Hot and Cold QCD White Paper from ALICE-USA: Input for 2023 U.S. Long Range Plan for Nuclear Science
The ALICE-USA collaboration presents its plans for the 2023 U.S. Long Range Plan for Nuclear Science.
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arXiv subjects
S. A. Voloshin
Publications and source records attributed to S. A. Voloshin.
Chiral Magnetic and Vortical Effects in High-Energy Nuclear Collisions --- A Status Report
The interplay of quantum anomalies with magnetic field and vorticity results in a variety of novel non-dissipative transport phenomena in systems with chiral fermions, including the quark-gluon plasma. Among them is the Chiral Magnetic Effect (CME) -- the generation of electric current along an external magnetic field induced by chirality imbalance. Because the chirality imbalance is related to the global topology of gauge fields, the CME current is topologically protected and hence non-dissipative even in the presence of strong interactions. As a result, the CME and related quantum phenomena affect the hydrodynamical and transport behavior of strongly coupled quark-gluon plasma, and can be studied in relativistic heavy ion collisions where strong magnetic fields are created by the colliding ions. Evidence for the CME and related phenomena has been reported by the STAR Collaboration at Relativistic Heavy Ion Collider at BNL, and by the ALICE Collaboration at the Large Hadron Collider at CERN. The goal of the present review is to provide an elementary introduction into the physics of anomalous chiral effects, to describe the current status of experimental studies in heavy ion physics, and to outline the future work, both in experiment and theory, needed to eliminate the existing uncertainties in the interpretation of the data.
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Charge-dependent directed flow in asymmetric nuclear collisions
The directed flow of identified hadrons is studied within the parton-hadron-string-dynamics (PHSD) approach for the asymmetric system Cu+Au in non-central collisions at $\sqrt{s_{NN}}$ = 200 GeV. It is emphasized that due to the difference in the number of protons of the colliding nuclei an electric field emerges which is directed from the heavy to the light nucleus. This strong electric field is only present for about 0.25 fm/c at $\sqrt{s_{NN}}$ = 200 GeV and leads to a splitting of the directed flow $v_1$ for particles with the same mass but opposite electric charges in case of an early presence of charged quarks and antiquarks. The microscopic calculations of the directed flow for $π^\pm, K^\pm, p$ and $\bar{p}$ are carried out in the PHSD by taking into account the electromagnetic field induced by the spectators as well as its influence on the hadronic and partonic quasiparticle trajectories. It is shown that the splitting of the directed flow as a function of pseudorapidity $η$ and in particular as a function of the transverse momentum $p_t$ provides a direct access to the electromagnetic response of the very early (nonequilibrium) phase of relativistic heavy-ion collisions and allows to shed light on the presence (and number) of electric charges in this phase.
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Collective properties of nucleus-nucleus collisions from AGS to LHC energies
The azimuthal anisotropies of the collective transverse flow of charged hadrons are investigated in a wide range of heavy-ion collision energies within the microscopic Parton-Hadron-String Dynamics (PHSD) transport approach which incorporates explicit partonic degrees-of-freedom in terms of strongly interacting quasiparticles (quarks and gluons) in line with an equation-of-state from lattice QCD as well as the dynamical hadronization and hadronic collision dynamics in the final reaction phase. The experimentally observed increase of the elliptic flow $v_2$ of charged hadrons with collision energy is successfully described in terms of the PHSD approach. The analysis of higher-order harmonics $v_3$ and $v_4$ in the azimuthal angular distribution shows a similar tendency of growing deviations between partonic and purely hadronic models with increasing collision energy. This demonstrates that the excitation functions of azimuthal anisotropies reflect the increasing role of quark-gluon degrees of freedom in the early phase of relativistic heavy-ion collisions. Furthermore, the specific variation of the ratio $v_4/(v_2)^2$ with respect to bombarding energy, centrality and transverse momentum is found to provide valuable information on the underlying partonic dynamics.
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Azimuthal anisotropies for Au+Au collisions in the parton-hadron transient energy range
The azimuthal anisotropies of the collective transverse flow of charged hadrons are investigated in a wide range of heavy-ion collision energies within the microscopic parton-hadron-string dynamics (PHSD) transport approach which incorporates explicit partonic degrees of freedom in terms of strongly interacting quasiparticles (quarks and gluons) in line with an equation of state from lattice QCD as well as the dynamical hadronization and hadronic collision dynamics in the final reaction phase. The experimentally observed increase of the elliptic flow $v_2$ of charged hadrons with collision energy is successfully described in terms of the PHSD approach. The PHSD scaling properties of various collective observables are confronted with experimental data as well as with hydrodynamic predictions. The analysis of higher-order harmonics $v_3$ and $v_4$ in the azimuthal angular distribution shows a similar tendency of growing deviations between partonic and purely hadronic models with increasing collision energy. This demonstrates that the excitation functions of azimuthal anisotropies reflect the increasing role of quark-gluon degrees of freedom in the early phase of relativistic heavy-ion collisions. Furthermore, the specific variation of the ratio $v_4/(v_2)^2$ with respect to bombarding energy, centrality and transverse momentum is found to provide valuable information on the underlying dynamics.
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Theoretical analysis of a possible observation of the chiral magnetic effect in Au + Au collisions within the RHIC beam energy scan program
In terms of the hadron-string-dynamics (HSD) approach we investigate the correlation function in the azimuthal angle $ψ$ of charged hadrons that is expected to be sensitive to a signal of local strong parity violation. Our analysis of Au+Au collisions is based on the recent STAR data within the RHIC Beam-Energy-Scan (BES) program. The HSD model reasonably reproduces STAR data for $\sqrt{s_{NN}}=$7.7 GeV, while there are some deviations from the experiment at the collision energy of 11.5 GeV and an increase of deviations between theory and experiment at $\sqrt{s_{NN}}=$39 GeV. For reference, the results for $\sqrt{s_{NN}}=$ 200 GeV are given as well. The role of the retarded electromagnetic field is discussed and a compensation effect for the action of its electric and magnetic components is pointed out. We conclude that the recent RHIC BES data at $\sqrt{s_{NN}}=$7.7 and 11.5 GeV can be understood on the hadronic level without involving the idea of a strong parity violation; however, at $\sqrt{s_{NN}}\sim$40 GeV and above one needs to take into consideration explicit partonic (quark-qluon) degrees-of-freedom for a proper treatment of the dynamics.
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Electromagnetic field evolution in relativistic heavy-ion collisions
The hadron string dynamics (HSD) model is generalized to include the creation and evolution of retarded electromagnetic fields as well as the influence of the magnetic and electric fields on the quasiparticle propagation. The time-space structure of the fields is analyzed in detail for non-central Au+Au collisions at $\sqrt{s_{NN}}=$200 GeV. It is shown that the created magnetic field is highly inhomogeneous but in the central region of the overlapping nuclei it changes relatively weakly in the transverse direction. For the impact parameter $b=$10 fm the maximal magnetic field - perpendicularly to the reaction plane - is obtained of order $eB_y/m_π^2\sim$5 for a very short time $\sim$ 0.2 fm/c, which roughly corresponds to the time of a maximal overlap of the colliding nuclei. We find that at any time the location of the maximum in the $eB_y$ distribution correlates with that of the energy density of the created particles. In contrast, the electric field distribution, being also highly inhomogeneous, has a minimum in the center of the overlap region. Furthermore, the field characteristics are presented as a function of the collision energy and the centrality of the collisions. To explore the effect of the back reaction of the fields on hadronic observables a comparison of HSD results with and without fields is exemplified. Our actual calculations show no noticeable influence of the electromagnetic fields - created in heavy-ion collisions - on the effect of the electric charge separation with respect to the reaction plane.
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Directed and Elliptic Flow in Pb+Pb collisions at 40 and 158 AGeV
Directed and elliptic flow are reported for charged pions and protons as a function of transverse momentum, rapidity, and centrality in 40 and 158 AGeV Pb + Pb collisions. The standard method of correlating particles with an event plane is used. The directed flow of protons is small and shows little variation near to midrapidity, but rises fast towards projectile rapidity in the 40 AGeV data. For most peripheral collisions the flat region becomes negative resulting in $v_1$ changing sign three times. Elliptic flow doesn't seem to change very much from 40 AGeV to 158 AGeV. The difference is smaller than anticipated from the overall energy dependence from AGS to RHIC.
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Anisotropic flow
Recent experimental results on directed and elliptic flow, theoretical developments, and new techniques for anisotropic flow analysis are reviewed.
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Effects of momentum conservation on the analysis of anisotropic flow
We present a general method for taking into account correlations due to momentum conservation in the analysis of anisotropic flow, either by using the two-particle correlation method or the standard flow vector method. In the latter, the correlation between the particle and the flow vector is either corrected through a redefinition (shift) of the flow vector, or subtracted explicitly from the observed flow coefficient. In addition, momentum conservation contributes to the reaction plane resolution. Momentum conservation mostly affects the first harmonic in azimuthal distributions, i.e., directed flow. It also modifies higher harmonics, for instance elliptic flow, when they are measured with respect to a first harmonic event plane such as one determined with the standard transverse momentum method. Our method is illustrated by application to NA49 data on pion directed flow.
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Anisotropic flow from AGS to RHIC
The recent results on anisotropic flow in ultrarelativistic nuclear collisions along with recent methodical developments and achievements in the understanding of the phenomena, are reviewed. The emphasis is given to the elliptic flow results.
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Novel rapidity dependence of directed flow in high energy heavy ion collisions
For high energy nucleus-nucleus collisions, we show that a combination of space-momentum correlations characteristic of radial expansion together with the correlation between the position of a nucleon in the nucleus and its stopping, results in a very specific rapidity dependence of directed flow: a reversal of sign in the mid-rapidity region. We support our argument by RQMD model calculations for Au+Au collisions at $\sqrt s$ = 200 AGeV.
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The physics of the centrality dependence of elliptic flow
The centrality dependence of elliptic flow and how it is related to the physics of expansion of the system created in high energy nuclear collisions is discussed. Since in the hydro limit the centrality dependence of elliptic flow is mostly defined by the elliptic anisotropy of the overlapping region of the colliding nuclei, and in the low density limit by the product of the elliptic anisotropy and the multiplicity, we argue that the centrality dependence of elliptic flow should be a good indicator of the degree of equilibration reached in the reaction. Then we analyze experimental data obtained at AGS and SPS energies. The observed difference in the centrality dependence of elliptic flow could imply a transition from a hadronic to a partonic nature of the system evolution. Finally we exploit the multiplicity dependence of elliptic flow to make qualitative predictions for RHIC and LHC.
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The relationship between particle freeze-out distributions and HBT radius parameters
The relationship between pion and kaon space-time freeze-out distributions and the HBT radius parameters in high-energy nucleus-nucleus collisions is investigated. We show that the HBT radius parameters in general do not reflect the R.M.S. deviations of the single particle production points. Instead, the HBT radius parameters are most closely related to the curvature of the two-particle space-time relative position distribution at the origin. We support our arguments by studies with a dynamical model (RQMD 2.4).
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Centrality Dependence of Directed and Elliptic Flow at the SPS
New data with a minimum bias trigger for 158 GeV/nucleon Pb + Pb have been analyzed. Directed and elliptic flow as a function of rapidity of the particles and centrality of the collision are presented. The centrality dependence of the ratio of elliptic flow to the initial space elliptic anisotropy is compared to models.
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Anisotropic Flow at STAR
We first review previous work on anisotropic flow at the AGS and SPS. Then the physics related to flow is discussed as well as the interaction of flow with other non-flow measurements. From 40k RQMD and 100k HIJING events predictions for anisotropic flow at RHIC are presented. Using the STAR detector acceptance, estimates for the resolution obtainable with STAR are shown. We conclude that it should be possible to obtain good measurements for elliptic flow with either the STAR main TPC or forward TPCs. Anisotropic flow should be easily one of the first results from STAR.
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Event-by-event fluctuations in collective quantities
We discuss an event-by-event fluctuation analysis of particle production in heavy ion collisions. We compare different approaches to the evaluation of the event-by-event dynamical fluctuations in quantities defined on groups of particles, such quantities as mean transverse momentum, transverse momentum spectra slope, strength of anisotropic flow, etc.. The direct computation of the dynamical fluctuations and the sub-event method are discussed in more detail. We also show how the fluctuation in different variables can be related to each other.
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Methods for analyzing anisotropic flow in relativistic nuclear collisions
The strategy and techniques for analyzing anisotropic flow (directed, elliptic, etc.) in relativistic nuclear collisions are presented. The emphasis is on the use of the Fourier expansion of azimuthal distributions. We present formulae relevant for this approach, and in particular, show how the event multiplicity enters into the event plane resolution. We also discuss the role of non-flow correlations and a method for introducing flow into a simulation.
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