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.
We report a new measurement of the production of electrons from open heavy-flavor hadron decays (HFEs) at mid-rapidity ($|y|<$ 0.7) in Au+Au collisions at $\sqrt{s_{\rm NN}}=200$ GeV. Invariant yields of HFEs are measured for the transverse momentum range of $3.5 < p_{\rm T} < 9$ GeV/$c$ in various configurations of the collision geometry. The HFE yields in head-on Au+Au collisions are suppressed by approximately a factor of 2 compared to that in $p$+$p$ collisions scaled by the average number of binary collisions, indicating strong interactions between heavy quarks and the hot and dense medium created in heavy-ion collisions. Comparison of these results with models provides additional tests of theoretical calculations of heavy quark energy loss in the quark-gluon plasma.
Ratios of NPE (non-photonic electron) to PHE (photonic electron) as a function of $p_{\rm T}$ in 0-10% central (yellow circles) and 40-80% peripheral (green squares) Au+Au collisions at $\sqrt{s_{\rm NN}}=200$ GeV. Vertical bars represent statistical uncertainties while boxes represent systematic uncertainties. Horizontal bars indicate the bin width.
Invariant yields of electrons from decays of prompt $J/\psi$ (dot-dashed line), $\Upsilon$ (dotted line), Drell-Yan (long dash-dotted line), light vector mesons (long dashed line) and the combined HDE (hadron decayed electron) contribution (solid line), estimated utilizing experimental measurements, theoretical calculations, and PYTHIA and $\rm E_{VT}G_{EN}$ event generators, in 0-10% central Au+Au collisions at $\sqrt{s_{\rm NN}}=200$ GeV. Color bands represent systematic uncertainties.
Invariant yields of electrons from decays of prompt $J/\psi$ (dot-dashed line), $\Upsilon$ (dotted line), Drell-Yan (long dash-dotted line), light vector mesons (long dashed line) and the combined HDE (hadron decayed electron) contribution (solid line), estimated utilizing experimental measurements, theoretical calculations, and PYTHIA and $\rm E_{VT}G_{EN}$ event generators, in 40-80% central Au+Au collisions at $\sqrt{s_{\rm NN}}=200$ GeV. Color bands represent systematic uncertainties.
The interpretation of cosmic antiproton flux measurements from space-borne experiments is currently limited by the knowledge of the antiproton production cross-section in collisions between primary cosmic rays and the interstellar medium. Using collisions of protons with an energy of 6.5 TeV incident on helium nuclei at rest in the proximity of the interaction region of the LHCb experiment, the ratio of antiprotons originating from antihyperon decays to prompt production is measured for antiproton momenta between 12 and 110 GeV/c. The dominant antihyperon contribution, namely $\bar{\Lambda} \to \bar{p} \pi^+$ decays from promptly produced $\bar{\Lambda}$ particles, is also exclusively measured. The results complement the measurement of prompt antiproton production obtained from the same data sample. At the energy scale of this measurement, the antihyperon contributions to antiproton production are observed to be significantly larger than predictions of commonly used hadronic production models.
Ratio of the antihyperon decays to prompt antiproton production (R_Hbar) in collisions of 6.5 TeV protons on He nuclei at rest in antiproton momentum and transverse momentum intervals. The average momentum and transverse momentum, as predicted by the EPOS-LHC generator for prompt antiprotons, are also listed for each interval. The uncertainty is split into an uncorrelated component, denoted with delta_uncorr, and a component that is fully correlated among the kinematic intervals, denoted delta_corr.
Ratio of the Lbar decays to prompt antiproton production (R_Lbar) in collisions of 6.5 TeV protons on He nuclei at rest in antiproton momentum and transverse momentum intervals. The average momentum and transverse momentum, as predicted by the EPOS-LHC generator for prompt antiprotons, are also listed for each interval. The uncertainty is split into an uncorrelated component, denoted with delta_uncorr, and a component that is fully correlated among the kinematic intervals, denoted delta_corr.
The measurement of direct photons from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=39$ and 62.4 GeV in the transverse-momentum range $0.4<p_T<3$ Gev/$c$ is presented by the PHENIX collaboration at the Relativistic Heavy Ion Collider. A significant direct-photon yield is observed in both collision systems. A universal scaling is observed when the direct-photon $p_T$ spectra for different center-of-mass energies and for different centrality selections at $\sqrt{s_{_{NN}}}=62.4$ GeV is scaled with $(dN_{\rm ch}/d\eta)^{\alpha}$ for $\alpha=1.21{\pm}0.04$. This scaling also holds true for direct-photon spectra from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV measured earlier by PHENIX, as well as the spectra from Pb$+$Pb at $\sqrt{s_{_{NN}}}=2760$ GeV published by ALICE. The scaling power $\alpha$ seems to be independent of $p_T$, center of mass energy, and collision centrality. The spectra from different collision energies have a similar shape up to $p_T$ of 2 GeV/$c$. The spectra have a local inverse slope $T_{\rm eff}$ increasing with $p_T$ of $0.174\pm0.018$ GeV/$c$ in the range $0.4<p_T<1.3$ GeV/$c$ and increasing to $0.289\pm0.024$ GeV/$c$ for $0.9<p_T<2.1$ GeV/$c$. The observed similarity of low-$p_T$ direct-photon production from $\sqrt{s_{_{NN}}}= 39$ to 2760 GeV suggests a common source of direct photons for the different collision energies and event centrality selections, and suggests a comparable space-time evolution of direct-photon emission.
$R_{\gamma}$ for minimum bias (0-86%) Au+Au collision at $\sqrt{s_{NN}} = 62.4$ GeV (a) and $39$ GeV (b). For $62.4$ GeV also centrality bins of 0-20% (c) and 20-40% (d) are shown. Data points are shown with statistical (bar) and systematic uncertainties (box)
$R_{\gamma}$ for minimum bias (0-86%) Au+Au collision at $\sqrt{s_{NN}} = 62.4$ GeV (a) and $39$ GeV (b). For $62.4$ GeV also centrality bins of 0-20% (c) and 20-40% (d) are shown. Data points are shown with statistical (bar) and systematic uncertainties (box)
Direct photon spectra for minimum bias (0-86%) Au+Au collision at $\sqrt{s_{NN}} = 62.4$ GeV (a) and $39$ GeV (b). For $62.4$ GeV also centrality bins of 0-20% (c) and 20-40% (d) are shown. Data points are shown with statistical and systematic uncertainties, unless the central value is negative (arrows) or is consistent with zero within the statistical uncertainties (arrows with data point). In these cases upper limit with CL = 95$%$ are given.
The PHENIX collaboration presents a systematic study of $\pi^0$ production from $p$ $+$ $p$, $p$ $+$Al, $p$ $+$Au, $d$ $+$Au, and $^{3}$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. Measurements were performed with different centrality selections as well as the total inelastic, 0%--100%, selection for all collision systems. For 0%--100% collisions, the nuclear modification factors, $R_{xA}$, are consistent with unity for $p_T$ above 8 GeV/$c$, but exhibit an enhancement in peripheral collisions and a suppression in central collisions. The enhancement and suppression characteristics are similar for all systems for the same centrality class. It is shown that for high-$p_T$-$\pi^0$ production, the nucleons in the $d$ and $^3$He interact mostly independently with the Au nucleus and that the counter intuitive centrality dependence is likely due to a physical correlation between multiplicity and the presence of a hard scattering process. These observations disfavor models where parton energy loss has a significant contribution to nuclear modifications in small systems. Nuclear modifications at lower $p_T$ resemble the Cronin effect -- an increase followed by a peak in central or inelastic collisions and a plateau in peripheral collisions. The peak height has a characteristic ordering by system size as $p$ $+$Au $>$ $d$ $+$Au $>$ $^{3}$He$+$Au $>$ $p$ $+$Al. For collisions with Au ions, current calculations based on initial state cold nuclear matter effects result in the opposite order, suggesting the presence of other contributions to nuclear modifications, in particular at lower $p_T$.
Differential cross section of $\pi^0$ in p+p collisions at $\sqrt{s}$ = 200 GeV
Invariant yield of $\pi^0$ from (a) p+Al, (b) p+Au, (c) d+Au, and (d) $^{3}$HeAu in different centrality selections at $\sqrt{s}$ = 200 GeV
Nuclear modification factors from inelastic (a) p+Al, (b) p+Au, (c) d+Au, and (d) $^{3}$HeAu collisions at $\sqrt{s}$ = 200 GeV. The right boxes are the $N_{coll}$ uncertainties from the Glauber model, while the left box represents the overall normalization uncertainty from p+p collisions
We present direct photon-hadron correlations in 200 GeV/A Au+Au, d+Au, and p+p collisions, for direct photon pT from 5–12 GeV/c, collected by the PHENIX Collaboration in the years from 2006 to 2011. We observe no significant modification of jet fragmentation in d+Au collisions, indicating that cold nuclear matter effects are small or absent. Hadrons carrying a large fraction of the quark's momentum are suppressed in Au+Au compared to p+p and d+Au. As the momentum fraction decreases, the yield of hadrons in Au+Au increases to an excess over the yield in p+p collisions. The excess is at large angles and at low hadron pT and is most pronounced for hadrons associated with lower momentum direct photons. Comparison to theoretical calculations suggests that the hadron excess arises from medium response to energy deposited by jets.
Per-trigger yield of hadrons associated to direct photons in Au+Au collisions for direct photon $p_T$ 5-9 GeV/$c$, compared with p+p baseline, in various $\xi$ bins.
Per-trigger yield of hadrons associated to direct photons in d+Au collisions for direct photon $p_T$ 7-9 GeV/$c$, compared with p+p baseline, in various $\xi$ bins.
Integrated away-side $\gamma_{dir}$-h per-trigger yields of Au+Au, d+Au, and p+p, as a function of $\xi$.
The cross-section for prompt antiproton production in collisions of protons with an energy of $6.5$ TeV incident on helium nuclei at rest is measured with the LHCb experiment from a data set corresponding to an integrated luminosity of $0.5\,nb^{-1}$. The target is provided by injecting helium gas into the LHC beam line at the LHCb interaction point. The reported results, covering antiproton momenta between $12$ and $110\,\mathrm{GeV/}c$, represent the first direct determination of the antiproton production cross-section in ${\rm p He}$ collisions, and impact the interpretation of recent results on antiproton cosmic rays from space-borne experiments.
Double-differential cross-section in antiproton momentum and transverse momentum for antiproton production in collisions of 6.5 TeV protons on He nuclei at rest. The antiproton momentum is defined in the laboratory frame. Results are averaged over the given kinematic range of each bin. The uncertainty is split into an uncertainty delta_uncorr, uncorrelated among the kinematic bins, and an uncertainty delta_corr, fully correlated among the kinematic bins. For both uncertainties, the systematic uncertainty, dominant for most bins, and the statistical uncertainty, are added in quadrature. The average value within each bin is also reported for the antiproton momentum, the transverse momentum, and x-Feynman xF=2 p_Z*/SQRT(S), where p_Z* is the longitudinal antiproton momentum in the center-of-mass system and SQRT(S) the nucleon-nucleon center-of-mass energy. These average values are obtained from simulation (based on EPOS LHC), to avoid biases from reconstruction effects and given the good agreement with data observed for the simulated kinematic spectra.
Double-differential cross-section in antiproton momentum and transverse momentum for antiproton production in collisions of 6.5 TeV protons on He nuclei at rest. The antiproton momentum is defined in the laboratory frame. Results are averaged over the given kinematic range of each bin. The uncertainty is split into an uncertainty delta_uncorr, uncorrelated among the kinematic bins, and an uncertainty delta_corr, fully correlated among the kinematic bins. For both uncertainties, the systematic uncertainty, dominant for most bins, and the statistical uncertainty, are added in quadrature. The average value within each bin is also reported for the antiproton momentum, the transverse momentum, and x-Feynman xF=2 p_Z*/SQRT(S), where p_Z* is the longitudinal antiproton momentum in the center-of-mass system and SQRT(S) the nucleon-nucleon center-of-mass energy. These average values are obtained from simulation (based on EPOS LHC), to avoid biases from reconstruction effects and given the good agreement with data observed for the simulated kinematic spectra.
The PHENIX collaboration presents first measurements of low-momentum ($0.4<p_T<3$ GeV/$c$) direct-photon yields from Au$+$Au collisions at $\sqrt{s_{_{NN}}}$=39 and 62.4 GeV. For both beam energies the direct-photon yields are substantially enhanced with respect to expectations from prompt processes, similar to the yields observed in Au$+$Au collisions at $\sqrt{s_{_{NN}}}$=200. Analyzing the photon yield as a function of the experimental observable $dN_{\rm ch}/d\eta$ reveals that the low-momentum ($>$1\,GeV/$c$) direct-photon yield $dN_{\gamma}^{\rm dir}/d\eta$ is a smooth function of $dN_{\rm ch}/d\eta$ and can be well described as proportional to $(dN_{\rm ch}/d\eta)^\alpha$ with $\alpha{\sim}$1.25. This new scaling behavior holds for a wide range of beam energies at the Relativistic Heavy Ion Collider and Large Hadron Collider, for centrality selected samples, as well as for different, $A$$+$$A$ collision systems. At a given beam energy the scaling also holds for high $p_T$ ($>5$\,GeV/$c$) but when results from different collision energies are compared, an additional $\sqrt{s_{_{NN}}}$-dependent multiplicative factor is needed to describe the integrated-direct-photon yield.
Direct photon spectra(Physical Review C87, 054907 (2013)) normalized by $(dN_{ch}/d\eta)^{1.25}$ for in p+p at $\sqrt{s_{NN}}$= 200 GeV.
Direct photon spectra(Physics Letters B94, 106 (1980)) normalized by $(dN_{ch}/d\eta)^{1.25}$ for in p+p at $\sqrt{s_{NN}}$= 62.4 GeV.
Direct photon spectra(Nucl. Part. Phys. 23, A1 (1997) and Sov. J. Nucl. Phys. 51, 836 (1990)) normalized by $(dN_{ch}/d\eta)^{1.25}$ for in p+p at $\sqrt{s_{NN}}$= 63 GeV.
We have measured direct photons for $p_T<5~$GeV/$c$ in minimum bias and 0\%--40\% most central events at midrapidity for Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The $e^{+}e^{-}$ contribution from quasi-real direct virtual photons has been determined as an excess over the known hadronic contributions in the $e^{+}e^{-}$ mass distribution. A clear enhancement of photons over the binary scaled $p$$+$$p$ fit is observed for $p_T<4$ GeV/$c$ in Cu$+$Cu data. The $p_T$ spectra are consistent with the Au$+$Au data covering a similar number of participants. The inverse slopes of the exponential fits to the excess after subtraction of the $p$$+$$p$ baseline are 285$\pm$53(stat)$\pm$57(syst)~MeV/$c$ and 333$\pm$72(stat)$\pm$45(syst)~MeV/$c$ for minimum bias and 0\%--40\% most central events, respectively. The rapidity density, $dN/dy$, of photons demonstrates the same power law as a function of $dN_{\rm ch}/d\eta$ observed in Au$+$Au at the same collision energy.
Direct photon fraction measured with the virtual photon method for different systems in $\sqrt{s_{NN}}$ = 200 GeV Cu+Cu collisions.
The direct photon spectra for Minimum Bias and 0-40% centrality in $\sqrt{s_{NN}}$ = 200 GeV Cu+Cu collisions.
We present measurements of azimuthal correlations of charged hadron pairs in $\sqrt{s_{_{NN}}}=200$ GeV Au$+$Au collisions after subtracting an underlying event using a model that includes higher-order azimuthal anisotropy $v_2$, $v_3$, and $v_4$. After subtraction, the away-side ($\Delta\phi\sim\pi)$ of the highest transverse-momentum trigger ($p_T>4$ GeV/$c$) correlations is suppressed compared to that of correlations measured in $p$$+$$p$ collisions. At the lowest associated particle $p_T$, the away-side shape and yield are modified. These observations are consistent with the scenario of radiative-jet energy loss. For the lowest-$p_T$ trigger correlations, an away-side yield exists and we explore the dependence of the shape of the away-side within the context of an underlying-event model. Correlations are also studied differentially versus event-plane angle $\Psi_n$. The angular correlations show an asymmetry when selecting the sign of the trigger-particle azimuthal angle with respect to the $\Psi_2$ event plane. This asymmetry and the measured suppression of the pair yield out of plane is consistent with a path-length-dependent energy loss. No $\Psi_3$ dependence can be resolved within experimental uncertainties.
Higher-order flow harmonics for charged hadrons at midrapidity in Au$+$Au collisions at $\sqrt{s_{NN}}$ and their systematics: $v_2$, $v_3$, $v_4$, and $v_4\{\Psi_2\}$. The source of systematic uncertainties are difference among RXN event-planes, matching cut width for CNT hadron tracks, and difference between $v_n$ measured with RXN and BBC event planes .
Per-trigger yields $Y(\Delta\phi)$ of dihadrons pairs measured in Au$+$Au collisions at$\sqrt{s_{NN}}$ after subtracting the underlying event model with several $p_T$ selections and centralities. Systematic uncertainties are due to track matching and the $v_n$ and due to ZYAM.
Per-trigger yields $Y(\Delta\phi)$ of dihadron pairs measured in Au$+$Au collisions after subtracting the underlying event-model with several $p_T$ selections of the trigger and associated particles ($p_T^{t,a}$ and several centralities. Systematic uncertainties are due to track matching and the $v_n$ and due to ZYAM.
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