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.
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.
The production of charged pions, kaons and (anti)protons has been measured at mid-rapidity ($-0.5 y 0$) in p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV using the ALICE detector at the LHC. Exploiting particle identification capabilities at high transverse momentum ($p_{\rm T}$), the previously published $p_{\rm T}$ spectra have been extended to include measurements up to 20 GeV/$c$ for seven event multiplicity classes. The $p_{\rm T}$ spectra for pp collisions at $\sqrt{s}=7$ TeV, needed to interpolate a pp reference spectrum, have also been extended up to 20 GeV/$c$ to measure the nuclear modification factor ($R_{\rm pPb}$) in non-single diffractive p-Pb collisions. At intermediate transverse momentum ($2 p_{\rm T} 10$\,GeV/$c$) the proton-to-pion ratio increases with multiplicity in p-Pb collisions, a similar effect is not present in the kaon-to-pion ratio. The $p_{\rm T}$ dependent structure of such increase is qualitatively similar to those observed in pp and heavy-ion collisions. At high $p_{\rm T}$ ($>10$ GeV/$c$), the particle ratios are consistent with those reported for pp and Pb-Pb collisions at the LHC energies. At intermediate $p_{\rm T}$ the (anti)proton $R_{\rm pPb}$ shows a Cronin-like enhancement, while pions and kaons show little or no nuclear modification. At high $p_{\rm T}$ the charged pion, kaon and (anti)proton $R_{\rm pPb}$ are consistent with unity within statistical and systematic uncertainties.
pT-differential invariant yield of charged pions in p-Pb collisions with centre-of-mass energy/nucleon=5.02 TeV, measured for different V0A multiplicity classes. The first uncertainty is statistical, the second one is the total systematic uncertainty, while the third one is the uncorrelated systematic uncertainty which is multiplicity dependent.
pT-differential invariant yield of charged pions in p-Pb collisions with centre-of-mass energy/nucleon=5.02 TeV, measured for NSD events. The first uncertainty is statistical, the second one is the total systematic uncertainty, while the third one is the uncorrelated systematic uncertainty which is multiplicity dependent.
pT-differential invariant yield of charged kaons in p-Pb collisions with centre-of-mass energy/nucleon=5.02 TeV, measured for different V0A multiplicity classes. The first uncertainty is statistical, the second one is the total systematic uncertainty, while the third one is the uncorrelated systematic uncertainty which is multiplicity dependent.
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.
Measurements of midrapidity charged particle multiplicity distributions, $dN_{\rm ch}/d\eta$, and midrapidity transverse-energy distributions, $dE_T/d\eta$, are presented for a variety of collision systems and energies. Included are distributions for Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$, 130, 62.4, 39, 27, 19.6, 14.5, and 7.7 GeV, Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=200$ and 62.4 GeV, Cu$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV, U$+$U collisions at $\sqrt{s_{_{NN}}}=193$ GeV, $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV, $^{3}$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV, and $p$$+$$p$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV. Centrality-dependent distributions at midrapidity are presented in terms of the number of nucleon participants, $N_{\rm part}$, and the number of constituent quark participants, $N_{q{\rm p}}$. For all $A$$+$$A$ collisions down to $\sqrt{s_{_{NN}}}=7.7$ GeV, it is observed that the midrapidity data are better described by scaling with $N_{q{\rm p}}$ than scaling with $N_{\rm part}$. Also presented are estimates of the Bjorken energy density, $\varepsilon_{\rm BJ}$, and the ratio of $dE_T/d\eta$ to $dN_{\rm ch}/d\eta$, the latter of which is seen to be constant as a function of centrality for all systems.
Transverse energy in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV
Multiplicity in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV
Transverse energy in Au+Au collisions at $\sqrt{s_{NN}}$ = 130 GeV
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.
The production of (anti-)deuteron and (anti-)$^{3}$He nuclei in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV has been studied using the ALICE detector at the LHC. The spectra exhibit a significant hardening with increasing centrality. Combined blast-wave fits of several particles support the interpretation that this behavior is caused by an increase of radial flow. The integrated particle yields are discussed in the context of coalescence and thermal-statistical model expectations. The particle ratios, $^3$He/d and $^3$He/p, in Pb-Pb collisions are found to be in agreement with a common chemical freeze-out temperature of $T_{\rm chem} \approx 156$ MeV. These ratios do not vary with centrality which is in agreement with the thermal-statistical model. In a coalescence approach, it excludes models in which nucleus production is proportional to the particle multiplicity and favors those in which it is proportional to the particle density instead. In addition, the observation of 31 anti-tritons in Pb-Pb collisions is reported. For comparison, the deuteron spectrum in pp collisions at $\sqrt{s} = 7$ TeV is also presented. While the p/$\pi$ ratio is similar in pp and Pb-Pb collisions, the d/p ratio in pp collisions is found to be lower by a factor of 2.2 than in Pb-Pb collisions.
Anti-deuteron over deuteron ratio versus pT per nucleon for various centrality classes for Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV.
Anti-deuteron over deuteron ratio versus pT per nucleon for various centrality classes for Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV.
Anti-$^{3}$He over $^{3}$He ratio versus pT per nucleon for 0-20% centrality class.
We report the measurement of cumulants ($C_n, n=1\ldots4$) of the net-charge distributions measured within pseudorapidity ($|\eta|<0.35$) in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=7.7-200$ GeV with the PHENIX experiment at the Relativistic Heavy Ion Collider. The ratios of cumulants (e.g. $C_1/C_2$, $C_3/C_1$) of the net-charge distributions, which can be related to volume independent susceptibility ratios, are studied as a function of centrality and energy. These quantities are important to understand the quantum-chromodynamics phase diagram and possible existence of a critical end point. The measured values are very well described by expectation from negative binomial distributions. We do not observe any nonmonotonic behavior in the ratios of the cumulants as a function of collision energy. The measured values of $C_1/C_2 = \mu/\sigma^2$ and $C_3/C_1 = S\sigma^3/\mu$ can be directly compared to lattice quantum-chromodynamics calculations and thus allow extraction of both the chemical freeze-out temperature and the baryon chemical potential at each center-of-mass energy.
Efficiency corrected cumulants of net-charge distributions as a function of $\langle N_{part} \rangle$ from Au+Au collisions at different collision energies.
Efficiency corrected cumulants of net-charge distributions as a function of $\langle N_{part} \rangle$ from Au+Au collisions at different collision energies.
Efficiency corrected cumulants of net-charge distributions as a function of $\langle N_{part} \rangle$ from Au+Au collisions at different collision energies.
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