Search for the Critical Point of Strongly Interacting Matter in Heavy-Ion Collisions

Event-by-event fluctuations of global observables in relativistic heavy-ion collisions are studied as probes for the QCD phase transition and as tools to search for critical phenomena near the phase boundary. Dynamical fluctuations in mean transverse momentum, identified particle ratios and conserved quantities (such as net-charge, net-baryon) are expected to provide signatures of a de-confined state of matter. Non-monotonic behavior in the higher-moments of conserved quantities as a function of beam energy and collision centrality are proposed as signatures of the QCD critical point. To study the QCD phase transition and locate the critical point, the STAR experiment at RHIC has collected a large amount of data for Au+Au collisions from = 7.7 - 200 GeV in the RHIC Beam Energy Scan (BES) program. We present the recent beam energy scan results on dynamical fluctuations of particle ratios and two-particle transverse momentum correlations at mid-rapidity. Higher-moments of the net-charge and net- proton multiplicity distributions as a function of beam energy will be presented. We give a summary of what has been learnt so far and future prospectives for the BES-II program.

Transition to hot quark matter in relativistic heavy-ion collision

Geometry and Dynamics in Heavy-Ion Collisions Seen by the Femtoscopy in the STAR Experiment

Geometry and dynamics of the particle-emitting source in heavy-ion collisions can be inferred via the femtoscopy method. Two-particle correlations at small relative momentum exploit Quantum Statistics (QS) and the Final State Interactions (FSI), which allow one to study the space-time characteristics of the source of the order of $10^{-15}$ m and $10^{-23}$ s. The RHIC Beam Energy Scan (BES) program covers a significant part of the QCD Phase Diagram using Au nuclei collisions for several beam energies from 3.0 to 200 GeV, where the baryon-rich region is studied via femtoscopy. Together with meson ones, baryon measurements provide complementary information about the source created in the final states of interactions. The results of non-identical particles enable studies of space-time asymmetries in the emission process. Besides, femtoscopy enables the investigation of FSI between hadrons. The femtoscopic measurements of various particle combinations for different collision energies and centralities will be shown in these proceedings.

One of the main goals of the RHIC Beam Energy Scan (BES) program is to study the QCD phase structure, which includes the search for the QCD critical point, over a wide range of chemical potential (μB). Theoretical calculations predict that fluctuations of conserved quantities, such as baryon number (B), charge (Q), and strangeness (S), are sensitive to the correlation length of the dynamical system. Experimentally, higher moments of multiplicity distributions have been utilized to search for the QCD critical point in heavy-ion collisions. In this paper, we report recent efficiency-corrected cumulants and cumulants ratios of the net- proton, net-kaon, and net-charge multiplicity distributions in Au+Au collisions at √sNN = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV collected in the years 2010, 2011, and 2014 with STAR at RHIC. The centrality and energy dependence of the cumulants up to the fourth order, as well as their ratios, are presented. Furthermore, the comparisons with baseline calculations (Poisson) and non-critical-point models (UrQMD) will also be discussed.

This paper presents the energy dependence of multiplicity and net-electric charge fluctuations in p+p interactions at beam momenta 20, 31, 40, 80, and 158 GeV/c. Results are corrected for the experimental biases and quantified with the use of cumulants and factorial cumulants. Cumulant ratios are an essential tool in the search for the critical point of strongly interacting matter in heavy ion collisions. Measurements performed in p+p interactions provide a vital baseline estimation in these studies. The measured signals are compared with the string hadronic models Epos1.99 and FTFP-BERT.

Studies of extremely dense matter in heavy-ion collisions at J-PARC

The NICA (Nuclotron-based Ion Collider fAcility) project is under realization at the Joint Institute for Nuclear Research (JINR, Dubna). The main goal of the project is an experimental study of hot and dense strongly interacting matter in heavy ion (up to Au) collisions at center-of-mass energies up to 11 GeV per nucleon. The physics program will be performed at two experiments, BM@N (Baryonic Matter at Nuclotron) at beams extracted from the Nuclotron, and MPD (Multi-Purpose Detector) at the NICA collider. The aim of the BM@N experiment is to study interactions of relativistic heavy ion beams with fixed targets. The scientific program comprises studies of nuclear matter in the intermediate energy range between experiments at the SIS and NICA/FAIR facilities. The BM@N experiment has recorded first experimental data in the carbon, argon and krypton beams of kinetic energy per nucleon ranging from 2.3 to 4.5 GeV per nucleon. The first measurement of short range correlations of nucleons in carbon nucleus was performed in inverse kinematics with the carbon beam and liquid hydrogen target. The MPD detector is under construction to study hot and baryon rich QCD matter in heavy ion collisions at the NICA collider in the energy range $\sqrt{s_{NN}} = 4 \div 11$ GeV. Physics program includes the study of collective phenomena, $\Lambda$ polarization, dilepton, hyperon and hypernuclei production under extreme conditions of highest baryonic density.

Thermalized matter created in noncentral relativistic heavy-ion collisions is expected to be tilted in the reaction plane with respect to the beam axis. The most notable consequence of this forward-backward symmetry breaking is the observation of rapidity-odd directed flow for charged particles. On the other hand, the production points for heavy quarks are forward-backward symmetric and shifted in the transverse plane with respect to the fireball. The drag on heavy quarks from the asymmetrically distributed thermalized matter generates substantial directed flow for heavy flavor mesons. We predict a very large rapidity-odd directed flow of D mesons in noncentral Au-Au collisions at sqrt[s_{NN}]=200 GeV, several times larger than for charged particles. A possible experimental observation of a large directed flow for heavy flavor mesons would represent an almost direct probe of the three-dimensional distribution of matter in heavy-ion collisions.

Lambda polarization in heavy ion collisions: from RHIC BES to LHC energies

Nuclear matter at extremely high temperature and high density is expected to undergo a phase transition to a different state of matter, that is made of deconfined quarks and gluons, such as Quark Gluon Plasma (QGP). The QGP is believed to have existed in early universe just after the Big Bang and/or inside neutron stars. High energy heavy-ion collisions have been carried out in various experiments at AGS, SPS, RHIC and LHC accelerator facilities at Brookhaven National Laboratory (BNL) and European Organization for Nuclear Research (CERN) to create and explore such new state of matter. The experimental results at the highest possible energy regions at RHIC and LHC indicate that the new state of matter has indeed been formed with a partonic degree of freedom, and the phase transition seems to be a smooth crossover that is also expected from the theoretical calculations in the high temperature region of the Quantum Chromo Dynamics (QCD) phase diagram. In the high density area of the QCD phase diagram, the transition is expected to be a first order phase transition and there could be a critical end point for the first order phase transition. The high density region can be reached by lowering the beam energy to a few GeV to a few 10 GeV. Experimental studies with heavy-ion collisions aiming at this high density region are currently being pursued at SPS and RHIC. Future facilities at FAIR, NICA, HIAF and J-PARC are planned. The STAR experiment has been extensively working on the beam energy scan program at RHIC especially around this beam energy region in order to find the first order phase transition as well as signatures from the critical end point in the QCD phase diagram. The recent experimental results of STAR collaboration from the RHIC beam energy scan program will be presented and discussed in this proceedings.

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.

The RHIC Beam Energy Scan Program: Results from the PHENIX Experiment

The PHENIX Experiment at RHIC has conducted a beam energy scan at several collision energies in order to search for signatures of the QCD critical point and the onset of deconfinement. PHENIX has conducted measurements of transverse energy production, muliplicity fluctuations, and the skewness and kurtosis of net charge distributions. The data analyzed to date show no significant indications of the presence of the critical point.

Fixed Target Collisions at STAR