Research Highlights

For the first time, evidence has been found for a significant spin alignment of vector mesons in heavy-ion collisions. The effect is strongest at low pT with respect to a vector perpendicular to the reaction plane and for midcentral (10–50%) collisions. These observations are qualitatively consistent with expectations from the effect of large initial angular momentum in noncentral heavy-ion collisions, which leads to quark polarization via spin-orbit coupling, subsequently transferred to hadronic degrees of freedom by hadronization via recombination.

We have studied the bulk properties of matter in the Beam Energy Scan program (BES) at RHIC. The BES program covers the energy range from 7.7 GeV to 39 GeV which along with top RHIC energy corresponds to the baryon chemical potential region of 20-400 MeV. The mid-rapidity yields of identified hadrons have been presented. They show the expected signatures of a high-baryon stopping region at lower energies. At high energies, the pair production mechanism dominates the particle production. At intermediate energies there is clearly a transition between these two regions, which is explored by the BES Program. The data have been used to analyse both chemical and kinetic freeze-out parameters. The results indicate in heavy-ion collisions a clear centrality dependence of the baryon chemical potential at lower energies. The centrality dependence of the freeze-out parameters provides an opportunity for the BES program at RHIC to enlarge the \((T, \mu_B)\) region of the phase diagram to search for the QCD critical point. The difference between chemical and kinetic freeze-out increases with increasing energy suggesting increasing hadronic interactions after chemical freeze-out at higher energies.

The \(K^*\) yields relative to charged kaons in Pb-Pb collisions show a suppression with respect to pp collisions, which increases with the system size. In contrast, no such suppression is observed for the \(\phi\) mesons. The lack of suppression for the \(\phi\) meson can be attributed to the fact that most of them decay outside the fireball because of its longer lifetime (\(46.3 \pm 0.4\) fm/c). Because of a shorter lifetime (\(K^* = 4.16 \pm 0.05\) fm/c), a significant number of produced \(K^*\) decays in the hadronic medium. The decay product(s) undergo interactions with other hadrons in the medium resulting in a significant change in their momentum, and no longer contributing to the \(K^*\) signal reconstructed in the experiment. Although both rescattering and regeneration are possible, the results presented here represent an experimental demonstration of the predominance of rescattering effects in the hadronic phase of the system produced in heavy-ion collisions.

We report the measurements of the higher moments and their products (Sσ and κσ2) of the net-proton distributions at midrapidity \((|y|\lt 0.5)\) within \(0.4\lt p_T \lt 0.8GeV\) in Au+Au collisions over a wide range of \(\sqrt{s_\mathrm{NN}}\) and \(\mu_B\) have been presented to search for a possible Critical Point (CP) and signals of a phase transition in the collisions. These observables show a centrality and energy dependence, which are neither reproduced by non-CP transport model calculations, nor by a hadron resonance gas model. For \(\sqrt{s_\mathrm{NN}}\gt 39\) GeV, \(S\sigma\) and \(\kappa\sigma^2\) values are simi-lar for central, peripheral Au+Au collisions and p+p collisions. Deviations for both \(\kappa\sigma^2\) and \(S\sigma\) from HRG and Skellam expectations are observed for \(\sqrt{s_\mathrm{NN}}\leq 27\) GeV. The data presented here also provides information to extract freeze-out conditions in heavy-ion collisions using QCD based approaches .

We report the first results of the moments of net-charge multiplicity distributions for\(|\eta|\lt 0.5\) as a function of centrality for Au+Au collisions at seven collision energies from \(\sqrt{s_\mathrm{NN}}=7.7\) to \(200\) GeV are presented. These data can be used to explore the nature of the QCD phase transition and to locate the QCD critical point. The measured moments of net-charge multiplicity distributions provide unique information about the freeze-out parameters by directly comparing with theoretical model calculations. Future measurements with high statistics data will be needed for precise determination of freeze-out conditions and to make definitive conclusions regarding the critical point.

We report the first measurement of the net-charge fluctuations in Pb-Pb collisions at \(\sqrt{s_\mathrm{NN}} = 2.76\) TeV, measured with the ALICE detector at the CERN Large Hadron Collider. The dynamical fluctuations per unit entropy are observed to decrease when going from peripheral to central collisions. An additional reduction in the amount of fluctuations is seen in comparison to the results from lower energies. We examine the dependence of fluctuations on the pseudorapidity interval, which may account for the dilution of fluctuations during the evolution of the system. We find that the fluctuations at the LHC are smaller compared to the measurements at the BNL Relativistic Heavy Ion Collider, and as such, closer to what has been theoretically predicted for the formation of a quark-gluon plasma.

We report the first observation of a beam-energy-dependent difference in elliptic flow versus transverse momentum between particles and corresponding antiparticles for minimum bias \(\sqrt{s_\mathrm{NN}}= 7.7-62.4\) GeV Au+Au collisions at midrapidity. The difference increases with decreasing beam energy. Baryons show a larger difference compared to mesons. The relative values of \(v_2\) for charged pions have the opposite trend to the values of charged kaons. It is concluded that, at the lower energies, particles and anti-particles are no longer consistent with the single NCQ scaling that was observed for \(\sqrt{s_\mathrm{NN}}=200\) GeV. However, for the group of particles the NCQ scaling holds within 10% while for the group of antiparticles the difference between baryon and meso \(v_2\) continues to decrease to lower energies. We further observed that the phi‑meson v2 atthe highest measured mT-m0 value is low compared to other hadrons at \(\sqrt{s_\mathrm{NN}}=7.7\)and \(11.5\) GeV with \(1.8\sigma\) and \(2.3\sigma\), respectively.

High-energy nuclear collisions create an energy density similar to that of the Universe microseconds after the Big Bang; in both cases, matter and antimatter are formed with comparable abundance. However, the relatively short-lived expansion in nuclear collisions allows antimatter to decouple quickly from matter, and avoid annihilation. Thus, a high-energy accelerator of heavy nuclei provides an efficient means of producing and studying antimatter. The antimatter helium-4 nucleus (\(\bar{^4\mathrm{He}}\)), also known as the anti-\(\alpha\) (\(\bar{\alpha}\)), consists of two antiprotons and two antineutrons (baryon number B = −4). It has not been observed previously, although the α-particle was identified a century ago by Rutherford and is present in cosmic radiation at the ten per cent level. Antimatter nuclei with \(B  \lt −1\) have been observed only as rare products of interactions at particle accelerators, where the rate of antinucleus production in high-energy collisions decreases by a factor of about 1,000 with each additional antinucleon. Here we report the observation of \(\bar{^4\mathrm{He}}\), the heaviest observed antinucleus to date. In total, 18 \(\bar{^4\mathrm{He}}\) counts were detected at the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) in \(10^9\) recorded gold-on-gold (Au+Au) collisions at centre-of-mass energies of 200 GeV and 62 GeV per nucleon–nucleon pair. The yield is consistent with expectations from thermodynamic and coalescent nucleosynthesis models, providing an indication of the production rate of even heavier antimatter nuclei and a benchmark for possible future observations of \(\bar{^4\mathrm{He}}\) in cosmic radiation.

We present the first measurements of identified hadron production, azimuthal anisotropy, and pion interferometry from Au+Au collisions below the nominal injection energy at the Relativistic Heavy-Ion Collider (RHIC) facility. The data were collected using the large acceptance STAR detector at \(\sqrt{s_\mathrm{NN}}=9.2\) GeV from a test run of the collider in the year 2008. Midrapidity results on multiplicity density (\(dN/dy\)) in rapidity (\(y\)), average transverse momentum, particle ratios, elliptic flow, and HBT radii are consistent with the corresponding results at similar \(\sqrt{s_\mathrm{NN}}\)​ from fixed target experiments. Directed flow measurements are presented for both midrapidity and forward rapidity regions. Furthermore the collision centrality dependence of identified particle \(dN/dy\), mean \(p_T\) , and particle ratios are discussed. These results also demonstrate the readiness of the STAR detector to undertake the proposed QCD critical point search and the exploration of the QCD phase diagram at RHIC.

We report \(K/\pi\) fluctuations from Au+Au collisions at \(\sqrt{s_\mathrm{NN}}= 19.6, 62.4, 130\) and \(200\) GeV using the STAR detector at the Relativistic Heavy Ion Collider. \(K/\pi\) fluctuations in central collisions show little dependence on incident energy and are on the same order as those from NA49 at the Super Proton Synchrotron in central Pb+Pb collisions at \(\sqrt{s_\mathrm{NN}}= 12.3\) and \(17.3\) GeV. We report results for the collision centrality dependence of \(K/\pi\) fluctuations and results for charge-separated fluctuations. We observe that the \(K/\pi\) fluctuations scale with the charged particle multiplicity density. These results may indicate that, due to later stage hadronic rescattering, the decay products of resonances are less likely to survive in central collisions than in peripheral collisions, or may be due to the onset of radial flow combined with our acceptance in \(p_T\).

We have presented a study of the energy dependence of \(\pi^\pm, p\) and \(\bar{p}\) production for Au+Au at \(\sqrt{s_\mathrm{NN}} = 62.4\) and \(200\) GeV. These measurements provide new experimental data for investigating the production of quarks, gluons and their interactions with the medium formed in heavy-ioncollisions and the interplay between coalescence of thermal partons and jet fragmentation.

We have presented the first results of photon multiplicity measurements at RHIC in the pseudorapidity region \(2.3\leq \eta\leq 3.7\). Photon production per participant pair is found to be approximately independent of centrality in this pseudorapidity region. Comparison with photon and charged pion data at RHIC and SPS energies shows, for the first time in heavy ion collisions, that photons and pions follow an energy independent limiting fragmentation behavior, as previously found for inclusive charged particles. Furthermore, photons are observed to follow a centrality independent limiting fragmentation scenario.