2023年

(9) Shi Pu, Bo-Wen Xiao, Jian Zhou, Ya-Jin Zhou, Coherent photons induced high energy reactions in ultraperipheral heavy ion collisions, Acta Phys.Sin. 72 (2023) 7, 072503, 收录专题：高能重离子碰撞过程的自旋与手征效应

We review the recent progress in the studies of coherent photons induced high energy reactions in ultraperipheral heavy ion collisions. The strong electromagnetic field created by a fast moving charged heavy ion can be effectively viewed as a flux of quasi-real coherent photons. In this paper, we mainly discuss two different type processes that coherent photons take part in: lepton pair production via photon fusion and diffractive vector meson production in UPCs. We focus on investigating the impact parameter dependent effect and the final state soft radiation effect. On the other hand, a series of recent work have revealed that coherent photons are highly linearly polarized with its polarization vector being parallel to its transverse momentum. It has been shown that the linearly polarized photons can lead to cos 4\phicos⁡4ϕazimuthal asymmetries in di-lepton production. This theoretical predication soon has been confirmed by the STAR measurement. With this new development from both theory and experiment sides, the linearly polarized photons provide a new experimental avenue to explore novel QCD phenomenology. For example, the linearly polarized photons can give rise to various different azimuthal asymmetries in diffractive vector meson production. These observables provide us unique chance to study two source interference effect in high energy scatterings, Coulomb-Nuclear interference effect as well as extracting gluon Wigner distribution. We will discuss these novel phenomenology studies and the possible future developments.

(8) Shi Pu, Xu-guang Huang, Relativistic spin hydrodynamics, Acta Phys.Sin. 72 (2023) 7, 071202  (Invited review) 收录专题：高能重离子碰撞过程的自旋与手征效应

In recent years, due to the discoveries of hyperon spin polarization and vector meson spin alignment in relativistic heavy-ion collision experiments, the spin transports in quark-gluon matter has received intensive studies. The relativistic spin hydrodynamics is one of the important theoretical frameworks to describe the spin transports, which encodes the spin degree of freedom into a hydrodynamic theory. The relativistic spin hydrodynamics have the conservation equations for energy-momentum tensor, currents and total angular momentum. In this article, we give an overview of the recent progresses of the relativistic spin hydrodynamics. We focus on the following topics: 1) The derivation of the relativistic spin hydrodynamic equations, including the phenomenological approach, the effective theory method, and the kinetic approach, 2) Some special properties of spin hydrodynamics, especially the asymmetric energy-momentum tensor and the pseudogauge transformation, and 3) The analytical solutions to the relativistic spin hydrodynamics for systems under Bjorken and Gubser expansion.

(7) Xiang-Yu Wu, Cong Yi, Guang-You Qin, Shi Pu, Global and local polarization of  Λ Λ hyperons across RHIC-BES energies, QM proceedings. 

(6) Shuo Fang, Shi Pu, Di-Lun Yang, Spin polarization and spin alignment from quantum kinetic theory with self-energy corrections, Phys.Rev.D 109 (2024) 3, 034034, arXiv: 2311.15197 

We derive the quantum kinetic theory for massive fermions with collision terms and self-energy corrections based on quantum field theory. We adopt an effective power counting scheme with ℏℏ expansion to obtain the leading-order perturbative solutions of the vector and axial Wigner functions and the corresponding kinetic equations. We observe that both the onshell relation and the structure of Wigner functions, along with the kinetic equations, are modified due to the presence of self-energies and their space-time gradients. We further apply our formalism to investigate the spin polarization phenomena in relativistic heavy ion collisions and derive the modification to the spin polarization spectrum of massive quarks. We find that the gradient of vector self-energy plays a similar role to the background electromagnetic fields, which induces a more dominant contribution than the collisional effects by a naive power counting in the gradient expansion and weak coupling. Our findings could further modify the spin polarization of strange quarks and spin alignment of ϕϕ mesons beyond local thermal equilibrium.

(5) Dong-Lin Wang, Shi Pu, Stability and causality criteria in linear mode analysis: stability means causality, Phys. Rev. D (Letter 109 (2024) 3, L031504

Causality and stability are fundamental requirements for the differential equations describing predictable relativistic many-body systems. In this work, we investigate the stability and causality criteria in linear mode analysis. We discuss the updated stability criterion in 3+1 dimensional systems and introduce the improved sufficient criterion for causality. Our findings clearly demonstrate that stability implies causality in linear mode analysis. Furthermore, based on the theorems present in this work, we conclude that if updated stability criterion and improved causality criterion are fulfilled in one inertial frame of reference (IFR), they hold for all IFR.

(4) Xin-li Sheng, Shi Pu, Qun Wang, Momentum dependence of the spin alignment of the phi meson, Phys.Rev.C 108 (2023) 5, 054902, arXiv: 2308.14038

We study the rapidity and azimuthal angle dependences of the global spin alignment ρ00 for ϕmesons with respect to the reaction plane in Au+Au collisions at RHIC by the relativistic coalescence model in the spin transport theory. The global spin alignment of ϕ mesons arises from local fluctuations of strong force fields whose values are extracted from the STAR's data. The calculated results show that ρ00<1/3 at the rapidity Y=0, and then it increases with rapidity and becomes ρ00>1/3 at Y=1. Such a rapidity dependence is dominated by the relative motion of the ϕ meson in the bulk matter. We also give prediction for the azimuthal angle dependence of ρ00 at different rapidities.

(3) Xin-Qing Xie, Dong-Lin Wang, Chen Yang, Shi Pu, Causality and stability analysis for the minimal causal spin hydrodynamics, Phys.Rev.D 108 (2023) 9, 094031, arXiv:2306.13880  

We perform the linear analysis of causality and stability for a minimal extended canonical spin hydrodynamics up to second order of the gradient expansion. The first order canonical spin hydrodynamics is proved to be acausal and unstable. To remove the unstable and acausal modes, we then formulate the minimal causal spin hydrodynamics up to second order of the gradient expansion. We derive that causality and stability conditions for this minimal causal spin hydrodynamics. Interestingly, the satisfaction of the stability condition relies on the equations of state for the spin density and chemical potentials. Moreover, different with the conventional relativistic dissipative hydrodynamics, the stability of the theory seems to be broken at the finite wave-vector when the stability conditions are fulfilled at small and large wave-vector limits. It implies that the linear stability conditions are necessary but may be insufficient.

(2) Cong Yi, Xiang-Yu Wu, Di-Lun Yang, Jian-Hua Gao, Shi Pu, Probing vortical structures in heavy-ion collisions at RHIC-BES energies through helicity polarization, Phys.Rev.C (Letter) 109, (2024)1, L011901

 

We investigate the hydrodynamic helicity polarization of Λ hyperons, defined as the projection of the spin polarization vector along the directions of particle momenta, at RHIC-BES energies by utilizing the relativistic (3+1)D CLVisc hydrodynamics framework with SMASH initial conditions. As opposed to local spin polarization at high energy collisions, our hydrodynamic simulations demonstrate that the helicity polarization induced by the kinetic vorticity dominates over other contributions at intermediate and low collision energies. Our findings provide an opportunity to probe the fine structure of local kinetic vorticity as a function of azimuthal angle at intermediate and low collision energies by mapping our predictions to the future measurements in experiments.

(1) Hao-Hao Peng, Xin-Li Sheng, Shi Pu, Qun Wang, Electric and magnetic conductivities in magnetized fermion systems, Phys.Rev.D 107 (2023) 11, 116006, arXiv: 2304.00519 

 

In Wigner function approach with relaxation time approximation, we calculate electric and magnetic conductivities of a fermion system in the strong magnetic field. The linear response has been calculated to the perturbation of electromagnetic fields on the background constant magnetic field. The Wigner function is separated into an equilibrium part in the background magnetic field and an off-equilibrium part induced by perturbative fields. The analytical expression for the equilibrium part and the corresponding equilibrium conditions are given. For the off-equilibrium part, we obtain the kinetic equation at the leading order in ħ from the master equation of the Wigner function. When perturbative fields only depend on the proper time, the off-equilibrium part can be analytically solved from which the vector and axial vector currents are obtained. We obtain the longitudinal and transverse Ohm conductivities as well as Hall conductivity as the linear response of the vector current to the perturbative electric field. The behaviors of these conductivities as functions of the evolving time, relaxation time, particle mass, and strength of the background magnetic field are investigated both analytically and numerically.