s_{NN}}=19.6$ and $27$ GeV in the RHIC Beam Energy Scan Phase II using the STAR detector, with an upper limit of $P_{\bar{\Lambda}}-P_{\Lambda}<0.24$% and $P_{\bar{\Lambda}}-P_{\Lambda}<0.35$%, respectively, at a 95% confidence level. We derive an upper limit on the na\"ive extraction of the late-stage magnetic field of $B<9.4\cdot10^{12}$ T and $B<1.4\cdot10^{13}$ T at $\sqrt{s_{NN}}=19.6$ and $27$ GeV, respectively, although more thorough derivations are needed. Differential measurements of $P_\mathrm{H}$ were performed with respect to collision centrality, transverse momentum, and rapidity. With our current acceptance of $|y|<1$ and uncertainties, we observe no dependence on transverse momentum and rapidity in this analysis. These results challenge multiple existing model calculations following a variety of different assumptions which have each predicted a strong dependence on rapidity in this collision-energy range.
The first-order event-plane resolution determined by the STAR EPD as a function of collision centrality is roughly doubled in comparison to previous analyses using the STAR BBC. We see $R_{\rm EP}^{(1)}$ peak for mid-central collisions.
The mid-central $P_{\rm H}$ measurements reported in this work are shown alongside previous measurements in the upper panel, and are consistent with previous measurements at the energies studied here. The difference between integrated $P_{\bar{\Lambda}}$ and $P_{\Lambda}$ is shown at $\sqrt{s_{\rm{NN}}}$=19.6 and 27 GeV alongside previous measurements in the lower panel. The splittings observed with these high-statistics data sets are consistent with zero. Statistical uncertainties are represented as lines while systematic uncertainties are represented as boxes. The previous $P_{\bar{\Lambda}}-P_{\Lambda}$ result at $\sqrt{s_{\rm NN}}=7.7$ GeV is outside the axis range, but is consistent with zero within $2\sigma$.
$P_{\rm H}$ measurements are shown as a function of collision centrality at $\sqrt{s_{\rm NN}}$=19.6 and 27 GeV. Statistical uncertainties are represented as lines while systematic uncertainties are represented as boxes. $P_{\rm H}$ increases with collision centrality at $\sqrt{s_{\rm NN}}$=19.6 and 27 GeV, as expected from an angular-momentum-driven phenomenon.
Global polarizations ($P$) of $\Lambda$ ($\bar{\Lambda}$) hyperons have been observed in non-central heavy-ion collisions. The strong magnetic field primarily created by the spectator protons in such collisions would split the $\Lambda$ and $\bar{\Lambda}$ global polarizations ($\Delta P = P_{\Lambda} - P_{\bar{\Lambda}} < 0$). Additionally, quantum chromodynamics (QCD) predicts topological charge fluctuations in vacuum, resulting in a chirality imbalance or parity violation in a local domain. This would give rise to an imbalance ($\Delta n = \frac{N_{\text{L}} - N_{\text{R}}}{\langle N_{\text{L}} + N_{\text{R}} \rangle} \neq 0$) between left- and right-handed $\Lambda$ ($\bar{\Lambda}$) as well as a charge separation along the magnetic field, referred to as the chiral magnetic effect (CME). This charge separation can be characterized by the parity-even azimuthal correlator ($\Delta\gamma$) and parity-odd azimuthal harmonic observable ($\Delta a_{1}$). Measurements of $\Delta P$, $\Delta\gamma$, and $\Delta a_{1}$ have not led to definitive conclusions concerning the CME or the magnetic field, and $\Delta n$ has not been measured previously. Correlations among these observables may reveal new insights. This paper reports measurements of correlation between $\Delta n$ and $\Delta a_{1}$, which is sensitive to chirality fluctuations, and correlation between $\Delta P$ and $\Delta\gamma$ sensitive to magnetic field in Au+Au collisions at 27 GeV. For both measurements, no correlations have been observed beyond statistical fluctuations.
Figure 1
Figure 2ab
Figure 2c
The deconfined quark-gluon plasma (QGP) created in relativistic heavy-ion collisions enables the exploration of the fundamental properties of matter under extreme conditions. Non-central collisions can produce strong magnetic fields on the order of $10^{18}$ Gauss, which offers a probe into the electrical conductivity of the QGP. In particular, quarks and anti-quarks carry opposite charges and receive contrary electromagnetic forces that alter their momenta. This phenomenon can be manifested in the collective motion of final-state particles, specifically in the rapidity-odd directed flow, denoted as $v_1(\mathsf{y})$. Here we present the charge-dependent measurements of $dv_1/d\mathsf{y}$ near midrapidities for $\pi^{\pm}$, $K^{\pm}$, and $p(\bar{p})$ in Au+Au and isobar ($_{44}^{96}$Ru+$_{44}^{96}$Ru and $_{40}^{96}$Zr+$_{40}^{96}$Zr) collisions at $\sqrt{s_{\rm NN}}=$ 200 GeV, and in Au+Au collisions at 27 GeV, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The combined dependence of the $v_1$ signal on collision system, particle species, and collision centrality can be qualitatively and semi-quantitatively understood as several effects on constituent quarks. While the results in central events can be explained by the $u$ and $d$ quarks transported from initial-state nuclei, those in peripheral events reveal the impacts of the electromagnetic field on the QGP. Our data put valuable constraints on the electrical conductivity of the QGP in theoretical calculations.
Directed flow of $p$ and $\bar{p}$ vs rapidity in Au+Au 200 GeV 50-80% centrality.
Directed flow of $p$ and $\bar{p}$ vs rapidity in Zr+Zr and Ru+Ru 200 GeV (combined) 50-80% centrality.
Directed flow of $p$ and $\bar{p}$ vs rapidity in Au+Au 27 GeV 50-80% centrality.
Density fluctuations near the QCD critical point can be probed via an intermittency analysis in relativistic heavy-ion collisions. We report the first measurement of intermittency in Au$+$Au collisions at $\sqrt{s_\mathrm{_{NN}}}$ = 7.7-200 GeV measured by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The scaled factorial moments of identified charged hadrons are analyzed at mid-rapidity and within the transverse momentum phase space. We observe a power-law behavior of scaled factorial moments in Au$+$Au collisions and a decrease in the extracted scaling exponent ($\nu$) from peripheral to central collisions. The $\nu$ is consistent with a constant for different collisions energies in the mid-central (10-40%) collisions. Moreover, the $\nu$ in the 0-5% most central Au$+$Au collisions exhibits a non-monotonic energy dependence that reaches a possible minimum around $\sqrt{s_\mathrm{_{NN}}}$ = 27 GeV. The physics implications on the QCD phase structure are discussed.
The scaled factorial moments, $F_{q}(M)$($q=$ 2-6), of identified charged hadrons ($h^{\pm}$) multiplicity in the most central (0-5\%) Au$+$Au collisions at $\sqrt{s_\mathrm{_{NN}}}$ = 7.7 GeV.
The scaled factorial moments, $F_{q}(M)$($q=$ 2-6), of identified charged hadrons ($h^{\pm}$) multiplicity in the most central (0-5\%) Au$+$Au collisions at $\sqrt{s_\mathrm{_{NN}}}$ = 19.6 GeV.
The scaled factorial moments, $F_{q}(M)$($q=$ 2-6), of identified charged hadrons ($h^{\pm}$) multiplicity in the most central (0-5\%) Au$+$Au collisions at $\sqrt{s_\mathrm{_{NN}}}$ = 39 GeV.
The linear and mode-coupled contributions to higher-order anisotropic flow are presented for Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 27, 39, 54.4, and 200 GeV and compared to similar measurements for Pb+Pb collisions at the Large Hadron Collider (LHC). The coefficients and the flow harmonics' correlations, which characterize the linear and mode-coupled response to the lower-order anisotropies, indicate a beam energy dependence consistent with an influence from the specific shear viscosity ($\eta/s$). In contrast, the dimensionless coefficients, mode-coupled response coefficients, and normalized symmetric cumulants are approximately beam-energy independent, consistent with a significant role from initial-state effects. These measurements could provide unique supplemental constraints to (i) distinguish between different initial-state models and (ii) delineate the temperature ($T$) and baryon chemical potential ($\mu_{B}$) dependence of the specific shear viscosity $\frac{\eta}{s} (T, \mu_B)$.
Comparison of the integrated three-particle correlators for Au+Au collisions at 54.4 GeV.
Comparison of the integrated three-particle correlators for Au+Au collisions at 39.0 GeV.
Comparison of the integrated three-particle correlators for Au+Au collisions at 27.0 GeV.
We report the measurement of $K^{*0}$ meson at midrapidity ($|y|<$ 1.0) in Au+Au collisions at $\sqrt{s_{\rm NN}}$~=~7.7, 11.5, 14.5, 19.6, 27 and 39 GeV collected by the STAR experiment during the RHIC beam energy scan (BES) program. The transverse momentum spectra, yield, and average transverse momentum of $K^{*0}$ are presented as functions of collision centrality and beam energy. The $K^{*0}/K$ yield ratios are presented for different collision centrality intervals and beam energies. The $K^{*0}/K$ ratio in heavy-ion collisions are observed to be smaller than that in small system collisions (e+e and p+p). The $K^{*0}/K$ ratio follows a similar centrality dependence to that observed in previous RHIC and LHC measurements. The data favor the scenario of the dominance of hadronic re-scattering over regeneration for $K^{*0}$ production in the hadronic phase of the medium.
$p_{\mathrm T}$-differential yield of $\mathrm{K^{*0}} + \bar{\mathrm{K^{*0}}}$ in AuAu collisions at $\sqrt{s_{\mathrm{NN}}}~=~$7.7 GeV (Multiplicity class 0-20%).
$p_{\mathrm T}$-differential yield of $\mathrm{K^{*0}} + \bar{\mathrm{K^{*0}}}$ in AuAu collisions at $\sqrt{s_{\mathrm{NN}}}~=~$7.7 GeV (Multiplicity class 20-40%).
$p_{\mathrm T}$-differential yield of $\mathrm{K^{*0}} + \bar{\mathrm{K^{*0}}}$ in AuAu collisions at $\sqrt{s_{\mathrm{NN}}}~=~$7.7 GeV (Multiplicity class 40-60%).
We report the triton ($t$) production in mid-rapidity ($|y| <$ 0.5) Au+Au collisions at $\sqrt{s_\mathrm{NN}}$= 7.7--200 GeV measured by the STAR experiment from the first phase of the beam energy scan at the Relativistic Heavy Ion Collider (RHIC). The nuclear compound yield ratio ($\mathrm{N}_t \times \mathrm{N}_p/\mathrm{N}_d^2$), which is predicted to be sensitive to the fluctuation of local neutron density, is observed to decrease monotonically with increasing charged-particle multiplicity ($dN_{ch}/d\eta$) and follows a scaling behavior. The $dN_{ch}/d\eta$ dependence of the yield ratio is compared to calculations from coalescence and thermal models. Enhancements in the yield ratios relative to the coalescence baseline are observed in the 0%-10% most central collisions at 19.6 and 27 GeV, with a significance of 2.3$\sigma$ and 3.4$\sigma$, respectively, giving a combined significance of 4.1$\sigma$. The enhancements are not observed in peripheral collisions or model calculations without critical fluctuation, and decreases with a smaller $p_{T}$ acceptance. The physics implications of these results on the QCD phase structure and the production mechanism of light nuclei in heavy-ion collisions are discussed.
Invariant yields of tritons at 7.7 GeV, all centralities. The first uncertainty is statistical uncertainty, the second is systematic uncertainty.
Invariant yields of tritons at 11.5 GeV, all centralities. The first uncertainty is statistical uncertainty, the second is systematic uncertainty.
Invariant yields of tritons at 14.5 GeV, all centralities. The first uncertainty is statistical uncertainty, the second is systematic uncertainty.
Azimuthal anisotropy of produced particles is one of the most important observables used to access the collective properties of the expanding medium created in relativistic heavy-ion collisions. In this paper, we present second ($v_{2}$) and third ($v_{3}$) order azimuthal anisotropies of $K_{S}^{0}$, $\phi$, $\Lambda$, $\Xi$ and $\Omega$ at mid-rapidity ($|y|<$1) in Au+Au collisions at $\sqrt{s_{\text{NN}}}$ = 54.4 GeV measured by the STAR detector. The $v_{2}$ and $v_{3}$ are measured as a function of transverse momentum and centrality. Their energy dependence is also studied. $v_{3}$ is found to be more sensitive to the change in the center-of-mass energy than $v_{2}$. Scaling by constituent quark number is found to hold for $v_{2}$ within 10%. This observation could be evidence for the development of partonic collectivity in 54.4 GeV Au+Au collisions. Differences in $v_{2}$ and $v_{3}$ between baryons and anti-baryons are presented, and ratios of $v_{3}$/$v_{2}^{3/2}$ are studied and motivated by hydrodynamical calculations. The ratio of $v_{2}$ of $\phi$ mesons to that of anti-protons ($v_{2}(\phi)/v_{2}(\bar{p})$) shows centrality dependence at low transverse momentum, presumably resulting from the larger effects from hadronic interactions on anti-proton $v_{2}$.
$v_{2}(p_{T})$ for $K_{S}^{0}$ (Centrality:0-10%)
$v_{2}(p_{T})$ for $K_{S}^{0}$ (Centrality:10-40%)
$v_{2}(p_{T})$ for $K_{S}^{0}$ (Centrality:40-80%)
Notwithstanding decades of progress since Yukawa first developed a description of the force between nucleons in terms of meson exchange, a full understanding of the strong interaction remains a major challenge in modern science. One remaining difficulty arises from the non-perturbative nature of the strong force, which leads to the phenomenon of quark confinement at distances on the order of the size of the proton. Here we show that in relativistic heavy-ion collisions, where quarks and gluons are set free over an extended volume, two species of produced vector (spin-1) mesons, namely $\phi$ and $K^{*0}$, emerge with a surprising pattern of global spin alignment. In particular, the global spin alignment for $\phi$ is unexpectedly large, while that for $K^{*0}$ is consistent with zero. The observed spin-alignment pattern and magnitude for the $\phi$ cannot be explained by conventional mechanisms, while a model with a connection to strong force fields, i.e. an effective proxy description within the Standard Model and Quantum Chromodynamics, accommodates the current data. This connection, if fully established, will open a potential new avenue for studying the behaviour of strong force fields.
Global spin alignment of $\phi$ and $K^{*0}$ vector mesons in heavy-ion collisions. The measured matrix element $\rho_{00}$ as a function of beam energy for the $\phi$ and $K^{*0}$ vector mesons within the indicated windows of centrality, transverse momentum ($p_T$) and rapidity ($y$). The open symbols indicate ALICE results for Pb+Pb collisions at 2.76 TeV at $p_{T}$ values of 2.0 and 1.4 GeV/c for the $\phi$ and $K^{*0}$ mesons, respectively, corresponding to the $p_{T}$ bin nearest to the mean $p_{T}$ for the 1.0 – 5.0 GeV/$c$ range assumed for each meson in the present analysis. The red solid curve is a fit to data in the range of $\sqrt{s_{NN}} = 19.6$ to 200 GeV, based on a theoretical calculation with a $\phi$-meson field. Parameter sensitivity of $\rho_{00}$ to the $\phi$-meson field is shown in Ref.5. The red dashed line is an extension of the solid curve with the fitted parameter $G_s^{(y)}$. The black dashed line represents $\rho_{00}=1/3.$
Global spin alignment of $\phi$ and $K^{*0}$ vector mesons in heavy-ion collisions. The measured matrix element $\rho_{00}$ as a function of beam energy for the $\phi$ and $K^{*0}$ vector mesons within the indicated windows of centrality, transverse momentum ($p_T$) and rapidity ($y$). The open symbols indicate ALICE results for Pb+Pb collisions at 2.76 TeV at $p_{T}$ values of 2.0 and 1.4 GeV/c for the $\phi$ and $K^{*0}$ mesons, respectively, corresponding to the $p_{T}$ bin nearest to the mean $p_{T}$ for the 1.0 – 5.0 GeV/$c$ range assumed for each meson in the present analysis. The red solid curve is a fit to data in the range of $\sqrt{s_{NN}} = 19.6$ to 200 GeV, based on a theoretical calculation with a $\phi$-meson field. Parameter sensitivity of $\rho_{00}$ to the $\phi$-meson field is shown in Ref.5. The red dashed line is an extension of the solid curve with the fitted parameter $G_s^{(y)}$. The black dashed line represents $\rho_{00}=1/3.$
Example of combinatorial background subtracted invariant mass distributions and the extracted yields as a function of $\cos \theta^*$ for $\phi$ and $K^{*0}$ mesons. \textbf{a)} example of $\phi \rightarrow K^+ + K^-$ invariant mass distributions, with combinatorial background subtracted, integrated over $\cos \theta^*$; \textbf{b)} example of $K^{*0} (\overline{K^{*0}}) \rightarrow K^{-} \pi^{+} (K^{+} \pi^{-})$ invariant mass distributions, with combinatorial background subtracted, integrated over $\cos \theta^*$; \textbf{c)} extracted yields of $\phi$ as a function of $\cos \theta^*$; \textbf{d)} extracted yields of $K^{*0}$ as a function of $\cos \theta^*$.
Elliptic flow measurements from two-, four- and six-particle correlations are used to investigate flow fluctuations in collisions of U+U at $\sqrt{s_{\rm NN}}$= 193 GeV, Cu+Au at $\sqrt{s_{\rm NN}}$= 200 GeV and Au+Au spanning the range $\sqrt{s_{\rm NN}}$= 11.5 - 200 GeV. The measurements show a strong dependence of the flow fluctuations on collision centrality, a modest dependence on system size, and very little if any, dependence on particle species and beam energy. The results, when compared to similar LHC measurements, viscous hydrodynamic calculations, and T$\mathrel{\protect\raisebox{-2.1pt}{R}}$ENTo model eccentricities, indicate that initial-state-driven fluctuations predominate the flow fluctuations generated in the collisions studied.
The Au+Au 200 GeV measurements of the two and four-particle elliptic flow and the elliptic flow fluctuations of the $\pi$ particle.
The Au+Au 200 GeV measurements of the two and four-particle elliptic flow and the elliptic flow fluctuations of the K particle.
The Au+Au 200 GeV measurements of the two and four-particle elliptic flow and the elliptic flow fluctuations of the p particle.
According to first-principle lattice QCD calculations, the transition from quark-gluon plasma to hadronic matter is a smooth crossover in the region μB ≤ T c. In this range the ratio, C6=C2, of net-baryon distributions are predicted to be negative. In this Letter, we report the first measurement of the midrapidity net-proton C6=C2 from 27, 54.4, and 200 GeV Au þ Au collisions at the Relativistic Heavy Ion Collider (RHIC). The dependence on collision centrality and kinematic acceptance in (p T , y) are analyzed. While for 27 and 54.4 GeV collisions the C6=C2 values are close to zero within uncertainties, it is observed that for 200 GeV collisions, the C6=C2 ratio becomes progressively negative from peripheral to central collisions. Transport model calculations without critical dynamics predict mostly positive values except for the most central collisions within uncertainties. These observations seem to favor a smooth crossover in the high-energy nuclear collisions at top RHIC energy.
Event by event net-proton multiplicity, $\Delta N_{p}$, distributions for Au+Au collisions at √sNN = 27, 54.4, and 200 GeV in 0-10% and 30-40% centralities at midrapidity (|y| < 0.5) for the transverse momentum range of 0.4 < $p_{T}$ (GeV/c) < 2.0. These distributions are normalized by the corresponding numbers of events and are not corrected for detector efficiencies. Statistical uncertainties are shown as vertical lines. The dashed lines show the Skellam distributions for each collision energy and centrality. The bottom panel shows the ratio of the data to the Skellam expectations.
Event by event net-proton multiplicity, $\Delta N_{p}$, distributions for Au+Au collisions at √sNN = 27, 54.4, and 200 GeV in 0-10% and 30-40% centralities at midrapidity (|y| < 0.5) for the transverse momentum range of 0.4 < $p_{T}$ (GeV/c) < 2.0. These distributions are normalized by the corresponding numbers of events and are not corrected for detector efficiencies. Statistical uncertainties are shown as vertical lines. The dashed lines show the Skellam distributions for each collision energy and centrality. The bottom panel shows the ratio of the data to the Skellam expectations.
Event by event net-proton multiplicity, $\Delta N_{p}$, distributions for Au+Au collisions at √sNN = 27, 54.4, and 200 GeV in 0-10% and 30-40% centralities at midrapidity (|y| < 0.5) for the transverse momentum range of 0.4 < $p_{T}$ (GeV/c) < 2.0. These distributions are normalized by the corresponding numbers of events and are not corrected for detector efficiencies. Statistical uncertainties are shown as vertical lines. The dashed lines show the Skellam distributions for each collision energy and centrality. The bottom panel shows the ratio of the data to the Skellam expectations.
We report a systematic measurement of cumulants, $C_{n}$, for net-proton, proton and antiproton multiplicity distributions, and correlation functions, $\kappa_n$, for proton and antiproton multiplicity distributions up to the fourth order in Au+Au collisions at $\sqrt{s_{\mathrm {NN}}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4 and 200 GeV. The $C_{n}$ and $\kappa_n$ are presented as a function of collision energy, centrality and kinematic acceptance in rapidity, $y$, and transverse momentum, $p_{T}$. The data were taken during the first phase of the Beam Energy Scan (BES) program (2010 -- 2017) at the BNL Relativistic Heavy Ion Collider (RHIC) facility. The measurements are carried out at midrapidity ($|y| <$ 0.5) and transverse momentum 0.4 $<$$p_{\rm T}$$<$ 2.0 GeV/$c$, using the STAR detector at RHIC. We observe a non-monotonic energy dependence ($\sqrt{s_{\mathrm {NN}}}$ = 7.7 -- 62.4 GeV) of the net-proton $C_{4}$/$C_{2}$ with the significance of 3.1$\sigma$ for the 0-5% central Au+Au collisions. This is consistent with the expectations of critical fluctuations in a QCD-inspired model. Thermal and transport model calculations show a monotonic variation with $\sqrt{s_{\mathrm {NN}}}$. For the multiparticle correlation functions, we observe significant negative values for a two-particle correlation function, $\kappa_2$, of protons and antiprotons, which are mainly due to the effects of baryon number conservation. Furthermore, it is found that the four-particle correlation function, $\kappa_4$, of protons plays a role in determining the energy dependence of proton $C_4/C_1$ below 19.6 GeV, which cannot be understood by the effect of baryon number conservation.
Reference charged particle multiplicity distributions using only pions and kaons ...
Reference charged particle multiplicity distributions using only pions and kaons ...
Reference charged particle multiplicity distributions using only pions and kaons ...
The measurements of particle multiplicity distributions have generated considerable interest in understanding the fluctuations of conserved quantum numbers in the Quantum Chromodynamics (QCD) hadronization regime, in particular near a possible critical point and near the chemical freeze-out. We report the measurement of efficiency and centrality bin width corrected cumulant ratios ($C_{2}/C_{1}$, $C_{3}/C_{2}$) of net-$\Lambda$ distributions, in the context of both strangeness and baryon number conservation, as a function of collision energy, centrality and rapidity. The results are for Au + Au collisions at five beam energies ($\sqrt{s_{NN}}$ = 19.6, 27, 39, 62.4 and 200 GeV) recorded with the Solenoidal Tracker at RHIC (STAR). We compare our results to the Poisson and negative binomial (NBD) expectations, as well as to Ultra-relativistic Quantum Molecular Dynamics (UrQMD) and Hadron Resonance Gas (HRG) model predictions. Both NBD and Poisson baselines agree with data within the statistical and systematic uncertainties. The ratios of the measured cumulants show no features of critical fluctuations. The chemical freeze-out temperatures extracted from a recent HRG calculation, which was successfully used to describe the net-proton, net-kaon and net-charge data, indicate $\Lambda$ freeze-out conditions similar to those of kaons. However, large deviations are found when comparing to temperatures obtained from net-proton fluctuations. The net-$\Lambda$ cumulants show a weak, but finite, dependence on the rapidity coverage in the acceptance of the detector, which can be attributed to quantum number conservation.
Centrality dependence of single cumulants C1, of net-lambda multiplicity distributions at Au + Au collision 19.6 GeV. Values are shown with NBD, Poisson and UrQMD predictions. Npart values are from Phys. Rev. C 104, 024902 (2021) and they are little different than the values shown in the original paper.
Centrality dependence of single cumulants C1, of net-lambda multiplicity distributions at Au + Au collision 27 GeV. Values are shown with NBD, Poisson and UrQMD predictions.Npart values are from Phys. Rev. C 104, 024902 (2021) and they are little different than the values shown in the original paper.
Centrality dependence of single cumulants C1, of net-lambda multiplicity distributions at Au + Au collision 39 GeV. Values are shown with NBD, Poisson and UrQMD predictions.Npart values are from Phys. Rev. C 104, 024902 (2021) and they are little different than the values shown in the original paper.
We report systematic measurements of bulk properties of the system created in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 14.5 GeV recorded by the STAR detector at the Relativistic Heavy Ion Collider (RHIC).The transverse momentum spectra of $\pi^{\pm}$, $K^{\pm}$ and $p(\bar{p})$ are studied at mid-rapidity ($|y| < 0.1$) for nine centrality intervals. The centrality, transverse momentum ($p_T$),and pseudorapidity ($\eta$) dependence of inclusive charged particle elliptic flow ($v_2$), and rapidity-odd charged particles directed flow ($v_{1}$) results near mid-rapidity are also presented. These measurements are compared with the published results from Au+Au collisions at other energies, and from Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV. The results at $\sqrt{s_{\mathrm{NN}}}$ = 14.5 GeV show similar behavior as established at other energies and fit well in the energy dependence trend. These results are important as the 14.5 GeV energy fills the gap in $\mu_B$, which is of the order of 100 MeV,between $\sqrt{s_{\mathrm{NN}}}$ =11.5 and 19.6 GeV. Comparisons of the data with UrQMD and AMPT models show poor agreement in general.
The $p_{T}$ spectra of proton measured at midrapidity (|y|<0.1) in Au+Au collisions at $\sqrt{s_{NN}}$ = 14.5 GeV. Spectra are plotted for nine centrality classes, with some spectra multiplied by a scale factor to improve clarity, as indicated in the legend
The $p_{T}$ spectra of antiproton measured at midrapidity (|y|<0.1) in Au+Au collisions at $\sqrt{s_{NN}}$ = 14.5 GeV. Spectra are plotted for nine centrality classes, with some spectra multiplied by a scale factor to improve clarity, as indicatedin the legend
The $p_{T}$ spectra of $\pi^{+}$ measured at midrapidity (|y|<0.1) in Au+Au collisions at $\sqrt{s_{NN}}$ = 14.5 GeV. Spectra are plotted for nine centrality classes, with some spectra multiplied by a scale factor to improve clarity, as indicatedin the legend
The two-particle angular correlation functions, $R_2$, of pions, kaons, and protons in Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV were measured by the STAR experiment at RHIC. These correlations were measured for both like-sign and unlike-sign charge combinations and versus the centrality. The correlations of pions and kaons show the expected near-side ({\it i.e.}, at small relative angles) peak resulting from short-range mechanisms. The amplitudes of these short-range correlations decrease with increasing beam energy. However, the proton correlation functions exhibit strong anticorrelations in the near-side region. This behavior is observed for the first time in an A+A collision system. The observed anticorrelation is $p_{T}$-independent and decreases with increasing beam energy and centrality. The experimental results are also compared to the Monte Carlo models UrQMD, Hijing, and AMPT.
Angular correlation function R2(∆y,∆φ) of like-sign pions in Au+Au collisions at mid centrality 30%-40% and 0.2 < pT < 2.0 GeV/c at 7.7 GeV
Angular correlation function R2(∆y,∆φ) of like-sign pions in Au+Au collisions at mid centrality 30%-40% and 0.2 < pT < 2.0 GeV/c at 11.5 GeV
Angular correlation function R2(∆y,∆φ) of like-sign pions in Au+Au collisions at mid centrality 30%-40% and 0.2 < pT < 2.0 GeV/c at 14.5 GeV
We present STAR measurements of strange hadron ($\mathrm{K}^{0}_{\mathrm S}$, $\Lambda$, $\overline{\Lambda}$, $\Xi^-$, $\overline{\Xi}^+$, $\Omega^-$, $\overline{\Omega}^+$, and $\phi$) production at mid-rapidity ($|y| < 0.5$) in Au+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}}$ = 7.7 - 39 GeV from the Beam Energy Scan Program at the Relativistic Heavy Ion Collider (RHIC). Transverse momentum spectra, averaged transverse mass, and the overall integrated yields of these strange hadrons are presented versus the centrality and collision energy. Antibaryon-to-baryon ratios ($\overline{\Lambda}$/$\Lambda$, $\overline{\Xi}^+$/$\Xi^-$, $\overline{\Omega}^+$/$\Omega^-$) are presented as well, and used to test a thermal statistical model and to extract the temperature normalized strangeness and baryon chemical potentials at hadronic freeze-out ($\mu_{B}/T_{\rm ch}$ and $\mu_{S}/T_{\rm ch}$) in central collisions. Strange baryon-to-pion ratios are compared to various model predictions in central collisions for all energies. The nuclear modification factors ($R_{\textrm{CP}}$) and antibaryon-to-meson ratios as a function of transverse momentum are presented for all collision energies. The $\mathrm{K}^{0}_{\mathrm S}$$R_{\textrm{CP}}$ shows no suppression for $p_{\rm T}$ up to 3.5 $\mathrm{GeV} / c$ at energies of 7.7 and 11.5 GeV. The $\overline{\Lambda}$/$\mathrm{K}^{0}_{\mathrm S}$ ratio also shows baryon-to-meson enhancement at intermediate $p_{\rm T}$ ($\approx$2.5 $\mathrm{GeV} / c$) in central collisions at energies above 19.6 GeV. Both observations suggest that there is likely a change of the underlying strange quark dynamics at collision energies below 19.6 GeV.
Data from STAR beam energy scan (Phase I) at RHIC, for mid-rapidity (|y|<0.5)
Data from STAR beam energy scan (Phase I) at RHIC, for mid-rapidity (|y|<0.5)
Data from STAR beam energy scan (Phase I) at RHIC, for mid-rapidity (|y|<0.5)
We report the energy dependence of mid-rapidity (anti-)deuteron production in Au+Au collisions at $\sqrt{s_\text{NN}} =\ $7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV, measured by the STAR experiment at RHIC. The yield of deuterons is found to be well described by the thermal model. The collision energy, centrality, and transverse momentum dependence of the coalescence parameter $B_2$ are discussed. We find that the values of $B_2$ for anti-deuterons are systematically lower than those for deuterons, indicating that the correlation volume of anti-baryons is larger than that of baryons at $\sqrt{s_\text{NN}}$ from 19.6 to 39 GeV. In addition, values of $B_2$ are found to vary with collision energy and show a broad minimum around $\sqrt{s_\text{NN}}=\ $20 to 40 GeV, which might imply a change of the equation of state of the medium in these collisions.
'transverse momentum spectra for deuterons in Au+Au collisions'
'transverse momentum spectra for deuterons in Au+Au collisions'
'transverse momentum spectra for deuterons in Au+Au collisions'
We report the first measurements of a complete second-order cumulant matrix of net-charge, net-proton, and net-kaon multiplicity distributions for the first phase of the beam energy scan program at RHIC. This includes the centrality and, for the first time, the pseudorapidity window dependence of both diagonal and off-diagonal cumulants in Au+Au collisions at \sNN~= 7.7-200 GeV. Within the available acceptance of $|\eta|<0.5$, the cumulants grow linearly with the pseudorapidity window. Relative to the corresponding measurements in peripheral collisions, the ratio of off-diagonal over diagonal cumulants in central collisions indicates an excess correlation between net-charge and net-kaon, as well as between net-charge and net-proton. The strength of such excess correlation increases with the collision energy. The correlation between net-proton and net-kaon multiplicity distributions is observed to be negative at \sNN~= 200 GeV and change to positive at the lowest collision energy. Model calculations based on non-thermal (UrQMD) and thermal (HRG) production of hadrons cannot explain the data. These measurements will help map the QCD phase diagram, constrain hadron resonance gas model calculations, and provide new insights on the energy dependence of baryon-strangeness correlations. An erratum has been added to address the issue of self-correlation in the previously considered efficiency correction for off-diagonal cumulant measurement. Previously considered unidentified (net-)charge correlation results ($\sigma^{11}_{Q,p}$ and $\sigma^{11}_{Q,k})$ are now replaced with identified (net-)charge correlation ($\sigma^{11}_{Q^{PID},p}$ and $\sigma^{11}_{Q^{PID},k}$)
The dependence of efficiency corrected second-order diagonal and off-diagonal cumulants on the width of the η-window. The filled and open circles represent 0-5% and 70-80% central collisions respectively. The shaded band represents the systematic uncertainty. The statistical uncertainties are within the marker size and solid lines are UrQMD calculations.
The dependence of efficiency corrected second-order diagonal and off-diagonal cumulants on the width of the η-window. The filled and open circles represent 0-5% and 70-80% central collisions respectively. The shaded band represents the systematic uncertainty. The statistical uncertainties are within the marker size and solid lines are UrQMD calculations.
Centrality dependence of efficiency corrected second-order diagonal cumulants of net-proton, net-kaon and net-pion (top to bottom) of the multiplicity distributions for Au+Au collisions at GeV (left to right) within kinematic range of |η| < 0.5 and 0.4 < pT < 1.6 GeV/c. The boxes represent the systematic error. The statistical error bars are within the marker size. The dashed lines represent scaling predicted by central limit theorem and the solid lines are UrQMD calculations.
We present two-particle $p_{\rm t}$ correlations as a function of event centrality for Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV at the Relativistic Heavy Ion Collider using the STAR detector. These results are compared to previous measurements from CERES at the Super Proton Synchrotron and from ALICE at the Large Hadron Collider. The data are compared with UrQMD model calculations and with a model based on a Boltzmann-Langevin approach incorporating effects from thermalization. The relative dynamical correlations for Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV show a power law dependence on the number of participant nucleons and agree with the results for Pb+Pb collisions at $\sqrt{s_{\rm NN}} = 2.76~ {\rm TeV}$ from ALICE. As the collision energy is lowered from $\sqrt{s_{\rm NN}}$ = 200 GeV to 7.7 GeV, the centrality dependence of the relative dynamical correlations departs from the power law behavior observed at the higher collision energies. In central collisions, the relative dynamical correlations increase with collision energy up to $\sqrt{s_{\rm NN}}$ = 200 GeV in contrast to previous measurements that showed little dependence on the collision energy.
'The relative dynamical correlation as a function of $N_{part}$'
'The relative dynamical correlation as a function of $N_{part}$'
'The relative dynamical correlation as a function of $N_{part}$'
New measurements of directed flow for charged hadrons, characterized by the Fourier coefficient \vone, are presented for transverse momenta $\mathrm{p_T}$, and centrality intervals in Au+Au collisions recorded by the STAR experiment for the center-of-mass energy range $\mathrm{\sqrt{s_{_{NN}}}} = 7.7 - 200$ GeV. The measurements underscore the importance of momentum conservation and the characteristic dependencies on $\mathrm{\sqrt{s_{_{NN}}}}$, centrality and $\mathrm{p_T}$ are consistent with the expectations of geometric fluctuations generated in the initial stages of the collision, acting in concert with a hydrodynamic-like expansion. The centrality and $\mathrm{p_T}$ dependencies of $\mathrm{v^{even}_{1}}$, as well as an observed similarity between its excitation function and that for $\mathrm{v_3}$, could serve as constraints for initial-state models. The $\mathrm{v^{even}_{1}}$ excitation function could also provide an important supplement to the flow measurements employed for precision extraction of the temperature dependence of the specific shear viscosity.
$v_{11}$ vs. $p_{T}^{b}$ for several selections of $p_{T}^{a}$ for 0-5 central Au+Au collisions at $\sqrt{s_{_{NN}}} = 200$ GeV. The curve shows the result of the simultaneous fit.
Extracted values of $v^{even}_{1}$ vs. $p_{T}$ for 0-10 central Au+Au collisions for several values of $\sqrt{s_{_{NN}}}$ as indicated; the $v^{even}_{1}$ values are obtained via fits. The curve in panel (a) shows the result from a viscous hydrodynamically based predictions.
(a) Centrality dependence of $v^{even}_{1}$ for $0.4 \lt p_{T} \lt 0.7$ GeV/c for Au+Au collisions at $\sqrt{s_{_{NN}}} = 200, 39$ and $19.6$ GeV; (b) $K$ vs. $\langle N_{ch} \rangle^{-1}$ for the $v^{even}_{1}$ values shown in (a). The $\langle N_{ch} \rangle$ values correspond to the centrality intervals indicated in panel (a).
Fluctuations of conserved quantities such as baryon number, charge, and strangeness are sensitive to the correlation length of the hot and dense matter created in relativistic heavy-ion collisions and can be used to search for the QCD critical point. We report the first measurements of the moments of net-kaon multiplicity distributions in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV. The collision centrality and energy dependence of the mean ($M$), variance ($\sigma^2$), skewness ($S$), and kurtosis ($\kappa$) for net-kaon multiplicity distributions as well as the ratio $\sigma^2/M$ and the products $S\sigma$ and $\kappa\sigma^2$ are presented. Comparisons are made with Poisson and negative binomial baseline calculations as well as with UrQMD, a transport model (UrQMD) that does not include effects from the QCD critical point. Within current uncertainties, the net-kaon cumulant ratios appear to be monotonic as a function of collision energy.
Raw $\Delta N_k$ distributions in Au+Au collisions at 7.7 GeV for 0–5%, 30–40%, and 70–80% collision centralities at midrapidity. The distributions are not corrected for the finite centrality bin width effect nor the reconstruction efficiency.
Raw $\Delta N_k$ distributions in Au+Au collisions at 11.5 GeV for 0–5%, 30–40%, and 70–80% collision centralities at midrapidity. The distributions are not corrected for the finite centrality bin width effect nor the reconstruction efficiency.
Raw $\Delta N_k$ distributions in Au+Au collisions at 14.5 GeV for 0–5%, 30–40%, and 70–80% collision centralities at midrapidity. The distributions are not corrected for the finite centrality bin width effect nor the reconstruction efficiency.
Rapidity-odd directed flow measurements at midrapidity are presented for $\Lambda$, $\bar{\Lambda}$, $K^\pm$, $K^0_s$ and $\phi$ at $\sqrt{s_{NN}} =$ 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV in Au+Au collisions recorded by the STAR detector at the Relativistic Heavy Ion Collider. These measurements greatly expand the scope of data available to constrain models with differing prescriptions for the equation of state of quantum chromodynamics. Results show good sensitivity for testing a picture where flow is assumed to be imposed before hadron formation and the observed particles are assumed to form via coalescence of constituent quarks. The pattern of departure from a coalescence-inspired sum-rule can be a valuable new tool for probing the collision dynamics.
Directed flow $v_1$ as a function of rapidity $y$ for $p$ in 5%–10% central Au+Au collisions at $\sqrt{s_{NN}} =$ 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV.
Directed flow $v_1$ as a function of rapidity $y$ for $K^{+}$ in 5%–10% central Au+Au collisions at $\sqrt{s_{NN}} =$ 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV.
Directed flow $v_1$ as a function of rapidity $y$ for $p$ in 10%–40% central Au+Au collisions at $\sqrt{s_{NN}} =$ 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV.
We report measurements of the nuclear modification factor, $R_{ \mathrm{CP}}$, for charged hadrons as well as identified $\pi^{+(-)}$, $K^{+(-)}$, and $p(\overline{p})$ for Au+Au collision energies of $\sqrt{s_{_{ \mathrm{NN}}}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, and 62.4 GeV. We observe a clear high-$p_{\mathrm{T}}$ net suppression in central collisions at 62.4 GeV for charged hadrons which evolves smoothly to a large net enhancement at lower energies. This trend is driven by the evolution of the pion spectra, but is also very similar for the kaon spectra. While the magnitude of the proton $R_{ \mathrm{CP}}$ at high $p_{\mathrm{T}}$ does depend on collision energy, neither the proton nor the anti-proton $R_{ \mathrm{CP}}$ at high $p_{\mathrm{T}}$ exhibit net suppression at any energy. A study of how the binary collision scaled high-$p_{\mathrm{T}}$ yield evolves with centrality reveals a non-monotonic shape that is consistent with the idea that jet-quenching is increasing faster than the combined phenomena that lead to enhancement.
Charged hadron RCP for RHIC BES energies. The uncertainty bands at unity on the right side of the plot correspond to the pT-independent uncertainty in Ncoll scaling with the color in the band corresponding to the color of the data points for that energy. The vertical uncertainty bars correspond to statistical uncertainties and the boxes to systematic uncertainties.
Identified particle (Pion Plus) RCP for RHIC BES energies. The colored shaded boxes describe the point-to-point systematic uncertainties. The uncertainty bands at unity on the right side of the plot correspond to the pT -independent uncertainty in Ncoll scaling with the color in the band corresponding to the color of the data points for that energy.
Identified particle (Pion Minus) RCP for RHIC BES energies. The colored shaded boxes describe the point-to-point systematic uncertainties. The uncertainty bands at unity on the right side of the plot correspond to the pT -independent uncertainty in Ncoll scaling with the color in the band corresponding to the color of the data points for that energy.
We present results on transverse momentum ($p_{\rm T}$) and rapidity ($y$) differential production cross sections, mean transverse momentum and mean transverse momentum square of inclusive J/$\psi$ and $\psi(2S)$ at forward rapidity ($2.5<y<4$) as well as $\psi(2S)$-to-J/$\psi$ cross section ratios. These quantities are measured in pp collisions at center of mass energies $\sqrt{s}=5.02$ and 13 TeV with the ALICE detector. Both charmonium states are reconstructed in the dimuon decay channel, using the muon spectrometer. A comprehensive comparison to inclusive charmonium cross sections measured at $\sqrt{s}=2.76$, 7 and 8 TeV is performed. A comparison to non-relativistic quantum chromodynamics and fixed-order next-to-leading logarithm calculations, which describe prompt and non-prompt charmonium production respectively, is also presented. A good description of the data is obtained over the full $p_{\rm T}$ range, provided that both contributions are summed. In particular, it is found that for $p_{\rm T}>15$ GeV/$c$ the non-prompt contribution reaches up to 50% of the total charmonium yield.
Differential production cross sections of $J/\psi$ as a function of $p_{\rm T}$.
Differential production cross sections of $J/\psi$ as a function of rapidity.
Differential production cross sections of $\psi(2S)$ as a function of $p_{\rm T}$.
We present measurements of bulk properties of the matter produced in Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7, 11.5, 19.6, 27, and 39 GeV using identified hadrons ($\pi^\pm$, $K^\pm$, $p$ and $\bar{p}$) from the STAR experiment in the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC). Midrapidity ($|y|<$0.1) results for multiplicity densities $dN/dy$, average transverse momenta $\langle p_T \rangle$ and particle ratios are presented. The chemical and kinetic freeze-out dynamics at these energies are discussed and presented as a function of collision centrality and energy. These results constitute the systematic measurements of bulk properties of matter formed in heavy-ion collisions over a broad range of energy (or baryon chemical potential) at RHIC.
The average number of participating nucleons (⟨Npart⟩) for various collision centralities in Au+Au collisions at √sNN = 7.7–39 GeV.
Midrapidity (|y| < 0.1) transverse momentum spectra for (b) π- in Au+Au collisions at √sNN = 7.7 GeV for different centralities. The spectra for centralities other than 0–5% are scaled for clarity as shown in the figure. The curves represent the Bose-Einstein, mT -exponential, and double-exponential function fits to 0–5% central data for pions, kaons, and (anti)protons, respectively. The uncertainties are statistical and systematic added in quadrature.
Midrapidity (|y| < 0.1) transverse momentum spectra for (a) π+ in Au+Au collisions at √sNN = 7.7 GeV for different centralities. The spectra for centralities other than 0–5% are scaled for clarity as shown in the figure. The curves represent the Bose-Einstein, mT -exponential, and double-exponential function fits to 0–5% central data for pions, kaons, and (anti)protons, respectively. The uncertainties are statistical and systematic added in quadrature.
The extreme temperatures and energy densities generated by ultra-relativistic collisions between heavy nuclei produce a state of matter with surprising fluid properties. Non-central collisions have angular momentum on the order of 1000$\hbar$, and the resulting fluid may have a strong vortical structure that must be understood to properly describe the fluid. It is also of particular interest because the restoration of fundamental symmetries of quantum chromodynamics is expected to produce novel physical effects in the presence of strong vorticity. However, no experimental indications of fluid vorticity in heavy ion collisions have so far been found. Here we present the first measurement of an alignment between the angular momentum of a non-central collision and the spin of emitted particles, revealing that the fluid produced in heavy ion collisions is by far the most vortical system ever observed. We find that $\Lambda$ and $\overline{\Lambda}$ hyperons show a positive polarization of the order of a few percent, consistent with some hydrodynamic predictions. A previous measurement that reported a null result at higher collision energies is seen to be consistent with the trend of our new observations, though with larger statistical uncertainties. These data provide the first experimental access to the vortical structure of the "perfect fluid" created in a heavy ion collision. They should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the Strong Force. Our results extend the recent discovery of hydrodynamic spin alignment to the subatomic realm.
Lambda and AntiLambda polarization as a function of collision energy. A 0.8% error on the alpha value used in the paper is corrected in this table. Systematic error bars include those associated with particle identification (negligible), uncertainty in the value of the hyperon decay parameter (2%) and reaction plane resolution (2%) and detector efficiency corrections (4%). The dominant systematic error comes from statistical fluctuations of the estimated combinatoric background under the (anti-)$\Lambda$ mass peak.
Lambda and AntiLambda polarization as a function of collision energy calculated using the new $\alpha_\Lambda=0.732$ updated on PDG2020. Systematic error bars include those associated with particle identification (negligible), uncertainty in the value of the hyperon decay parameter (2%) and reaction plane resolution (2%) and detector efficiency corrections (4%). The dominant systematic error comes from statistical fluctuations of the estimated combinatoric background under the (anti-)$\Lambda$ mass peak.
We present measurements of three-particle correlations for various harmonics in Au+Au collisions at energies ranging from $\sqrt{s_{{\rm NN}}}=7.7$ to 200 GeV using the STAR detector. The quantity $\langle\cos(m\phi_1+n\phi_2-(m+n)\phi_3)\rangle$ is evaluated as a function of $\sqrt{s_{{\rm NN}}}$, collision centrality, transverse momentum, $p_T$, pseudo-rapidity difference, $\Delta\eta$, and harmonics ($m$ and $n$). These data provide detailed information on global event properties like the three-dimensional structure of the initial overlap region, the expansion dynamics of the matter produced in the collisions, and the transport properties of the medium. A strong dependence on $\Delta\eta$ is observed for most harmonic combinations consistent with breaking of longitudinal boost invariance. Data reveal changes with energy in the two-particle correlation functions relative to the second-harmonic event-plane and provide ways to constrain models of heavy-ion collisions over a wide range of collision energies.
The centrality dependence of the C$_{m,n,m+n}$ correlations versus N$_{part}$ for charged hadrons with p$_{T}>0.2$ GeV/c and $\eta<1$ from 200 GeV Au+Au collisions.
The centrality dependence of the C$_{m,n,m+n}$ correlations versus N$_{part}$ for charged hadrons with p$_{T}>0.2$ GeV/c and $\eta<1$ from 62.4 GeV Au+Au collisions.
The centrality dependence of the C$_{m,n,m+n}$ correlations versus N$_{part}$ for charged hadrons with p$_{T}>0.2$ GeV/c and $\eta<1$ from 39 GeV Au+Au collisions.
The inclusive $J/\psi$ transverse momentum ($p_{T}$) spectra and nuclear modification factors are reported at midrapidity ($|y|<1.0$) in Au+Au collisions at $\sqrt{s_{NN}}=$ 39, 62.4 and 200 GeV taken by the STAR experiment. A suppression of $J/\psi$ production, with respect to {\color{black}the production in $p+p$ scaled by the number of binary nucleon-nucleon collisions}, is observed in central Au+Au collisions at these three energies. No significant energy dependence of nuclear modification factors is found within uncertainties. The measured nuclear modification factors can be described by model calculations that take into account both suppression of direct $J/\psi$ production due to the color screening effect and $J/\psi$ regeneration from recombination of uncorrelated charm-anticharm quark pairs.
J/psi invariant yields in Au+Au collisions = 39 GeV as a function of pT for different centralities.
J/psi invariant yields in Au+Au collisions = 62.4 GeV as a function of pT for different centralities.
J/psi invariant yields in Au+Au collisions = 200 GeV as a function of pT for different centralities.
We present measurements of 2$^{nd}$ order azimuthal anisotropy ($v_{2}$) at mid-rapidity $(|y|<1.0)$ for light nuclei d, t, $^{3}$He (for $\sqrt{s_{NN}}$ = 200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV) and anti-nuclei $\bar{\rm d}$ ($\sqrt{s_{NN}}$ = 200, 62.4, 39, 27, and 19.6 GeV) and $^{3}\bar{\rm He}$ ($\sqrt{s_{NN}}$ = 200 GeV) in the STAR (Solenoidal Tracker at RHIC) experiment. The $v_{2}$ for these light nuclei produced in heavy-ion collisions is compared with those for p and $\bar{\rm p}$. We observe mass ordering in nuclei $v_{2}(p_{T})$ at low transverse momenta ($p_{T}<2.0$ GeV/$c$). We also find a centrality dependence of $v_{2}$ for d and $\bar{\rm d}$. The magnitude of $v_{2}$ for t and $^{3}$He agree within statistical errors. Light-nuclei $v_{2}$ are compared with predictions from a blast wave model. Atomic mass number ($A$) scaling of light-nuclei $v_{2}(p_{T})$ seems to hold for $p_{T}/A < 1.5$ GeV/$c$. Results on light-nuclei $v_{2}$ from a transport-plus-coalescence model are consistent with the experimental measurements.
Mid-rapidity v2(pT) for d,anti-d,t,He,anti-He from minimum bias (0-80%) Au+Au collisions 200 GeV (d data points are also shown in Fig 5).
Mid-rapidity v2(pT) for d,anti-d,t,He from minimum bias (0-80%) Au+Au collisions 62.4 GeV.
Mid-rapidity v2(pT) for d,anti-d,t,He from minimum bias (0-80%) Au+Au collisions 39 GeV.
We present results from a harmonic decomposition of two-particle azimuthal correlations measured with the STAR detector in Au+Au collisions for energies ranging from $\sqrt{s_{NN}}=7.7$ GeV to 200 GeV. The third harmonic $v_3^2\{2\}=\langle \cos3(\phi_1-\phi_2)\rangle$, where $\phi_1-\phi_2$ is the angular difference in azimuth, is studied as a function of the pseudorapidity difference between particle pairs $\Delta\eta = \eta_1-\eta_2$. Non-zero {\vthree} is directly related to the previously observed large-$\Delta\eta$ narrow-$\Delta\phi$ ridge correlations and has been shown in models to be sensitive to the existence of a low viscosity Quark Gluon Plasma (QGP) phase. For sufficiently central collisions, $v_3^2\{2\}$ persist down to an energy of 7.7 GeV suggesting that QGP may be created even in these low energy collisions. In peripheral collisions at these low energies however, $v_3^2\{2\}$ is consistent with zero. When scaled by pseudorapidity density of charged particle multiplicity per participating nucleon pair, $v_3^2\{2\}$ for central collisions shows a minimum near {\snn}$=20$ GeV.
Representative results on $v_3^2\{2\}$ from Au+Au collisions as a function of $\Delta\eta$ for charged hadrons with pT > 0.2 GeV/c and |$\eta$| < 1.
Representative results on $v_3^2\{2\}$ from Au+Au collisions as a function of $\Delta\eta$ for charged hadrons with pT > 0.2 GeV/c and |$\eta$| < 1.
Representative results on $v_3^2\{2\}$ from Au+Au collisions as a function of $\Delta\eta$ for charged hadrons with pT > 0.2 GeV/c and |$\eta$| < 1.
Elliptic flow (v_2) values for identified particles at midrapidity in Au + Au collisions measured by the STAR experiment in the Beam Energy Scan at the Relativistic Heavy Ion Collider at sqrt{s_{NN}}= 7.7--62.4 GeV are presented for three centrality classes. The centrality dependence and the data at sqrt{s_{NN}}= 14.5 GeV are new. Except at the lowest beam energies we observe a similar relative v_2 baryon-meson splitting for all centrality classes which is in agreement within 15% with the number-of-constituent quark scaling. The larger v_2 for most particles relative to antiparticles, already observed for minimum bias collisions, shows a clear centrality dependence, with the largest difference for the most central collisions. Also, the results are compared with A Multiphase Transport Model and fit with a Blast Wave model.
No description provided.
The difference in $v_{2}$ between particles (X) and their corresponding antiparticles $\bar{X}$ (see legend) as a function of $\sqrt{s_{NN}}$ for 10%-40% central Au + Au collisions. The systematic errors are shown by the hooked error bars. The dashed lines in the plot are fits with a power-law function.
No description provided.
Balance functions have been measured in terms of relative pseudorapidity ($\Delta \eta$) for charged particle pairs at the Relativistic Heavy-Ion Collider (RHIC) from Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7 GeV to 200 GeV using the STAR detector. These results are compared with balance functions measured at the Large Hadron Collider (LHC) from Pb+Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV by the ALICE Collaboration. The width of the balance function decreases as the collisions become more central and as the beam energy is increased. In contrast, the widths of the balance functions calculated using shuffled events show little dependence on centrality or beam energy and are larger than the observed widths. Balance function widths calculated using events generated by UrQMD are wider than the measured widths in central collisions and show little centrality dependence. The measured widths of the balance functions in central collisions are consistent with the delayed hadronization of a deconfined quark gluon plasma (QGP). The narrowing of the balance function in central collisions at $\sqrt{s_{\rm NN}}$ = 7.7 GeV implies that a QGP is still being created at this relatively low energy.
The balance function in terms of $\Delta \eta$ for all charged particles with $0.2 < p_{T} < 2.0$ GeV/$c$ from central Au+Au collisions (0-5%) for $\sqrt{s_{NN}}=7.7$ GeV. The data are the measured balance functions corrected by subtracting balance functions calculated using mixed events. Also shown are balance functions calculated using shuffled events.
The balance function in terms of $\Delta \eta$ for all charged particles with $0.2 < p_{T} < 2.0$ GeV/$c$ from central Au+Au collisions (0-5%) for $\sqrt{s_{NN}}=11.5$ GeV. The data are the measured balance functions corrected by subtracting balance functions calculated using mixed events. Also shown are balance functions calculated using shuffled events.
The balance function in terms of $\Delta \eta$ for all charged particles with $0.2 < p_{T} < 2.0$ GeV/$c$ from central Au+Au collisions (0-5%) for $\sqrt{s_{NN}}=19.6$ GeV. The data are the measured balance functions corrected by subtracting balance functions calculated using mixed events. Also shown are balance functions calculated using shuffled events.
We present measurements of $\Omega$ and $\phi$ production at mid-rapidity from Au+Au collisions at nucleon-nucleon center-of-mass energies $\sqrt{s_{NN}}$ = 7.7, 11.5, 19.6, 27 and 39 GeV by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). Motivated by the coalescence formation mechanism for these strange hadrons, we study the ratios of $N(\Omega^{-}+\Omega^{+})/(2N(\phi))$. These ratios as a function of transverse momentum ($p_T$) fall on a consistent trend at high collision energies, but start to show deviations in peripheral collisions at $\sqrt{s_{NN}}$ = 19.6, 27 and 39 GeV, and in central collisions at 11.5 GeV in the intermediate $p_T$ region of 2.4-3.6 GeV/c. We further evaluate empirically the strange quark $p_T$ distributions at hadronization by studying the $\Omega/\phi$ ratios scaled by the number of constituent quarks. The NCQ-scaled $\Omega/\phi$ ratios show a suppression of strange quark production in central collisions at 11.5 GeV compared to $\sqrt{s_{NN}} >= 19.6$ GeV. The shapes of the presumably thermal strange quark distributions in 0-60% most central collisions at 7.7 GeV show significant deviations from those in 0-10% most central collisions at higher energies. These features suggest that there is likely a change of the underlying strange quark dynamics in the transition from quark-matter to hadronic matter at collision energies below 19.6 GeV.
Phi Meson Spectra.
Phi Meson Spectra.
Phi Meson Spectra.
We present measurements of $\pi^-$ and $\pi^+$ elliptic flow, $v_2$, at midrapidity in Au+Au collisions at $\sqrt{s_{_{\rm NN}}} =$ 200, 62.4, 39, 27, 19.6, 11.5 and 7.7 GeV, as a function of event-by-event charge asymmetry, $A_{ch}$, based on data from the STAR experiment at RHIC. We find that $\pi^-$ ($\pi^+$) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins at $\sqrt{s_{_{\rm NN}}} = \text{27 GeV}$ and higher. At $\sqrt{s_{_{\rm NN}}} = \text{200 GeV}$, the slope of the difference of $v_2$ between $\pi^-$ and $\pi^+$ as a function of $A_{ch}$ exhibits a centrality dependence, which is qualitatively similar to calculations that incorporate a chiral magnetic wave effect. Similar centrality dependence is also observed at lower energies.
The distribution of observed charge asymmetry from STAR data.
Pion $v_2${2} as a function of observed charge asymmetry.
$v_2$ difference between $\pi^-$ and $\pi^+$ as a function of charge asymmetry with the tracking efficiency correction, for 30-40% central Au+Au collisions at 200 GeV. The errors are statistical only.
The acceptance-corrected dielectron excess mass spectra, where the known hadronic sources have been subtracted from the inclusive dielectron mass spectra, are reported for the first time at mid-rapidity $|y_{ee}|<1$ in minimum-bias Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened $\rho$ spectral function for $M_{ee}<1.1$ GeV/$c^{2}$. The integrated dielectron excess yield at $\sqrt{s_{NN}}$ = 19.6 GeV for $0.4<M_{ee}<0.75$ GeV/$c^2$, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in In+In collisions at $\sqrt{s_{NN}}$ = 17.3 GeV. For $\sqrt{s_{NN}}$ = 200 GeV, the normalized excess yield in central collisions is higher than that at $\sqrt{s_{NN}}$ = 17.3 GeV and increases from peripheral to central collisions. These measurements indicate that the lifetime of the hot, dense medium created in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV is longer than those in peripheral collisions and at lower energies.
Reconstructed dielectron unlike-sign pairs, like-sign pairs and signal distributions, together with the signal to background ratio (S/B). All columns are presented as a function of dielectron invariant mass in Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 GeV.
Dielectron invariant mass spectrum in the STAR acceptance (|$y_{ee}$| < 1, 0.2 < $p_T^e$ < 3 GeV/c, |$\eta^e$ | < 1) after efficiency correction in Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 GeV.
Hadronic cocktail consisting of the decays of light hadrons and correlated decays of charm in Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 GeV.
A search for the quantum chromodynamics (QCD) critical point was performed by the STAR experiment at the Relativistic Heavy Ion Collider, using dynamical fluctuations of unlike particle pairs. Heavy-ion collisions were studied over a large range of collision energies with homogeneous acceptance and excellent particle identification, covering a significant range in the QCD phase diagram where a critical point may be located. Dynamical $K\pi$, $p\pi$, and $Kp$ fluctuations as measured by the STAR experiment in central 0-5\% Au+Au collisions from center-of-mass collision energies $\rm \sqrt{s_{NN}}$ = 7.7 to 200 GeV are presented. The observable $\rm \nu_{dyn}$ was used to quantify the magnitude of the dynamical fluctuations in event-by-event measurements of the $K\pi$, $p\pi$, and $Kp$ pairs. The energy dependences of these fluctuations from central 0-5\% Au+Au collisions all demonstrate a smooth evolution with collision energy.
$p\pi$, Kp, and $K\pi$ fluctuations as a function of collision energy, expressed as $v_{dyn,p\pi}$, $v_{dyn,Kp}$, and $v_{dyn,K\pi}$ respectively. Shown are data from central (0-5%) Au+Au collisions at energies from $\sqrt{s_{\rm NN}}$ = 7.7 to 200 GeV from the STAR experiment.
We present measurements of elliptic flow ($v_2$) of electrons from the decays of heavy-flavor hadrons ($e_{HF}$) by the STAR experiment. For Au+Au collisions at $\sqrt{s_{\rm NN}} = $ 200 GeV we report $v_2$, for transverse momentum ($p_T$) between 0.2 and 7 GeV/c using three methods: the event plane method ($v_{2}${EP}), two-particle correlations ($v_2${2}), and four-particle correlations ($v_2${4}). For Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 62.4 and 39 GeV we report $v_2${2} for $p_T< 2$ GeV/c. $v_2${2} and $v_2${4} are non-zero at low and intermediate $p_T$ at 200 GeV, and $v_2${2} is consistent with zero at low $p_T$ at other energies. The $v_2${2} at the two lower beam energies is systematically lower than at $\sqrt{s_{\rm NN}} = $ 200 GeV for $p_T < 1$ GeV/c. This difference may suggest that charm quarks interact less strongly with the surrounding nuclear matter at those two lower energies compared to $\sqrt{s_{\rm NN}} = 200$ GeV.
Signal-to-background (S/B) ratio as a function of transverse momentum, Au+Au 200 GeV, 0-60% central events with minimum bias trigger
Signal-to-background (S/B) ratio as a function of transverse momentum, Au+Au 200 GeV, 0-60% central events with with High Tower (high pT) trigger
Signal-to-background (S/B) ratio as a function of transverse momentum, Au+Au 39 GeV, 0-60% central events with minimum bias trigger
Local parity-odd domains are theorized to form inside a Quark-Gluon-Plasma (QGP) which has been produced in high-energy heavy-ion collisions. The local parity-odd domains manifest themselves as charge separation along the magnetic field axis via the chiral magnetic effect (CME). The experimental observation of charge separation has previously been reported for heavy-ion collisions at the top RHIC energies. In this paper, we present the results of the beam-energy dependence of the charge correlations in Au+Au collisions at midrapidity for center-of-mass energies of 7.7, 11.5, 19.6, 27, 39 and 62.4 GeV from the STAR experiment. After background subtraction, the signal gradually reduces with decreased beam energy, and tends to vanish by 7.7 GeV. The implications of these results for the CME will be discussed.
The three-point correlator, $\gamma$, as a function of centrality for Au+Au collisions at 62.4 GeV.
The three-point correlator, $\gamma$, as a function of centrality for Au+Au collisions at 39 GeV.
The three-point correlator, $\gamma$, as a function of centrality for Au+Au collisions at 27 GeV.
We report the beam energy (\sqrt s_{NN} = 7.7 - 200 GeV) and collision centrality dependence of the mean (M), standard deviation (\sigma), skewness (S), and kurtosis (\kappa) of the net-proton multiplicity distributions in Au+Au collisions. The measurements are carried out by the STAR experiment at midrapidity (|y| < 0.5) and within the transverse momentum range 0.4 < pT < 0.8 GeV/c in the first phase of the Beam Energy Scan program at the Relativistic Heavy Ion Collider. These measurements are important for understanding the Quantum Chromodynamic (QCD) phase diagram. The products of the moments, S\sigma and \kappa\sigma^{2}, are sensitive to the correlation length of the hot and dense medium created in the collisions and are related to the ratios of baryon number susceptibilities of corresponding orders. The products of moments are found to have values significantly below the Skellam expectation and close to expectations based on independent proton and anti-proton production. The measurements are compared to a transport model calculation to understand the effect of acceptance and baryon number conservation, and also to a hadron resonance gas model.
$\Delta N_p$ multiplicity distributions in Au+Au collisions at $\sqrt{S_{NN}}=7.7$ GeV for 0-5 percent, 30-40 percent and 70-80 percent collision centralities at midrapidity.
$\Delta N_p$ multiplicity distributions in Au+Au collisions at $\sqrt{S_{NN}}=11.5$ GeV for 0-5 percent, 30-40 percent and 70-80 percent collision centralities at midrapidity.
$\Delta N_p$ multiplicity distributions in Au+Au collisions at $\sqrt{S_{NN}}=19.6$ GeV for 0-5 percent, 30-40 percent and 70-80 percent collision centralities at midrapidity.
Elliptic flow ($v_{2}$) values for identified particles at mid-rapidity in Au+Au collisions, measured by the STAR experiment in the Beam Energy Scan at RHIC at $\sqrt{s_{NN}}=$ 7.7--62.4 GeV, are presented. A beam-energy dependent difference of the values of $v_{2}$ between particles and corresponding anti-particles was observed. The difference increases with decreasing beam energy and is larger for baryons compared to mesons. This implies that, at lower energies, particles and anti-particles are not consistent with the universal number-of-constituent-quark (NCQ) scaling of $v_{2}$ that was observed at $\sqrt{s_{NN}}=$ 200 GeV.
The elliptic flow $v_{2}$ of protons and anti-protons as a function of the transverse momentum, $p_{T}$, for 0–80$\%$ central Au+Au collisions. The lower panels show the difference in $v_{2}(p_{T})$ between the particles and anti-particles. The solid curves are fits with a horizontal line. The shaded areas depict the magnitude of the systematic errors.
The elliptic flow $v_{2}$ of protons and anti-protons as a function of the transverse momentum, $p_{T}$, for 0–80$\%$ central Au+Au collisions. The lower panels show the difference in $v_{2}(p_{T})$ between the particles and anti-particles. The solid curves are fits with a horizontal line. The shaded areas depict the magnitude of the systematic errors.
The elliptic flow $v_{2}$ of protons and anti-protons as a function of the transverse momentum, $p_{T}$, for 0–80$\%$ central Au+Au collisions. The lower panels show the difference in $v_{2}(p_{T})$ between the particles and anti-particles. The solid curves are fits with a horizontal line. The shaded areas depict the magnitude of the systematic errors.
We present a study of the average transverse momentum ($p_t$) fluctuations and $p_t$ correlations for charged particles produced in Cu+Cu collisions at midrapidity for $\sqrt{s_{NN}} =$ 62.4 and 200 GeV. These results are compared with those published for Au+Au collisions at the same energies, to explore the system size dependence. In addition to the collision energy and system size dependence, the $p_t$ correlation results have been studied as functions of the collision centralities, the ranges in $p_t$, the pseudorapidity $\eta$, and the azimuthal angle $\phi$. The square root of the measured $p_t$ correlations when scaled by mean $p_t$ is found to be independent of both colliding beam energy and system size studied. Transport-based model calculations are found to have a better quantitative agreement with the measurements compared to models which incorporate only jetlike correlations.
Event-by-event $\langle p_{t}\rangle$ distributions for data and mixed events in central Cu+Cu collisions at $\sqrt{s_{NN}}$ = 200 and 62.4 GeV.
Comparison of dynamical $\langle p_{t}\rangle$ fluctuations in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$ = 62.4 and 200 GeV as a function of the number of participanting nucleons.
Comparison of dynamical $\langle p_{t}\rangle$ fluctuations in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$ = 62.4 and 200 GeV as a function of the number of participanting nucleons.
Measurements of the elliptic flow, $v_{2}$, of identified hadrons ($\pi^{\pm}$, $K^{\pm}$, $K_{s}^{0}$, $p$, $\bar{p}$, $\phi$, $\Lambda$, $\bar{\Lambda}$, $\Xi^{-}$, $\bar{\Xi}^{+}$, $\Omega^{-}$, $\bar{\Omega}^{+}$) in Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7, 11.5, 19.6, 27, 39 and 62.4 GeV are presented. The measurements were done at mid-rapidity using the Time Projection Chamber and the Time-of-Flight detectors of the STAR experiment during the Beam Energy Scan program at RHIC. A significant difference in the $v_{2}$ values for particles and the corresponding anti-particles was observed at all transverse momenta for the first time. The difference increases with decreasing center-of-mass energy, $\sqrt{s_{NN}}$ (or increasing baryon chemical potential, $\mu_{B}$) and is larger for the baryons as compared to the mesons. This implies that particles and anti-particles are no longer consistent with the universal number-of-constituent quark (NCQ) scaling of $v_{2}$ that was observed at $\sqrt{s_{NN}}=$ 200 GeV. However, for the group of particles NCQ scaling at $(m_{T}-m_{0})/n_{q}>$ 0.4 GeV/$c^{2}$ is not violated within $\pm$10%. The $v_{2}$ values for $\phi$ mesons at 7.7 and 11.5 GeV are approximately two standard deviations from the trend defined by the other hadrons at the highest measured $p_{T}$ values.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
A systematic study is presented for centrality, transverse momentum ($p_T$) and pseudorapidity ($\eta$) dependence of the inclusive charged hadron elliptic flow ($v_2$) at midrapidity($|\eta| < 1.0$) in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 19.6, 27 and 39 GeV. The results obtained with different methods, including correlations with the event plane reconstructed in a region separated by a large pseudorapidity gap and 4-particle cumulants ($v_2{4}$), are presented in order to investigate non-flow correlations and $v_2$ fluctuations. We observe that the difference between $v_2{2}$ and $v_2{4}$ is smaller at the lower collision energies. Values of $v_2$, scaled by the initial coordinate space eccentricity, $v_{2}/\varepsilon$, as a function of $p_T$ are larger in more central collisions, suggesting stronger collective flow develops in more central collisions, similar to the results at higher collision energies. These results are compared to measurements at higher energies at the Relativistic Heavy Ion Collider ($\sqrt{s_{NN}}$ = 62.4 and 200 GeV) and at the Large Hadron Collider (Pb + Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV). The $v_2(p_T)$ values for fixed $p_T$ rise with increasing collision energy within the $p_T$ range studied ($< 2 {\rm GeV}/c$). A comparison to viscous hydrodynamic simulations is made to potentially help understand the energy dependence of $v_{2}(p_{T})$. We also compare the $v_2$ results to UrQMD and AMPT transport model calculations, and physics implications on the dominance of partonic versus hadronic phases in the system created at Beam Energy Scan (BES) energies are discussed.
The event plane resolutions for Au + Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 19.6, 27 and 39 GeV as a function of collision centrality.
The comparison of $v_2$ as a function of $p_T$ between GF-cumulant and Q-cumulant methods in Au+Au collisions at $\sqrt{s_{NN}}$ = 39 GeV.
The $p_T$ (> 0.2 GeV/c) and $\eta$ ($∣\eta∣$ < 1) integrated $v_2$ as a function of collision centrality for Au + Au collisions at $\sqrt{s_{NN}}$ = 7.7 GeV, 11.5 GeV, 19.6 GeV, 27 GeV and 39 GeV.
We present STAR measurements of azimuthal anisotropy by means of the two- and four-particle cumulants $v_2$ ($v_2\{2\}$ and $v_2\{4\}$) for Au+Au and Cu+Cu collisions at center of mass energies $\sqrt{s_{_{\mathrm{NN}}}} = 62.4$ and 200 GeV. The difference between $v_2\{2\}^2$ and $v_2\{4\}^2$ is related to $v_{2}$ fluctuations ($\sigma_{v_2}$) and nonflow $(\delta_{2})$. We present an upper limit to $\sigma_{v_2}/v_{2}$. Following the assumption that eccentricity fluctuations $\sigma_{\epsilon}$ dominate $v_2$ fluctuations $\frac{\sigma_{v_2}}{v_2} \approx \frac{\sigma_{\epsilon}}{\epsilon}$ we deduce the nonflow implied for several models of eccentricity fluctuations that would be required for consistency with $v_2\{2\}$ and $v_2\{4\}$. We also present results on the ratio of $v_2$ to eccentricity.
The two-particle cumulant $v_2\{2\}^2$ for Au+Au collisions at 200 and 62.4 GeV. Results are shown with like-sign combinations (LS) and charge-independent results (CI) for $0.15 < p_T < 2.0$ GeV/$c$.
The two-particle cumulant $v_2\{2\}^2$ for Au+Au collisions at 200 and 62.4 GeV. Results are shown with like-sign combinations (LS) and charge-independent results (CI) for $0.15 < p_T < 2.0$ GeV/$c$.
The same as the left but for Cu+Cu collisions. The systematic errors are shown as thin lines with wide caps at the ends and statistical errors are shown as thick lines with small caps at the end. Statistical and systematic errors are very small.
Two-particle azimuthal ($\Delta\phi$) and pseudorapidity ($\Delta\eta$) correlations using a trigger particle with large transverse momentum ($p_T$) in $d$+Au, Cu+Cu and Au+Au collisions at $\sqrt{s_{{NN}}}$ =\xspace 62.4 GeV and 200~GeV from the STAR experiment at RHIC are presented. The \ns correlation is separated into a jet-like component, narrow in both $\Delta\phi$ and $\Delta\eta$, and the ridge, narrow in $\Delta\phi$ but broad in $\Delta\eta$. Both components are studied as a function of collision centrality, and the jet-like correlation is studied as a function of the trigger and associated $p_T$. The behavior of the jet-like component is remarkably consistent for different collision systems, suggesting it is produced by fragmentation. The width of the jet-like correlation is found to increase with the system size. The ridge, previously observed in Au+Au collisions at $\sqrt{s_{{NN}}}$ = 200 GeV, is also found in Cu+Cu collisions and in collisions at $\sqrt{s_{{NN}}}$ =\xspace 62.4 GeV, but is found to be substantially smaller at $\sqrt{s_{{NN}}}$ =\xspace 62.4 GeV than at $\sqrt{s_{{NN}}}$ = 200 GeV for the same average number of participants ($ \langle N_{\mathrm{part}}\rangle$). Measurements of the ridge are compared to models.
Parameterizations of the transverse momentum dependence of the reconstruction efficiency of charged particles in the TPC in various collision systems, energies and centrality bins for the track selection cuts used in this analysis.
The raw correlation in $\Delta\eta$ for di-hadron correlations for 3 $<$ $p_T^{trigger}$ $<$ 6 GeV/$c$ and 1.5 GeV/$c$ $<$ $p_T^{associated}$ $<$ $p_T^{trigger}$ for 0-12% central \Au collisions for $|\Delta\phi|<$ 0.78 before and after the track merging correction is applied. The data have been reflected about $\Delta\eta$=0.
Sample correlations in $\Delta\eta$ ($|\Delta\phi|<$ 0.78) for 3 $<$ $p_T^{trigger}$ $<$ 6 GeV/$c$ and 1.5 GeV/$c$ $<$ $p_T^{associated}$ $<$ $p_T^{trigger}$ for 0-60% Cu+Cu at $\sqrt{s_{NN}}$ = 62.4 GeV, 0-80% Au+Au at $\sqrt{s_{NN}}$ = 62.4 GeV, 0-95% $d$+Au at $\sqrt{s_{NN}}$ = 200 GeV, 0-60% Cu+Cu at $\sqrt{s_{NN}}$ = 200 GeV, 40-80% Au+Au at $\sqrt{s_{NN}}$ = 200 GeV, and 0-12% central Au+Au at $\sqrt{s_{NN}}$ = 200 GeV. The data are averaged between positive and negative $\Delta\eta$. 5% systematic uncertainty due to track reconstruction efficiency not listed below.
We present two-dimensional (2D) two-particle angular correlations on relative pseudorapidity $\eta$ and azimuth $\phi$ for charged particles from Au-Au collisions at $\sqrt{s_{\rm NN}} = 62$ and 200 GeV with transverse momentum $p_t \geq 0.15$ GeV/$c$, $|\eta| \leq 1$ and $2\pi$ azimuth. Observed correlations include a {same-side} (relative azimuth $< \pi/2$) 2D peak, a closely-related away-side azimuth dipole, and an azimuth quadrupole conventionally associated with elliptic flow. The same-side 2D peak and away-side dipole are explained by semihard parton scattering and fragmentation (minijets) in proton-proton and peripheral nucleus-nucleus collisions. Those structures follow N-N binary-collision scaling in Au-Au collisions until mid-centrality where a transition to a qualitatively different centrality trend occurs within a small centrality interval. Above the transition point the number of same-side and away-side correlated pairs increases rapidly {relative to} binary-collision scaling, the $\eta$ width of the same-side 2D peak also increases rapidly ($\eta$ elongation) and the $\phi$ width actually decreases significantly. Those centrality trends are more remarkable when contrasted with expectations of jet quenching in a dense medium. Observed centrality trends are compared to {\sc hijing} predictions and to the expected trends for semihard parton scattering and fragmentation in a thermalized opaque medium. We are unable to reconcile a semihard parton scattering and fragmentation origin for the observed correlation structure and centrality trends with heavy ion collision scenarios which invoke rapid parton thermalization. On the other hand, if the collision system is effectively opaque to few-GeV partons the observations reported here would be inconsistent with a minijet picture.
FIG. 1. (Color online) Perspective views of $2 \mathrm{D}$ charge-independent angular correlations $\Delta \rho / \sqrt{\rho_{\mathrm{ref}}}$ on $\left(\eta_{\Delta}, \phi_{\Delta}\right)$ for Au-Au collisions at $\sqrt{s_{N N}}=200$ and $62 \mathrm{GeV}$ (top and bottom rows, respectively). Centrality increases left to right from most peripheral to most central. Corrected total cross-section fractions are (left to right) $84 \%-93 \%, 55 \%-64 \%, 18 \%-28 \%,$ and $0 \%-5 \%$ for the $200-\mathrm{GeV}$ data and $84 \%-95 \%, 56 \%-65 \%$ $18 \%-28 \%,$ and $0 \%-5 \%$ for the $62 \mathrm{GeV}$ data (see Tables III and IV).
FIG. 1. (Color online) Perspective views of $2 \mathrm{D}$ charge-independent angular correlations $\Delta \rho / \sqrt{\rho_{\mathrm{ref}}}$ on $\left(\eta_{\Delta}, \phi_{\Delta}\right)$ for Au-Au collisions at $\sqrt{s_{N N}}=200$ and $62 \mathrm{GeV}$ (top and bottom rows, respectively). Centrality increases left to right from most peripheral to most central. Corrected total cross-section fractions are (left to right) $84 \%-93 \%, 55 \%-64 \%, 18 \%-28 \%,$ and $0 \%-5 \%$ for the $200-\mathrm{GeV}$ data and $84 \%-95 \%, 56 \%-65 \%$ $18 \%-28 \%,$ and $0 \%-5 \%$ for the $62 \mathrm{GeV}$ data (see Tables III and IV).
FIG. 1. (Color online) Perspective views of $2 \mathrm{D}$ charge-independent angular correlations $\Delta \rho / \sqrt{\rho_{\mathrm{ref}}}$ on $\left(\eta_{\Delta}, \phi_{\Delta}\right)$ for Au-Au collisions at $\sqrt{s_{N N}}=200$ and $62 \mathrm{GeV}$ (top and bottom rows, respectively). Centrality increases left to right from most peripheral to most central. Corrected total cross-section fractions are (left to right) $84 \%-93 \%, 55 \%-64 \%, 18 \%-28 \%,$ and $0 \%-5 \%$ for the $200-\mathrm{GeV}$ data and $84 \%-95 \%, 56 \%-65 \%$ $18 \%-28 \%,$ and $0 \%-5 \%$ for the $62 \mathrm{GeV}$ data (see Tables III and IV).
This paper reports results for directed flow $v_{1}$ and elliptic flow $v_{2}$ of charged particles in Cu+Cu collisions at $\sqrt{s_{NN}}=$ 22.4 GeV at the Relativistic Heavy Ion Collider. The measurements are for the 0-60% most central collisions, using charged particles observed in the STAR detector. Our measurements extend to 22.4 GeV Cu+Cu collisions the prior observation that $v_1$ is independent of the system size at 62.4 and 200 GeV, and also extend the scaling of $v_1$ with $\eta/y_{\rm beam}$ to this system. The measured $v_2(p_T)$ in Cu+Cu collisions is similar for $\sqrt{s_{NN}} = 22.4-200$ GeV. We also report a comparison with results from transport model (UrQMD and AMPT) calculations. The model results do not agree quantitatively with the measured $v_1(\eta), v_2(p_T)$ and $v_2(\eta)$.
The event plane resolution measured using the TPC (second order) and using the BBC (first order) are shown as a function of collision centrality for Cu+Cu collisions at $\sqrt{s_{NN}}$ = 22.4 GeV. Errors are statistical only.
Charged hadron $v_{1}${BBC} vs. $\eta$ for 0-60% centrality Cu+Cu collisions at $\sqrt{s_{NN}}$ = 22.4 GeV. The errors shown are statistical. Systematic errors are discussed in Section III.C. Results are compared to $v_{1}$ from 0-40% centrality Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 GeV from the PHOBOS collaboration [10].
Comparison of the measured $v_{1}${BBC} as a function of η in 0-60% Cu+Cu collisions at $\sqrt{s_{NN}}$ = 22.4 GeV with model predictions. The inset shows the central $\eta$ region in more detail. The errors are statistical only.
Vector mesons may be photoproduced in relativistic heavy-ion collisions when a virtual photon emitted by one nucleus scatters from the other nucleus, emerging as a vector meson. The STAR Collaboration has previously presented measurements of coherent $\rho^0$ photoproduction at center of mass energies of 130 GeV and 200 GeV in AuAu collisions. Here, we present a measurement of the cross section at 62.4 GeV; we find that the cross section for coherent $\rho^0$ photoproduction with nuclear breakup is $10.5\pm1.5\pm 1.6$ mb at 62.4 GeV. The cross-section ratio between 200 GeV and 62.4 GeV is $2.8\pm0.6$, less than is predicted by most theoretical models. It is, however, proportionally much larger than the previously observed $15\pm 55$% increase between 130 GeV and 200 GeV.
Acceptance corrected invariant mass distributions for the coherently produced $\rho^0$ candidates collected with trigger A (left) and B (right). The fit function (solid) encompasses the Breit-Wigner (dashed), the mass independent contribution from direct $\pi^+\pi^-$ production (dash-dotted), and the interference term (dotted). The hatched area is the contribution from the combinatorial background. The statistical errors are shown.
Acceptance corrected invariant mass distributions for the coherently produced $\rho^0$ candidates collected with trigger A (left) and B (right). The fit function (solid) encompasses the Breit-Wigner (dashed), the mass independent contribution from direct $\pi^+\pi^-$ production (dash-dotted), and the interference term (dotted). The hatched area is the contribution from the combinatorial background. The statistical errors are shown.
Transverse momentum distribution of the $\rho^0$ candidates (open distribution) overlaid by the combinatorial background estimated with like-sign pairs (not corrected to the acceptance and reconstruction efficiency) and scaled to match in the high transverse momentum region, $p_T$ ≥ 250 MeV/$c$ (hatched distribution). The plot is based on the dataset collected with trigger B.
The first measurement of two-pion Bose-Einstein correlations in central Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV at the Large Hadron Collider is presented. We observe a growing trend with energy now not only for the longitudinal and the outward but also for the sideward pion source radius. The pion homogeneity volume and the decoupling time are significantly larger than those measured at RHIC.
Projections of the correlation function C.
Projections of the correlation function C.
Projections of the correlation function C.
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