RHIC Beam Energy Scan Program Research Articles

K*0 production in Au+Au collisions at sNN=7.7 , 11.5, 14.5, 19.6, 27, and 39 GeV from the RHIC beam energy scan

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.

Global spin alignment of ɸ and K*0 vector mesons in Au+Au collisions from RHIC BES-II program

Global spin alignment is a preferential alignment of a particle’s spin along the orbital angular momentum produced in heavy-ion collisions. The global spin alignment of vector mesons (JP = 1−) ϕ and K*0 may be sensitive to the vorticity in the medium and hadronization mechanisms. The second phase of RHIC Beam Energy Scan (BES-II) program provides higher statistics data sets for Au+Au collisions at √SNN = 7.7 - 19.6 GeV. From this data, we can make high precision measurements of ϕ and K*0 global spin alignment, allowing for more differential studies not possible with the BES-I data. We can also compare global spin alignment between ϕ and K*0, where the lifetime of ϕ is roughly ten times larger than that of K*0 and the latter is more sensitive to hadronic rescattering. We report high precision measurements for the global spin alignment of ϕ at √SNN = 19.6 GeV from BES-II.

Dynamic modeling for heavy-ion collisions

Recent theory progress in (3+1)D dynamical descriptions of relativistic nuclear collisions at finite baryon density are reviewed. Heavy-ion collisions at different collision energies produce strongly coupled nuclear matter to probe the phase structure of Quantum Chromodynamics (QCD). Dynamical frameworks serve as a quantitative tool to study properties of hot QCD matter and map collisions to the QCD phase diagram. Outstanding challenges are highlighted when confronting theoretical models with the current and forthcoming experimental measurements from the RHIC beam energy scan program.

Strange Hadron Production in Au+Au Collisions at √SNN = 54.4 GeV

The RHIC Beam Energy Scan (BES) program was proposed to study the QCD phase transition. As a sensitive probe to the transition from the hadron gas to the Quark-Gluon Plasma, the particle ratios of strange hadrons are measured to study the properties of nuclear matter. Recently, the production of strange hadrons (KS0, Λ, Ξ, Ω, and ϕ) at mid-rapidity (|y|&lt;0.5) in Au+Au collisions at √SNN = 54.4 GeV were measured by the STAR experiment at RHIC. In this paper nuclear modification factor, baryon-to-meson ratio, as well as the overall integrated yields of these strange hadrons at √SNN = 54.4 GeV are presented. In particular, the multi-strange baryon-to-meson ratio N(Ω− + Ω¯+)/[2N(ϕ)] is studied and compared to previous measurements from other BES energies. The strange-hadron-to-pion ratios versus charged hadron multiplicity are also studied and compared to the measurements in heavy-ion collisions at higher energies.

Probing early-time longitudinal dynamics with the Λ hyperon's spin polarization in relativistic heavy-ion collisions

We systematically study the hyperon global polarization's sensitivity to the collision systems' initial longitudinal flow velocity in hydrodynamic simulations. By explicitly imposing local energy-momentum conservation when mapping the initial collision geometry to macroscopic hydrodynamic fields, we study the evolution of systems' orbital angular momentum (OAM) and fluid vorticity. We find that a simultaneous description of the $\Lambda$ hyperons' global polarization and the slope of pion's directed flow can strongly constrain the size of longitudinal flow at the beginning of hydrodynamic evolution. We extract the size of the initial longitudinal flow and the fraction of orbital angular momentum in the produced QGP fluid as a function of collision energy with the STAR measurements in the RHIC Beam Energy Scan program. We find that there is about 100-200 $\hbar$ OAM that remains in the mid-rapidity fluid at the beginning of hydrodynamic evolution. We further exam the effects of different hydrodynamic gradients on the spin polarization of $\Lambda$ and $\bar{\Lambda}$. The gradients of $\mu_B/T$ can change the ordering between $\Lambda$'s and $\bar{\Lambda}$'s polarization.

Benchmark of initial state models for heavy-ion collisions at sNN=27 and 62 GeV

Description of relativistic heavy-ion collisions at the energies of RHIC Beam Energy Scan program with fluid dynamic approach poses several challenges, one of which being a complex geometry and a longer duration of the pre-hydrodynamic stage. Therefore, existing fluid dynamic models for heavy-ion collisions at the RHIC Beam Energy Scan energies rely on rather complex initial states, such as UrQMD cascade or multi-fluid dynamics. In this study, we show that functionally simpler, non-dynamical initial states can be employed for the fluid dynamical simulations of Au-Au collisions at $\mbox{$\sqrt{s_{_{\rm NN}}}$}=27$ and 62.4~GeV. We adapt the initial states based on Monte Carlo Glauber model (GLISSANDO 2) and $\sqrt{T_A T_B}$ ansatz based on reduced thickness (T$_{\rm R}$ENTo $p=0$), extended into the longitudinal direction and finite baryon density. We find that both initial states, when coupled to a 3D event-by-event viscous fluid dynamic + cascade model, result in an overall fair reproduction of basic experimental data: pseudorapidity distributions, transverse momentum spectra and elliptic flow, at both collision energies. This is a rather surprising, given that the $\sqrt{T_A T_B}$ ansatz is functionally similar to the EKRT and IP-Glasma models, which are successful at much larger energies and rely on a partonic picture of the initial state.

Probing thermal nature of matter formed at RHIC via fluctuations

Arguments for thermalization of the QCD matter created in high-energy nuclear collisions has dominantly come from the agreement of the measured yields of produced hadrons with those from statistical thermal models. Ideally for a thermalized system, in addition to mean, the higher orders of the moments of the multiplicity distribution of produced particles should also show agreement with thermal models. In this respect, simultaneously studying the moments of the event-by-event distributions of conserved quantities like net-baryon, net-strangeness and net-charge number is best suited. We present a systematic study of comparing the results from a thermal hadron resonance gas (HRG) model with data on higher moments of net-proton, net-kaon and net-charge distributions measured at RHIC beam energy scan program. The experimental acceptances in terms of rapidity and transverse momentum are used in the model calculations which also include resonance decay. For the first time, the HRG model results are found to explain the measurements up to third order of moment with a common temperature and baryonic chemical potential. These calculations have tested the thermal nature of produced net-particle distributions up to third order, thereby providing evidence for thermalization of the QCD matter formed in such high energy heavy-ion collisions.

Self-Similarity of Negative Particle Production in Au + Au Collisions at STAR

Results of a new analysis of negative particle yields in Au + Au collisions obtained by the STAR Collaboration in the first phase of the RHIC Beam Energy Scan (BES) program in the framework of $$z$$ -scaling approach are presented. The spectra were measured over a wide range of collision energy $$\sqrt {{{s}_{{NN}}}} = 7.7 - 200$$ GeV and transverse momentum of produced particles for different centralities at $$\left| \eta \right| < 0.5.$$ The concept of the $$z$$ -scaling based on self-similarity, locality, and fractality of hadron interactions at a constituent level is verified. The constituent energy loss as a function of energy and centrality of collision and transverse momentum of inclusive particle was estimated.

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

STAR collaboration at RHIC has recently measured the polarization of Λ hyperons in non-central heavy ion collisions in the RHIC Beam Energy Scan (BES) program. The magnitude of the polarization was found to decrease from few percents at the lowest BES energies to ≈ 0.2% at the top RHIC energy. The polarization signal has been reproduced in different hydrodynamic calculations assuming a thermodynamic spin-vorticity coupling mechanism at the Cooper-Frye hypersurface [1].In this work an extension of our existing calculations [2] of the Λ polarization in the RHIC BES program to the top RHIC and 2.76 TeV LHC energies is presented. The longitudinal component of the Λ polarization, which is the dominant component of the polarization at the LHC energies, is discussed. Finally we show that the global polarization of Λ originates dominantly from the relativistic analogue of the classical vorticity, whereas the quadrupole longitudinal component originates from the gradients of temperature and acceleration of the medium when the Λs are produced out of the fluid.

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

The geometry and the dynamics of the particle-emitting source in high-energy heavy-ion collisions can be inferred via femtoscopy measurement. Two-particle correlations at small relative momentum exploit quantum statistics and the final-state interactions which allow one to study the space-time characteristics of the source of the order of 10−15 m and 10−23 s, respectively. The RHIC Beam Energy Scan (BES) program covers a significant part of the QCD Phase Diagram using collisions of Au nuclei at eight beam energies in the range from 7.7 to 200 GeV, covering the baryon-rich region, which can be studied via baryon femtoscopy. Thus, two-baryon measurements together with two-meson and meson-baryon correlations provide complementary information about the source characteristics.In this proceedings, the STAR preliminary results on femtoscopic observables of various particle combinations of protons, pions and kaons from Au+Au collisions at BES energies are presented. Determining how the properties of the particle-emitting source depend on collision energy is an important step towards understanding the physics of heavy-ion collisions. The BES program provides the possibility to study the energy dependence of the source sizes for various collision centralities. In addition to the source size, the measurements of non-identical particle combinations provide information about space-time asymmetries in emission process.

Estimation of the freeze-out parameters reachable in a fixed-target experiment at the CERN Large Hadron Collider

Estimation of the freeze-out parameters as a function of the rapidity in Pb + Pb collisions at $\sqrt{{s}_{NN}}=72\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$ in the AFTER@LHC project is performed. The conventional hadron resonance gas model is used for analysis of the events generated by the UrQMD model. The results indicate that one may obtain at least a 2.5 times increase in the baryon chemical potential ${\ensuremath{\mu}}_{B}$ in the forward rapidity range compared to the midrapidity range. The ${\ensuremath{\mu}}_{B}$ values in the rapidity range of $0&lt;y&lt;4.5$ for AFTER@LHC are comparable to those covered by the RHIC Beam Energy Scan program. Thus, a rapidity scan in the AFTER@LHC project provides a complementary approach for study of the QCD phase diagram.

Chiral magnetic effect in the Dirac–Heisenberg–Wigner formalism

In this paper the emergence of the Chiral Magnetic Effect (CME) and the related anomalous current is investigated using the real time Dirac–Heisenberg–Wigner formalism. This method is widely used for describing strong field physics and QED vacuum tunneling phenomena as well as pair production in heavy-ion collisions. We extend earlier investigations of the CME in constant flux tube configuration by considering time dependent electric and magnetic fields. In this model we can follow the formation of axial charge separation, formation of axial current and then the emergence of the anomalous electric current. Qualitative results are shown for special field configurations that help to interpret the predictions of CME related effects in heavy-ion collisions at the RHIC Beam Energy Scan program.

Recent results on small μB QCD from the lattice

I will briefly review our current understanding of the phase diagram of QCD, from the point of view of recent first principle lattice calculations. Whenever possible, I will also make contact with the experimental efforts of the RHIC Beam Energy Scan Program.

Measurement of the cumulants of net-proton multiplicity distribution in Au+Au collisions at [formula omitted] – 200 GeV from the STAR experiment

One of the main goals of the RHIC Beam Energy Scan program is to search for the QCD Critical Point (CP) and phase transition in heavy-ion collisions. Fluctuations of conserved quantities are highly sensitive to the correlation length, and are directly connected to the susceptibilities in the Lattice QCD. Therefore, they are ideal observables for finding the CP and phase transition signatures. We present measurements of the cumulants of net-proton distributions from Au+Au collisions at sNN=7.7,11.5,14.5,19.6,27,39,62.4 (up to the fourth-order) and 200 GeV (up to the sixth-order) measured by the STAR experiment at RHIC. We also present the multiparticle correlation functions extracted from the proton and anti-proton cumulants. It is observed that the four-particle correlations are positive and show non-monotonic energy dependence in the fourth-order fluctuations. Finally, we discuss the corresponding physics implications.

Recent STAR heavy-ion results

The recent STAR heavy-ion results obtained in the first phase of the RHIC Beam Energy Scan program are presented. The measurements of particle spectra have been performed over a wide range of collision energy √sNN =7.7-200 GeV, centrality and transverse momentum of produced particles. The fixed target mode in heavy-ion collisions at the STAR experiment also extends considerably the range of search for the new physics. Heavy quarks provide an exceptional probe in understanding properties of the hot and dense medium created in such collisions. The Heavy Flavor Tracker (HFT) and Muon Telescope Detector (MTD) upgrades at the STAR experiment at RHIC significantly improved the experimental capabilities of TPC, ToF and EMC detectors in measuring both open and hidden heavy flavor hadrons in heavy-ion collisions.

Fixed Target Collisions at STAR

The RHIC Beam Energy Scan (BES) program was proposed to look for the turn-off of signatures of the quark gluon plasma (QGP), search for a possible QCD critical point, and study the nature of the phase transition between hadronic and partonic matter. Previous results have been used to claim that the onset of deconfinement occurs at a center-of-mass energy of 7 GeV. Data from lower energies are needed to test if this onset occurs. The goal of the STAR Fixed-Target Program is to extend the collision energy range in BES II to energies that are likely below the onset of deconfinement. Currently, STAR has inserted a gold target into the beam pipe and conducted test runs at center-of-mass energies of 3.9 and 4.5 GeV. Tests have been done with both Au and Al beams. First physics results from a Coulomb potential analysis of Au + Au fixed-target collisions are presented and are found to be consistent with results from previous experiments. Furthermore, the Coulomb potential, which is sensitive to the Z of the projectile and degree of baryonic stopping, will be compared to published results from the AGS.

Higher Moments of Net-Particle Multiplicity Distributions

Studying fluctuations of conserved quantities, such as baryon number, strangeness, and charge, provides insights into the properties of matter created in high-energy nuclear collisions. Lattice QCD calculations suggest that higher moments of these quantities are sensitive to the phase structure of the hot and dense nuclear matter created in such collisions. In this paper, we present first experimental results of volume and temperature independent cumulant ratios of net-charge and net-proton distributions in Au+Au collisions at sNN=14.5 GeV completing the first RHIC Beam Energy Scan (BES-I) program for sNN=7.7 to 200GeV, together with the first measurement of fully corrected net-kaon results, measured with the STAR detector at RHIC at mid-rapidity and a transverse momentum up to pT = 2GeV/c. The pseudorapidity dependence of the sNN=14.5 GeV net-charge cumulant ratios is discussed. The estimated uncertainties on the ratio c4/c2, the most statistics-hungry of the present observables, at sNN=7.7 GeV in the upcoming RHIC BES-II program will also be presented.

32nd Winter Workshop on Nuclear Dynamics

These are the proceedings of the 32nd Winter Workshop on Nuclear Dynamics, which was held in Guadeloupe, Caribbean, in February/March 2016. As in previous years, the unique character of this conference series has allowed us to bring together nuclear scientists with very different interests to discuss recent progress and scientific achievements.Out of the 44 contributions at WWND 2016 we have selected these 27 representative manuscripts. The topics capture the range of theoretical and experimental advances in our field, from very exciting results from the RHIC beam energy scan program to the small systems measurements at the LHC. On the theory side this system size dependence led to a detailed evaluation of the initial conditions and subsequent hydrodynamical or transport approaches, as well as a profound discussion on how small a QGP system can be. These results were complemented by the most recent continuum extrapolated data from lattice in order to model the complete evolution of the relativistic heavy ion system.These proceedings of the 32nd Winter Workshop on Nuclear Dynamics again provide a snapshot of the status of the field. The articles, many of which were written by some of the most promising young scientists in the field, are documenting the excitement and achievements that are characteristic for modern day nuclear science.Joerg Aichelin, SUBATECH, Ecole des Mines, FranceRene Bellwied, University of Houston, USAElena Bratkovskaya, FIAS, Johann Wolfgang Goethe-Universitaet, GermanyCarsten Greiner, Johann Wolfgang Goethe-Universitaet, GermanyChristina Markert, University of Texas at Austin, USA

Energy Dependence of Moments of Net-Proton, Net-Kaon, and Net-Charge Multiplicity Distributions at STAR

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.

The fixed-target experiment at STAR

The RHIC Beam Energy Scan (BES-I) program, which covers center-of-mass energies 7.7 GeV to 39 GeV, was proposed to look for the turn-off of signatures of the quark gluon plasma, search for a possible QCD critical point, and study the nature of the phase transition between hadronic and partonic matter. RHIC BES-I has shown that the partonic interactions are dominant at center-of-mass energies above 20 GeV. Several observables, including v1 of protons and Lambdas, v2 of all identified hadrons, and net-proton higher moments, show interesting behavior below 20 GeV and could suggest a transition to a hadron interaction dominated regime. Data from energies lower than 7 GeV could help determine whether these behaviors are indicative of phase transitions or criticality. The goal of the STAR Fixed-Target Program is to extend the collision energy range in BES II to lower energies than what is feasible at RHIC with colliding beams. First physics results from test runs of Au + Au fixed-target collisions will be presented. The results will be compared with published results from similar collision energies at the AGS. The implications for the fixed-target program after the completion of the inner TPC (iTPC) and endcap TOF (eTOF) detector upgrades will also be discussed.