Non-central Heavy-ion Collisions Research Articles

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.

Λ( Unknown node mover found in MathML fragment. $ \bar {\Lambda } $ ) polarization in Au+Au collisions at RHIC

Initial large global angular momentum in non-central relativistic heavy-ion collisions can produce strong vorticity, and through the spin-orbit couping, causes the spin of particles to align with the system’s global angular momentum. We present the azimuthal angle dependent (relative to the first-order plane) global polarization for Λ hyperons in midcentral Au+Au collisions at $ \sqrt {S_{NN} } $ = 200 GeV. We also present the polarization of Λ hyperons along the beam direction as a function of Λ hyperons’ emission angle relative to the second-order event plane at $ \sqrt {S_{NN} } $ = 200 GeV. This longitudinal polarization is found to increase in more peripheral collision. The implications of the results are discussed.

Probing the topological charge in QCD matter via multiplicity up–down asymmetry

Relativistic heavy ion collisions provide the possibility to study the topological charge in QCD matter through the event-by-event fluctuating net axial charge or nonequal numbers of left- and right-handed quarks they generate in the produced quark–gluon plasma. Based on the chiral kinetic approach for nearly massless quarks and antiquarks in the strong vorticity field produced along the normal direction of the reaction plane of non-central heavy ion collisions, we show that a unique signal for the topological charge in QCD matter can be identified from the asymmetric distribution of particles with momenta pointing to the upper and lower hemispheres of the reaction plane as a result of the fluctuating net axial charge.

Tensor polarizability of the vector mesons from SU(3) lattice gauge theory

The magnetic dipole polarizabilities of the vector ρ0 and ρ± mesons in SU(3) pure gauge theory are calculated in the article. Based on this the authors explore the contribution of the dipole magnetic polarizabilities to the tensor polarization of the vector mesons in external abelian magnetic field. The tensor polarization leads to the dilepton asymmetry observed in non-central heavy ion collisions and can be also estimated in lattice gauge theory.

Global polarization of Λ hyperons in Au + Au collisions at sNN=200 GeV

Global polarization of $\Lambda$ hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV, with no significant difference between $\Lambda$ and $\bar{\Lambda}$. These new results reveal the collision energy dependence of the global polarization together with the results previously observed at $\sqrt{s_{_{NN}}}$ = 7.7 -- 62.4 GeV and indicate noticeable vorticity of the medium created in non-central heavy-ion collisions at the highest RHIC collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from the AMPT (A Multi-Phase Transport) model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperon's transverse momentum and pseudorapidity $\eta^H$ within $|\eta^H|<1$. An indication of the polarization dependence on the event-by-event charge asymmetry is observed at the $2\sigma$ level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field.

Large Directed Flow of Open Charm Mesons Probes the Three-Dimensional Distribution of Matter in Heavy-Ion Collisions.

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.

Anomalous effects of dense matter under rotation

We study the anomaly induced effects of dense baryonic matter under rotation. We derive the anomalous terms that account for the chiral vortical effect in the low-energy effective theory for light Nambu-Goldstone modes. The anomalous terms lead to new physical consequences, such as the anomalous Hall energy current and spontaneous generation of angular momentum in a magnetic field (or spontaneous magnetization by rotation). In particular, we show that, due to the presence of such anomalous terms, the ground state of the quantum chromodynamics (QCD) under sufficiently fast rotation becomes the “chiral soliton lattice” of neutral pions that has lower energy than the QCD vacuum and nuclear matter. We briefly discuss the possible realization of the chiral soliton lattice induced by a fast rotation in noncentral heavy ion collisions.

ϕ Meson Spin Alignment and the Azimuthal Angle Dependence of Λ (Λ) Polarization in Au+Au collisions at RHIC

Initial large global angular momentum in non-central relativistic heavy-ion collisions can produce strong vorticity, and through the spin-orbit coupling, causes the spin of particles to align with the system’s global angular momentum. We present the azimuthal angle dependent (relative to the reaction plane) polarization for Λ and Λ in mid-central Au+Au collisions at [see formula in PDF] = 200 GeV. We also present the ϕ meson spin alignment parameter, ρ00 in Au+Au collisions at [see formula in PDF] = 19.6, 27, 39, 62.4 and 200 GeV. The implications of the results are discussed.

Study of Lambda polarization at RHIC BES and LHC energies

In hydrodynamic approach to relativistic heavy ion collisions, hadrons with nonzero spin, produced out of the hydrodynamic medium, can acquire polarization via spin-vorticity thermodynamic coupling mechanism. The hydrodynamical quantity steering the polarization is the thermal vorticity, that is minus the antisymmetric part of the gradient of four-temperature field. Based on this mechanism there have been several calculations in hydrodynamic and non-hydrodynamic models for non-central heavy ion collisions in the RHIC Beam Energy Scan energy range, showing that the amount of polarization of produced Λ hyperons ranges from few percents to few permille, and decreases with collision energy. We report on an extension of our existing calculation of global Λ polarization in UrQMD+vHLLE model to full RHIC and LHC energies, and discuss the component of polarization along the beam direction, which is the dominant one at high energies.

Effects of magnetic field on plasma evolution in relativistic heavy-ion collisions

Very strong magnetic fields can arise in non-central heavy-ion collisions at ultrarelativistic energies, which may not decay quickly in a conducting plasma. We carry out relativistic magnetohydrodynamics (RMHD) simulations to study the effects of this magnetic field on the evolution of the plasma and on resulting flow fluctuations in the ideal RMHD limit. Our results show that magnetic field leads to enhancement in elliptic flow for small impact parameters while it suppresses it for large impact parameters (which may provide a signal for initial stage magnetic field). Interestingly, we find that magnetic field in localized regions can temporarily increase in time as evolving plasma energy density fluctuations lead to reorganization of magnetic flux. This can have important effects on chiral magnetic effect. Magnetic field has non-trivial effects on the power spectrum of flow fluctuations. For very strong magnetic field case one sees a pattern of even-odd difference in the power spectrum of flow coefficients arising from reflection symmetry about the magnetic field direction if initial state fluctuations are not dominant. We discuss the situation of nontrivial magnetic field configurations arising from collision of deformed nuclei and show that it can lead to anomalous elliptic flow. Special (crossed body-body) configurations of deformed nuclei collision can lead to presence of quadrupolar magnetic field which can have very important effects on the rapidity dependence of transverse expansion (similar to {\it beam focusing} from quadrupole fields in accelerators).

Λ hyperon polarization in relativistic heavy ion collisions from a chiral kinetic approach

Based on the chiral kinetic approach using initial conditions from a multiphase transport model, we study the spin polarizations of quarks and antiquarks in non-central heavy ion collisions at the Relativistic Heavy Ion Collider. Because of the non-vanishing vorticity field in these collisions, quarks and antiquarks are found to acquire appreciable spin polarizations in the direction perpendicular to the reaction plane. Converting quarks and antiquarks to hadrons via the coalescence model, we further calculate the spin polarizations of Lambda and anti-Lambda hyperons and find their values comparable to those measured in experiments by the STAR Collaboration.

Open Heavy Flavor Dynamics: T-dependent drag and Initial Magnetic Field

Simultaneous description of heavy quark nuclear modification factor RAA and the elliptic flow v2 is a top challenge for all the existing models. We highlight how the temperature dependence of the energy loss/transport coefficients is responsible for addressing a large part of such a puzzle along with the full solution of the Boltzmann collision integral for the momentum evolution of heavy quarks in the medium. At the same time Ultra-relativistic Heavy-Ion Collision (HIC) also generates very strong initial magnetic field (B→). The high B→ influences the evolution dynamics that is opposed by the large Faraday current due to electric field generated by the time varying B→. Since the heavy quark are produced at the early stage of HICs, we argue that their dynamics will be affected by such a strong magnetic field and we demonstrate that the directed flow, v1, of heavy quarks is a superior probe to estimate the magnetic field generated in non-central HICs

Vorticity in heavy-ion collisions at the JINR Nuclotron-based Ion Collider fAcility

Vorticity of matter generated in noncentral heavy-ion collisions at energies of the Nuclotron-based Ion Collider fAcility (NICA) at the Joint Institute for Nuclear Research (JINR) in Dubna is studied. Simulations are performed within the model of the three-fluid dynamics (3FD) which reproduces the major part of bulk observables at these energies. Comparison with earlier calculations is done. Qualitative pattern of the vorticity evolution is analyzed. It is demonstrated that the vorticity is mainly located at the border between participants and spectators. In particular, this implies that the relative $\Lambda$-hyperon polarization should be stronger at rapidities of the fragmentation regions than that in the midrapidity region.

Event-by-event generation of vorticity in heavy-ion collisions

In a noncentral heavy-ion collision, the two colliding nuclei have a finite angular momentum in the direction perpendicular to the reaction plane. After the collision, a fraction of the total angular momentum is retained in the produced hot quark-gluon matter and is manifested in the form of fluid shear. Such fluid shear creates finite flow vorticity. We study some features of such generated vorticity, including its strength, beam energy dependence, centrality dependence, and spatial distribution.

Possible formation of high temperature superconductor at an early stage of heavy-ion collisions

We investigate the effect of the inverse magnetic catalysis (IMC) on the charged $\rho$ meson condensation at finite temperature in the framework of the Nambu--Jona-Lasinio model, where mesons are calculated to the leading order of $1/N_c$ expansion by summing up infinity quark-loops. IMC for chiral condensate has been considered in three different ways, i.e. fitting Lattice data, using the running coupling constant and introducing the chiral chemical potential, respectively. It is observed that, with no IMC effect included, the critical magnetic field $eB_c$ for charged $\rho$ condensation increases monotonically with the temperature. However, including IMC substantially affects the polarized charged $\rho$ condensation around the critical temperature $T_c$ of chiral phase transition, the critical magnetic field $eB_c$ for charged $\rho$ condensation decreases with the temperature firstly, reaches to a minimum value around $T_c$, then increases with the temperature. Our calculation indicates that the charged $\rho$ condensation can exist in the temperature region of $1-1.5 T_c$ with critical magnetic field $eB_c\sim 0.15-0.3 {\rm GeV}^2$, which suggests that high temperature superconductor might be created through non-central heavy ion collisions at LHC energies. We also show that a growing electric conductivity in early stage of non-central heavy-ion collisions substantially delays the decay of strong magnetic field, which is helpful for the formation of the high temperature superconductor.

Anomalous transport model study of chiral magnetic effects in heavy ion collisions

Using an anomalous transport model for massless quarks and antiquarks, we study the effect of a magnetic field on the elliptic flows of quarks and antiquarks in relativistic heavy ion collisions. With initial conditions from a blast wave model and assuming that the strong magnetic field produced in non-central heavy ion collisions can last for a sufficiently long time, we obtain an appreciable electric quadrupole moment in the transverse plane of a heavy ion collision. The electric quadrupole moment subsequently leads to a splitting between the elliptic flows of quarks and antiquarks. The slope of the charge asymmetry dependence of the elliptic flow difference between positively and negatively charged particles is positive, which is expected from the chiral magnetic wave formed in the produced QGP and observed in experiments at the BNL Relativistic Heavy Ion Collider, only if the Lorentz force acting on the charged particles is neglected and the quark-antiquark scattering is assumed to be dominated by the chirality changing channel.

Thermal relaxation, electrical conductivity, and charge diffusion in a hot QCD medium

The response of electromagnetic (EM) fields that are produced in non-central heavy-ion collisions to electromagnetically charged quark gluon plasma can be understood in terms of charge transport and charge diffusion in the hot QCD medium. This article presents a perspective on these processes by investigating the temperature behavior of the related transport coefficients, {\it viz.} electrical conductivity and the charge diffusion coefficients along with charge susceptibility. In the process of estimating them, thermal relaxation times for quarks and gluons have been determined first. These transport coefficients have been studied by solving the relativistic transport equation in the Chapman-Enskog method. For the analysis, $2\rightarrow 2$, quark-quark, quark-gluon and gluon-gluon scattering processes are taken into account along with an effective description of hot QCD Equations of state (EOSs) in terms of temperature dependent effective fugacities of quasi-quarks (anti-quarks) and quasi-gluons. Both improved perturbative hot QCD EOSs at high temperature and a lattice QCD EOS are included for the analysis. The hot QCD medium effects entering through the quasi-particle momentum distributions along with an effective coupling, are seen to have significant impact on the temperature behavior of these transport parameters along with the thermal relaxation times for the quasi-gluons and quasi-quarks

Observation of Global Hyperon Polarization in Ultrarelativistic Heavy Ion Collisions

Non-central heavy-ion collisions provide a system with non-zero total angular momentum which can be transferred, in part, to the fireball via baryon stopping. It has been predicted that a net spin of emitted particles aligned with the system angular momentum may emerge through coupling with the bulk material. Due to its parity violating decay the ⋀ baryon is self-analyzing, which allows us to associate the daughter proton decay direction with ⋀ particle spin. Ultimately this allows us to use them as a probe of net-particle spin. In preliminary STAR measurements of the net ⋀ baryon polarization from Au+Au collisions at 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV we find that both ⋀ and ⊼ particles are polarized in the direction of the system angular momentum. Including previously published STAR results we see a polarization of about 2% for mid-central collisions when √Snn < 100GeV.

P-odd effects in heavy ion collisions at NICA

Experimental manifestation of P-odd effects related to the vorticity and hydrodynamic helicity in non-central heavy ion collisions at MPD@NICA and BM@N detectors is discussed. For the NICA and FAIR energy range characterized by the large baryonic charge of the forming medium the effect should manifest itself in the specific neutron asymmetries. The polarization of strange particles probing the vorticity and helicity is also discussed.

Hydrodynamic helicity and strange hyperon polarization in heavy-ion collisions

We study the P-odd effects related to the vorticity of the medium formed in noncentral heavy ion collisions. Using the kinetic Quark-Gluon String Model, we perform numerical simulations of the vorticity and hydrodynamical helicity for various atomic numbers, energies and centralities. We observe vortical structures typically occupying a relatively small fraction of the fireball volume. In the course of numerical simulations a noticeable hydrodynamic helicity was observed manifesting a specific mirror behaviour with respect to the reaction plane. The effect is maximal at the NICA and FAIR energy range.