We report on the first measurements of J/$\psi$ production at very low transverse momentum ($p_{T} <$ 0.2 GeV/c) in hadronic Au+Au collisions at $\sqrt{s_{\rm{NN}}} =$ 200 GeV and U+U collisions at $\sqrt{s_{\rm{NN}}} =$ 193 GeV. Remarkably, the inferred nuclear modification factor of J/$\psi$ at mid-rapidity in Au+Au (U+U) collisions reaches about 24 (52) for $p_{T} <$ 0.05 GeV/c in the 60-80$\%$ collision centrality class. This noteworthy enhancement cannot be explained by hadronic production accompanied by cold and hot medium effects. In addition, the $dN/dt$ distribution of J/$\psi$ for the very low $p_{T}$ range is presented for the first time. The distribution is consistent with that expected from the Au nucleus and shows a hint of interference. Comparison of the measurements to theoretical calculations of coherent production shows that the excess yield can be described reasonably well and reveals a partial disruption of coherent production in semi-central collisions, perhaps due to the violent hadronic interactions. Incorporating theoretical calculations, the results strongly suggest that the dramatic enhancement of J/$\psi$ yield observed at extremely low $p_{T}$ originates from coherent photon-nucleus interactions. In particular, coherently produced J/$\psi$'s in violent hadronic collisions may provide a novel probe of the quark-gluon-plasma.
'$J/\Psi$ invariant yields for Au+Au collisions 200 GeV as a function of $p_T$'
'$J/\Psi$ invariant yields for U+U collisions 193 GeV as a function of $p_T$'
'$J/\Psi$ $R_{AA}$ for Au+Au collisions 200 GeV as a function of $p_T$'
The Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC) was extended to energies below $\sqrt{\textit{s}_{NN}}$ = 7.7 GeV in 2015 by successful implementation of the fixed-target mode of operation in the STAR (Solenoidal Track At RHIC) experiment. In the fixed-target mode, ions circulate in one ring of the collider and interact with a stationary target at the entrance of the STAR Time Projection Chamber. The first results for Au+Au collisions at $\sqrt{\textit{s}_{NN}}$ = 4.5 GeV are presented, including directed and elliptic flow of identified hadrons, and radii from pion femtoscopy. The proton flow and pion femtoscopy results agree quantitatively with earlier measurements by Alternating Gradient Synchrotron experiments at similar energies. This validates running the STAR experiment in the fixed-target configuration. Pion directed and elliptic flow are presented for the first time at this beam energy. Pion and proton elliptic flow show behavior which hints at constituent quark scaling, but large error bars preclude reliable conclusions. The ongoing second phase of BES (BES-II) will provide fixed-target data sets with 100 times more events at each of several energies down to $\sqrt{\textit{s}_{NN}}$ = 3.0 GeV.
Centrality selection for STAR FXT sqrt(sNN) = 4.5 GeV Au+Au collisions
Rapidity dependence of directed flow, v1(y), for protons with transverse momentum 0.4 < pT < 2.0 GeV/c from events with 10-25% centrality.
Rapidity dependence of directed flow, v1(y), for negative pions with transverse momentum pT > 0.2 GeV/c and total momentum magnitude |p| < 1.6 GeV/c from events within 10-30% centrality. Here, the BBC-based Event Plane method is used. Plotted error bars are statistical only, and systematic errors are of comparable size.
Quark interactions with topological gluon configurations can induce local chirality imbalance and parity violation in quantum chromodynamics, which can lead to the chiral magnetic effect (CME) -- an electric charge separation along the strong magnetic field in relativistic heavy-ion collisions. The CME-sensitive azimuthal correlator observable ($\Delta\gamma$) is contaminated by background arising, in part, from resonance decays coupled with elliptic anisotropy ($v_{2}$). We report here differential measurements of the correlator as a function of the pair invariant mass ($m_{\rm inv}$) in 20-50% centrality Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$= 200 GeV by the STAR experiment at RHIC. Strong resonance background contributions to $\Delta\gamma$ are observed. At large $m_{\rm inv}$ where this background is significantly reduced, the $\Delta\gamma$ value is found to be significantly smaller. An event-shape-engineering technique is deployed to determine the $v_{2}$ background shape as a function of $m_{\rm inv}$. We extract a $v_2$-independent and $m_{\rm inv}$-averaged signal $\Delta\gamma_{\rm sig}$ = (0.03 $\pm$ 0.06 $\pm$ 0.08) $\times10^{-4}$, or $(2\pm4\pm5)\%$ of the inclusive $\Delta\gamma(m_{\rm inv}>0.4$ GeV/$c^2$)$ =(1.58 \pm 0.02 \pm 0.02) \times10^{-4}$, within pion $p_{T}$ = 0.2 - 0.8~\gevc and averaged over pseudorapidity ranges of $-1 < \eta < -0.05$ and $0.05 < \eta < 1$. This represents an upper limit of $0.23\times10^{-4}$, or $15\%$ of the inclusive result, at $95\%$ confidence level for the $m_{\rm inv}$-integrated CME contribution.
The $m_{\rm inv}$ dependences of the OS and SS pion pair multiplicities in 20-50$\%$ Au+Au collisions at 200 GeV.
The $m_{\rm inv}$ dependences of the $\gamma_{OS}$, $\gamma_{SS}$ in 20-50$\%$ Au+Au collisions at 200 GeV.
$m_{\rm inv}$ dependences of the relative excess of OS over SS pion pairs in 20-50$\%$ Au+Au collisions at 200 GeV.
Flow harmonics ($\textit{v}_{n}$) of the Fourier expansion for the azimuthal distributions of hadrons are commonly employed to quantify the azimuthal anisotropy of particle production relative to the collision symmetry planes. While lower order Fourier coefficients ($\textit{v}_{2}$ and $\textit{v}_{3}$) are more directly related to the corresponding eccentricities of the initial state, the higher-order flow harmonics ($\textit{v}_{n>3}$) can be induced by a mode-coupled response to the lower-order anisotropies, in addition to a linear response to the same-order anisotropies. These higher-order flow harmonics and their linear and mode-coupled contributions can be used to more precisely constrain the initial conditions and the transport properties of the medium in theoretical models. The multiparticle azimuthal cumulant method is used to measure the linear and mode-coupled contributions in the higher-order anisotropic flow, the mode-coupled response coefficients, and the correlations of the event plane angles for charged particles as functions of centrality and transverse momentum in Au+Au collisions at nucleon-nucleon center-of-mass energy \roots = 200 GeV. The results are compared to similar LHC measurements as well as to several viscous hydrodynamic calculations with varying initial conditions.
3-particle integrated correlators
The integrated $v_{4}$ and $v_{5}$
The integrated $\chi_{4,22}$, $\rho_{4,22}$, $\chi_{5,23}$ and $\rho_{5,23}$
We present a measurement of the first-order azimuthal anisotropy, $v_1(y)$, of deuterons from Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV recorded with the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The energy dependence of the $v_1(y)$ slope, $dv_{1}/dy|_{y=0}$, for deuterons, where $y$ is the rapidity, is extracted for semi-central collisions (10-40\% centrality) and compared to that of protons. While the $v_1(y)$ slopes of protons are generally negative for $\sqrt{s_{NN}} >$ 10 GeV, those for deuterons are consistent with zero, a strong enhancement of the $v_1(y)$ slope of deuterons is seen at the lowest collision energy (the largest baryon density) at $\sqrt{s_{NN}} =$ 7.7 GeV. In addition, we report the transverse momentum dependence of $v_1$ for protons and deuterons. The experimental results are compared with transport and coalescence models.
The 1st-order event plane ($\Psi_{1}$) resolution as a function of centrality of Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV. The $\Psi_{1}$ is reconstructed with the BBC detectors and its resolution is estimated by the correlation of sub-$\Psi_{1}$ from east BBC and west BBC. Data presented later (10-40\% centrality) is indicated by the dashed-line box.
Rapidity dependene of $v_1$ for protons(open squares) in 10-40 \%Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV. The lines, dashed-dot line for proton and dashed line for deuteron, at the midrapidity ($|y| < 0.6$) are the fit with linear functions to extract the slopes. The plotted uncertainties are the statistical only
Rapidity dependene of $v_1$ for deuterons(solid circles) in 10-40 \%Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV. The lines, dashed-dot line for proton and dashed line for deuteron, at the midrapidity ($|y| < 0.6$) are the fit with linear functions to extract the slopes. The plotted uncertainties are the statistical only
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.
We report the first measurement of the inclusive jet and the dijet longitudinal double-spin asymmetries, $A_{LL}$, at midrapidity in polarized $pp$ collisions at a center-of-mass energy $\sqrt{s} = 510$ GeV. The inclusive jet $A_{LL}$ measurement is sensitive to the gluon helicity distribution down to a gluon momentum fraction of $x\approx 0.015$, while the dijet measurements, separated into four jet-pair topologies, provide constraints on the $x$ dependence of the gluon polarization. Both results are consistent with previous measurements made at $\sqrt{s}= 200$ GeV in the overlapping kinematic region, $x > 0.05$, and show good agreement with predictions from recent next-to-leading order global analyses.
Fractions of the next-to-leading-order cross section for inclusive jet production arising from quark-quark, quark-gluon and gluon-gluon scattering in pp collisions (200GeV)
Fractions of the next-to-leading-order cross section for inclusive jet production arising from quark-quark, quark-gluon and gluon-gluon scattering in pp collisions (500GeV)
The upper panel shows the jet yield vs detector jet pT in data and simulation for each of the three trigger categories. The lower three panels show the relative differences between data and simulation (JP0)
We report high-precision measurements of the longitudinal double-spin asymmetry, $A_{LL}$, for midrapidity inclusive jet and dijet production in polarized $pp$ collisions at a center-of-mass energy of $\sqrt{s}=200\,\mathrm{GeV}$. The new inclusive jet data are sensitive to the gluon helicity distribution, $\Delta g(x,Q^2)$, for gluon momentum fractions in the range from $x \simeq 0.05$ to $x \simeq 0.5$, while the new dijet data provide further constraints on the $x$ dependence of $\Delta g(x,Q^2)$. The results are in good agreement with previous measurements at $\sqrt{s}=200\,\mathrm{GeV}$ and with recent theoretical evaluations of prior world data. Our new results have better precision and thus strengthen the evidence that $\Delta g(x,Q^2)$ is positive for $x > 0.05$.
Jet yield versus jet transverse momentum $p_{T}$ at the detector level and at the parton level. Table includes data for the JP2 trigger conditions and the corresponding simulations.
Jet yield versus jet transverse momentum $p_{T}$ at the detector level and at the parton level. Table includes data for the JP1 trigger conditions and the corresponding simulations.
Dijet yield versus the dijet $M_{inv}$ at the detector level and at the parton level. Table includes data for the JP1 and JP2 trigger conditions and the corresponding simulations.
According to the CPT theorem, which states that the combined operation of charge conjugation, parity transformation and time reversal must be conserved, particles and their antiparticles should have the same mass and lifetime but opposite charge and magnetic moment. Here, we test CPT symmetry in a nucleus containing a strange quark, more specifically in the hypertriton. This hypernucleus is the lightest one yet discovered and consists of a proton, a neutron, and a $\Lambda$ hyperon. With data recorded by the STAR detector{\cite{TPC,HFT,TOF}} at the Relativistic Heavy Ion Collider, we measure the $\Lambda$ hyperon binding energy $B_{\Lambda}$ for the hypertriton, and find that it differs from the widely used value{\cite{B_1973}} and from predictions{\cite{2019_weak, 1995_weak, 2002_weak, 2014_weak}}, where the hypertriton is treated as a weakly bound system. Our results place stringent constraints on the hyperon-nucleon interaction{\cite{Hammer2002, STAR-antiH3L}}, and have implications for understanding neutron star interiors, where strange matter may be present{\cite{Chatterjee2016}}. A precise comparison of the masses of the hypertriton and the antihypertriton allows us to test CPT symmetry in a nucleus with strangeness for the first time, and we observe no deviation from the expected exact symmetry.
Measurements of relative mass-to-charge ratio differences between nuclei and antinuclei (d and antid)
Measurements of relative mass-to-charge ratio differences between nuclei and antinuclei (He and antiHe)
Measurements of relative mass-to-charge ratio differences between nuclei and antinuclei (hypertriton and antihypertriton)
The transversity distribution, which describes transversely polarized quarks in transversely polarized nucleons, is a fundamental component of the spin structure of the nucleon, and is only loosely constrained by global fits to existing semi-inclusive deep inelastic scattering (SIDIS) data. In transversely polarized $p^\uparrow+p$ collisions it can be accessed using transverse polarization dependent fragmentation functions which give rise to azimuthal correlations between the polarization of the struck parton and the final state scalar mesons. This letter reports on spin dependent di-hadron correlations measured by the STAR experiment. The new dataset corresponds to 25 pb$^{-1}$ integrated luminosity of $p^\uparrow+p$ collisions at $\sqrt{s}=500$ GeV, an increase of more than a factor of ten compared to our previous measurement at $\sqrt{s}=200$ GeV. Non-zero asymmetries sensitive to transversity are observed at a $Q^2$ of several hundred GeV and are found to be consistent with the former measurement and a model calculation. %we observe consistent with the former measurement are observed.} We expect that these data will enable an extraction of transversity with comparable precision to current SIDIS datasets but at much higher momentum transfers where subleading effects are suppressed.
Squared 4-momentum transfer $Q^2$ vs x coverage of STAR .
$A_{UT}$ as a function of $\eta$ for $<p_{T}>$ = 13 GeV/c and $<M_{inv}>$ = 1 GeV/($c^2$) (Upper panel of the fig. 3). Kinematic variables $<x>$, $<z>$ as a function of $\eta$ for $<p_{T}>$ = 13 GeV/c and $<M_{inv}>$ = 1 GeV/($c^2$) (Lower panel of the fig. 3). In addition to statistical uncertainties, systematic uncertainties originating from PID and trigger bias systematic uncertainties are also mentioned for $A_{UT}$.
$A_{UT}$ as a function of $<M_{inv}>$ for pT bin $<p_{T}>$ = 4 GeV/c for $\eta > 0$ and $\eta < 0$. In addition to statistical uncertainties, systematic uncertainties originating from PID and trigger bias systematic uncertainties are also mentioned.
Forum