Shiyong Li
We review the recent progress in formulating a quantum kinetic theory for the polarization of spin-$\frac{1}{2}$ massive quarks from the leading-log order of perturbative QCD.
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We present the analytical calculation of the contribution of order m_q^2 α_s^4 n_f^2 to the absorptive part of the vacuum polarization function of vector currents. This term constitutes an important gauge-invariant part of the full O(m_q^2 alpha_s^4) correction to the total cross-section of e^+ e^- annihilation into hadrons. The results are compared to predictions following from various optimization schemes.
I review recent progress on exclusive hadronic B meson decays in the perturbative QCD approach, with focus on puzzles in the branching ratios and the CP asymmetries of the B -> pi K and B -> pi pi modes, and polarization fractions in B -> VV modes.
We investigate the Lambda_b polarization problem in the inclusive semileptonic decays of a polarized Lambda_b baryon, using the modified perturbative QCD formalism which includes Sudakov suppression. According to HQEFT, we show that, at the leading order in the 1/M_b expansion, the polarized and unpolarized distribution functions become one single universal distribution function. To explore the mechanisms which determine the spin properties of a polarized Lambda_b baryon, we construct four formalisms which are the naive quark model (QM), the modified quark model (MQM), the naive parton model (…
Within the framework of perturbative QCD approach, we study the charmless two-body decays into final states involving one axial-vector (A), $a_1(1260)$ or $b_1(1235)$, and one vector (V), namely $ρ(ω,ϕ)$. Using the decays constants and the light-cone distribution amplitudes for these mesons derived from the QCD sum rule method, we find the following results: (a) Except the decays $\bar B^0\to a^{0}_1ρ^0(ω)$, other tree-dominated decays $B\to a_1ρ(ω)$ have larger branching ratios, at the order of $10^{-5}$. (b)Except the decays $\bar B\to b^+_1ρ^-$ and $B^-\to b^0_1ρ^-$, other $B\to b_1ρ(ω)$ de…
We ``quantify'' the role of elastic as well as inelastic gg<->ggg pQCD processes in kinetic equilibration within a pQCD inspired parton cascade. The contributions of different processes to kinetic equilibration are manifested by the transport collision rates. We find that in a central Au+Au collision at RHIC energy pQCD Bremstrahlung processes are much more efficient for momentum isotropization compared to elastic scatterings. For the parameters chosen the ratio of their transport collision rates amounts to 5:1.
The flavor and spin structure for the quark distributions of the $Λ$-baryon is studied in a perturbative QCD (pQCD) analysis and in the SU(6) quark-diquark model, and then applied to calculate the $Λ$-polarization of semi-inclusive $Λ$ production in $e^+e^-$-annihilation near the $Z$-pole. It is found that the quark-diquark model gives very good description of the available experimental data. The pQCD model can also give good description of the data by taking into account the suppression of quark helicities compared to the naive SU(6) quark model spin distributions. Further information is requ…
In the one-photon exchange approximation we discuss questions related to the interpretation of unexpected results of the JLab polarization experiments to measure the Sachs form factors ratio $G_E/G_M$ in the region $1. 0 \leq Q^2 \leq 8.5$ GeV$^2$. For this purpose, we developed an approach which essentially is a generalization of the constituent-counting rules of the perturbative QCD (pQCD) for the case of massive quarks. We assume that at the lower boundary of the considered region the hard-scattering mechanism of pQCD is realized. Within the framework of the developed approach we calculated…
We calculate the CP-averaged branching ratios and CP-violating asymmetries of a number of two-body charmless hadronic decays $\bar{B_s^0} \to PP, PV, VV$ in the perturbative QCD (pQCD) approach to leading order in $α_s$ (here $P$ and $V$ denote light pseudoscalar and vector mesons, respectively). The mixing-induced CP violation parameters are also calculated for these decays. We also predict the polarization fractions of $B_s \to VV$ decays and find that the transverse polarizations are enhanced in some penguin dominated decays such as $\bar{B_s^0} \to K ^{*}\bar{K^{*}}$, $K ^{*}ρ$. Some of th…
Recent evaluations of the hadronic vacuum polarization contributions to the effective fine-structure constant alpha_{em}(M_Z) are summarized and commented. A new update based on corrected CMD-2 data is presented. My new estimates are Delta alpha_{had}^{(5)}(M_Z) = 0.027773 +/- 0.000354 (e^+e^- - data based) and Delta alpha_{had}^{(5)}(M_Z) = 0.027664 +/- 0.000173 (via the Adler-function and extended use of pQCD). Prospects of further possible progress is discussed.
We calculate, within pQCD parton model at leading orders, the expression of the longitudinal polarization of Lambda baryons produced in polarized semi-inclusive DIS, including weak interaction effects. We present some numerical estimates in few cases for which data are or will soon be available and discuss how to gather new information on polarized fragmentation functions.
Branching fraction, $CP$ asymmetry, and polarization in $B\toρρ$ decays with the modified perturbative QCD approach
本研究通过引入临界尺度$μ_c$（约1 GeV）的红外截断改进微扰QCD方法，结合相对论势模型的$B$介子分布振幅及色八重态贡献，提升了$B\toρρ$衰变分支比等可观测量的理论计算可靠性与实验吻合度。
We investigate the dynamical spin polarization of a massless electron probing an electron plasma in locally thermal equilibrium via the Moller scattering from the quantum kinetic theory. We derive an axial kinetic equation delineating the dynamical spin evolution in the presence of the collision term with quantum corrections up to $\mathcal{O}(\hbar)$ and the leading-logarithmic order in coupling by using the hard-thermal-loop (HTL) approximation, from which we extract the spin-polarization rate induced by the spacetime gradients of the medium. When the electron probe approaches local equilibr…
We critically compare thermodynamic and kinetic approaches, that have been recently used to study relations between the spin polarization and fluid vorticity in systems consisting of spin-one-half particles. The thermodynamic approach refers to general properties of global thermal equilibrium with a rigid-like rotation and demonstrates that the spin-polarization and thermal-vorticity tensors are equal. On the other hand, the kinetic approach uses the concept of the Wigner function and its semi-classical expansion. In most of the works done so far, the Wigner functions satisfy kinetic equations…
We present the quantum kinetic equation for spin polarization of massive quarks in leading log order of perturbative QCD, which describes time evolution of the spin density matrix in momentum space of a massive quark interacting with a background QCD plasma. We find that the time evolution operator of the spin density matrix, or the quantum kinetic collision terms, are universally of order $α_s^2\log(1/α_s)$ in terms of the QCD coupling constant $α_s=g^2/(4π)$. Our quantum kinetic equation is valid for an arbitrary quark mass $m\gg m_D\sim gT$, where $m_D$ is the Debye mass, and can be used to…
The total angular momentum is conserved in the evolution of the Quark-Gluon Plasma (QGP) created in heavy-ion collision, and consists of two sectors: the orbital angular momentum (OAM) caused by kinetic motion, and the spin, an intrinsic degree of freedom of quarks and gluons. Microscopic scattering processes allow the conversion between these two components, hence the spin density could eventually have non-trivial influence on the QGP evolution. A hydrodynamic theory, with the spin polarization effect properly taken into account, is required to quantitatively study the polarization rate for o…
In this work, we start with chiral kinetic theory and construct the spin hydrodynamic framework for a chiral spinor system. Using the 14-moment expansion formalism, we obtain the equations of motion of second-order dissipative relativistic fluid dynamics with non-trivial spin polarization density. In a chiral spinor system, the spin alignment effect could be treated in the same framework as the Chiral Vortical Effect (CVE). However, the quantum corrections due to fluid vorticity induce not only CVE terms in the vector/axial charge currents but also corrections to the stress tensor. In this fra…
Quarkonium Polarization Kinetic Equation from Open Quantum Systems and Effective Field Theories
本研究利用pNRQCD和开放量子系统框架，推导了矢量夸克偶素在夸克胶子等离子体中的极化依赖玻尔兹曼输运方程及Lindblad方程，为理解重离子碰撞中的自旋排列提供了新工具。
We derive a quantum kinetic theory for QED based on Kadanoff-Baym equations for Wigner functions. By assuming parity invariance and considering a complete set of self-energy diagrams, we find the resulting kinetic theory expanded to lowest order in $\hbar$ generalizes the well-known classical kinetic theory to massive case. It contains elastic and inelastic collision terms and integrates screening effect naturally. For a given solution to the classical kinetic theory, we find at next order in $\hbar$ a non-dynamical quantum correction to Wigner functions for both fermions and photons, which gi…
The kinetic equation of non - Markovian type for description of the vacuum creation of parton - antiparton pairs under action of a space homogeneous time - dependent chromo - electric field of the arbitrary polarization is obtained on the strict non - pertubative foundation in the framework of the oscillator representation. A comparison of the effectiveness of vacuum creation with the case of linear polarization one is fulfilled.
Spin polarization and spin alignment from quantum kinetic theory with self-energy corrections
基于量子场论推导了包含碰撞项和自能修正的有质量费米子量子动力学理论，发现矢量自能梯度在弱耦合和梯度展开下对自旋极化的贡献大于碰撞效应，进而修正了重离子碰撞中夸克极化与介子自旋排列。
A power expansion scheme is set up to determine the Wigner function that satisfies the quantum kinetic equation for spin-1/2 charged fermions in a background electromagnetic field. Vector and axial-vector current induced by magnetic field and vorticity are obtained simultaneously from the Wigner function. The chiral magnetic and vortical effect and chiral anomaly are shown as natural consequences of the quantum kinetic equation. The axial-vector current induced by vorticity is argued to lead to a local polarization effect along the vorticity direction in heavy-ion collisions.
Probing vortical structures in heavy-ion collisions at RHIC-BES energies through helicity polarization
本文利用CLVisc流体力学与SMASH初始条件研究RHIC-BES能区Λ超子螺旋度极化，发现中低能下动涡量贡献主导，为实验探测局域涡旋精细结构提供理论预测。
The observation of jets in a variety of hard-scattering processes has allowed the quantitative study of perturbative quantum chromodynamics (PQCD) by comparing detailed theoretical predictions with a wide range of experimental data. This paper examines how some important, nonperturbative, facets of QCD involving the internal dynamical structure of jets can be studied by measuring the spin orientation of Lambda particles produced in these jets. The measurement of the transverse polarization for an individual Lambda within a QCD jet permits the definition of spin-directed asymmetries in local qu…
Quantum Kinetic Theory for Quantum Chromodynamics
本文提出了一种量子色动力学的量子动力学理论，用于描述夸克胶子等离子体中夸克和胶子的自旋极化，并探讨了自旋与轨道角动量的转换机制。
Polarization fractions and helicity-dependent CP asymmetries in $B_{(s)} \to ρρ, ρK^\ast$ and $K^\ast K^\ast$ decays
研究利用 pQCD 计算对 $B_{(s)} \to \rho\rho、\rho K^\ast$ 和 $K^\ast K^\ast$ 衰变进行唯象分析，扩展了先前研究。对部分模式的分支比和极化分数预测与实验吻合，但存在差异，凸显解决两体 $B$ 衰变极化难题需进一步理论研究。
Collisional corrections to spin polarization from quantum kinetic theory using Chapman-Enskog expansion
本文利用Chapman-Enskog展开的量子动理学理论推导含Møller散射的自旋Boltzmann方程，在硬热圈近似下分析两种情形中自旋极化赝矢量的碰撞修正，为后续数值研究奠定理论基础。
We present a detailed analysis of the self-consistent system of kinetic equations (KEs) describing electron-positron pair production from vacuum under the action of a spatially homogeneous time-dependent electric field of arbitrary polarization. The physical significance of all the basic functions of the kinetic theory is ascertained. It is demonstrated that the total system of the KEs consists of two coupled quasiparticle and spin subsystems with their integrals of motion. A projection method is proposed in order to obtain the KE system in two particular cases: linearly polarized external ele…
We present the systematic results for three dimensional fragmentation functions of spin one hadrons defined via quark-quark correlator. There are totally 72 such fragmentation functions, among them 18 are twist-2, 36 are twist-3 and 18 are twist-4. We also present the relationships between the twist-3 parts and those defined via quark-gluon-quark correlator obtained from the QCD equation of motion. We show that two particle semi-inclusive hadron production process $e^+e^-\to VπX$ at high energies is one of the best places to study the three-dimensional tensor polarization dependent fragmentati…
Spin kinetic theory with a nonlocal relaxation time approximation
本文提出含自旋动力学理论的非局域弛豫时间近似，证明其极化结果可与 Zubarev 形式体系对应，并通过旋转圆柱验证动理学与流体力学描述等价。
The flavor and helicity distributions of the $Λ$ and $\barΛ$ hyperons for both valence and sea quarks are calculated in a perturbative QCD (pQCD) based model. We relate these quark distributions to the fragmentation functions of the $Λ$ and $\barΛ$, and calculate the $z$-dependence of the longitudinal spin transfer to the $Λ$ and $\barΛ$ in lepton deep-inelastic scattering (DIS). It is shown that the spin transfer to the $Λ$ is compatible with the first HERMES results at DESY and further tests are suggested. We also make predictions for the $z$-dependence of the $Λ$ and $\barΛ$ longitudinal po…
Vacuum polarization and pair production in time-dependent electric fields: A quantum-kinetic-equation approach
研究时变电场中真空极化与对偶产生，采用量子动力学方程方法计算可观测量并提供理论基础。
We systematically derive the collision term for the axial kinetic theory, a quantum kinetic theory delineating the coupled dynamics of the vector/axial charges and spin transport carried by the massive spin-1/2 fermions traversing a medium. We employ the Wigner-function approach and propose a consistent power-counting scheme where the axial-charge distribution function, a non-conserved quantity for massive particles, is accounted as the first-order quantity in the $ \hbar $ expansion, while the vector-charge distribution function the zeroth-order quantity. This specific power-counting scheme a…
It is shown that a perturbative QCD (pQCD) based analysis and the SU(6) quark-diquark model give significant different predictions concerning the flavor and spin structure for the quark distributions of the $Λ$-baryon near $x=1$. Detailed predictions for the ratios $u(x)/s(x)$ of unpolarized quark distributions, $Δs(x)/s(x)$ of valence strange quark, and $Δu(x)/u(x)$ of valence up and down quarks of the $Λ$ are given from the quark-diquark model and from a pQCD based model. It is found that the up and down quarks are positively polarized at large $x$, even though their net spin contributions t…
We give a theoretical perspective on the vorticity and spin polarization in heavy-ion collisions. We discuss the recent progress in spin hydrodynamics and spin kinetic theory. We also discuss other effects caused by vorticity including the chiral vortical effect and rotation-induced phase transitions.
Perfect spin hydrodynamics at all orders in spin polarization
本文比较了两种完美自旋流体力学框架，发现其守恒电流形式相同，仅在乘法因子上存在差异。
Self-consistent constraints on the collision terms in quantum kinetic theory
本文利用维格纳函数形式推导量子动理学理论中碰撞项的自洽约束条件，给出满足约束的具体解，并在弛豫时间近似下发展该理论，进而讨论相对论重离子碰撞中的自旋极化与电荷分离效应。
We derive the quantum kinetic equations for massive and massless quarks coupled with the background chromo-electromagnetic fields from the Wigner-function approach with the $\hbar$ expansion and effective power-counting scheme. For each case, one obtains coupled color-singlet and color-octet kinetic equations, which also involve the scalar and axial-vector components for the charge and spin transport. These kinetic equations delineate entangled evolution of the corresponding distribution functions decomposed in color space. At weak coupling, we derive the close form of the color-singlet kineti…
The role of dynamical chiral condensate in spin polarization is investigated in a kinetic theory framework. Transport equations for quark matter are derived in the mean-field approximation for the Nambu--Jona-Lasinio model. The dynamical condensate carries part of the energy momentum tensor (EMT) and the angular momentum tensor (AMT), the conservation of EMT and AMT can be proved from the kinetic equations as required by the symmetry. The transport equations of vector and axial-vector components are derived taking the spin decomposition as well as semi-classical expansion. Inhomogeneous mass i…
Longitudinal spin polarization in a thermal model with dissipative corrections
本文结合含自旋自由度的相对论动理学理论与冻结参数化，将热剪切和热涡度梯度导致的耗散修正纳入Pauli-Lubanski矢量，以解释重离子碰撞中Λ超子的纵向自旋极化，并估计自旋弛豫时间τ_s约为5 fm/c。
Is the shear induced spin polarization non-dissipative?
本文通过熵流、H定理和Zubarev方法分析剪切诱导极化与反常霍尔效应，指出坐标空间时间反演分析不足以判定耗散性，并揭示这些效应可能具有耗散性质。
We give a brief overview of recent theoretical and experimental results on the chiral magnetic effect and spin polarization effect in heavy-ion collisions. We present updated experimental results for the chiral magnetic effect and related phenomena. The time evolution of the magnetic fields in different models is discussed. The newly developed quantum kinetic theory for massive fermions is reviewed. We present theoretical and experimental results for the polarization of $Λ$ hyperons and the $ρ_{00}$ value of vector mesons.
We consider vacuum polarization effects in the one-photon annihilation channel within a kinetic description of the e+ e- plasma produced from the vacuum in the focal spot of counter-propagating laser beams. This entails essential changes in the structure of the photon kinetic equation. We investigate the domain of large adiabaticity parameters gamma >> 1 where the photon radiation turns out to be very small. A more thorough examination of the domain gamma < 1 needs separate investigation. However, an exploratory study has shown that the one-photon annihilation channel can lead for some domains…
We develop a covariant kinetic theory for massive fermions in curved spacetime and external electromagnetic field based on quantum field theory. We derive four coupled semi-classical kinetic equations accurate at $O(\hbar)$, which describe the transports of particle number and spin degrees of freedom. The relation with the chiral kinetic theory is discussed. As an application, we study the spin polarization in the presence of finite Riemann curvature and electromagnetic field in both local and global equilibrium states.
Photon quantum kinetic equations and collective modes in an axion background
该研究建立轴子背景下无碰撞光子的量子动力学理论，推导经典对应关系，得到偏振分量的色散与输运方程，并发现轴子背景会打破电子-正电子等离子体中光子横向集体模式的简并。
Polarization dynamics from moment equations
该研究从自旋动力学理论推导有质量粒子膨胀系统的局域横向极化表达式，建立自旋矩的封闭运动方程组，以描述从自由流动到局域平衡的全过程并用于重离子碰撞极化研究。
We compute the fermion spin distribution in the vortical fluid created in off-central high-energy heavy-ion collisions. We employ the event-by-event (3+1)D viscous hydrodynamic model. The spin polarization density is proportional to the local fluid vorticity in quantum kinetic theory. As a result of strong collectivity, the spatial distribution of the local vorticity on the freeze-out hyper-surface strongly correlates to the rapidity and azimuthal angle distribution of fermion spins. We investigate the sensitivity of the local polarization to the initial fluid velocity in the hydrodynamic mode…
Generalized Chiral Kinetic Equations
本文推导适用于任意质量费米子的广义手征动力学方程，揭示磁矩分布导致自旋极化与电荷分离，以及涡旋和电磁场在全局平衡中诱导相关分布的机制。
P and CP violation in cosmology can be manifested as large-scale helical velocity flows in the ambient plasma and as primordial helical magnetic fields. We show that kinetic helicity at last scattering leads to temperature-polarization correlations ($C_l^{TB}$ and $C_l^{EB}$) in the cosmic microwave background radiation (CMBR) and calculate the magnitude of the effect. Helical primordial magnetic fields, expected from cosmic events such as electroweak baryogenesis, can lead to helical velocity flows and hence to non-vanishing correlations of the temperature and B-type polarization. However we …
Kinetic theory for spin-polarized relativistic plasmas
该研究针对高场科学中自旋极化等离子体的描述需求，推导了纳入QED效应的相对论动理学理论，进一步得到自旋极化量子辐射反作用方程，明确了自旋相关辐射反作用项及辐射对自旋动力学的贡献。
As the core ingredient for spin polarization, the local equilibrium spin distribution function is derived from the detailed balance principle. The kinetic theory for interacting fermionic systems is applied to the Nambu--Jona-Lasinio model at quark level. Under the semi-classical expansion with respect to $\hbar$ and non-perturbative expansion with respect to $N_c$, the kinetic equations for the vector and axial-vector distribution functions are derived with collision terms. It is found that, for an initially unpolarized system, non-zero spin polarization can be generated at the order of $\hba…
We show that the standard model Higgs field can realize the quadratic chaotic inflation, if the kinetic term is significantly modified at large field values. This is a simple realization of the so-called running kinetic inflation. The point is that the Higgs field respects an approximate shift symmetry at high energy scale. The tensor-to-scalar ratio is predicted to be $r \simeq 0.13 - 0.16$, which nicely explains the primordial B-mode polarization, $r=0.20^{+0.07}_{-0.05}$, recently discovered by the BICEP2 experiment. In particular, allowing small modulations induced by the shift symmetry br…
We derive a kinetic equation for chiral matter at non-zero chemical potential that governs the response of the parity odd part of the distribution function to perturbations of the Robertson-Walker metric. The derivation is based on a recent evaluation of the gravitational polarization tensor at non-zero chemical potential. We also provide the equations for gravity waves that follow from the anisotropic stress tensor describing the lepton asymmetry. These equations can be used to assess the effects that a non-zero neutrino chemical potential would have on the evolution of cosmological perturbat…
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