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Xin-Li Sheng

Publications and source records attributed to Xin-Li Sheng.

At least 19 recordsLinked to original sources
arXiv2026

Spectral Functions of $J/ψ$ Meson in Rotating Thermal Background from Holography

We investigate the spectral functions of the $J/ψ$ meson in a rotating thermal background within the soft-wall holographic model. The global rotation is implemented through a rotating AdS-like metric, while a local inertial frame is introduced in which the vector field can be decomposed into different spin states. We solve the equations of motion of the vector field in the bulk with incoming wave condition near the horizon and compute the retarded Green function, from which we extract the invariant-mass spectral functions for $J/ψ$. When the momentum is parallel to the rotation axis, the peak energies shift by $-ΩJ_z$, as expected by a coupling between rotation and angular momentum, while the widths are nearly independent to $Ω$. When the momentum is in perpendicular direction, the spectral functions for spin-$\pm1$ states deviate significantly from the single-peak behavior and the energy shifts depart from $-ΩJ_z$. The resulting triplet splittings of the spectral functions provides a holographic perspective on spin-dependent vector meson properties in rotating systems.

hep-th↗
arXiv2026

An improved formula for Wigner function and spin polarization in a decoupling relativistic fluid at local thermodynamic equilibrium

We present an upgraded formula for Wigner function and spin polarization of fermions emitted by a relativistic fluid at local thermodynamic equilibrium at the decoupling which improves the one obtained in refs. [1, 2] and used in numerical simulations of relativistic nuclear collisions. By using a new expansion method, applicable to decoupling hypersurfaces with arbitrary geometry, we reproduce the known term proportional to thermal vorticity and obtain an upgraded form of the spin-shear term which captures the dependence on the geometry. The new method also includes additional contributions whose physical nature is to be assessed. The new expression also naturally excludes contributions from space-time gradients in the normal direction of the hypersurface, providing a theoretical justification for the isothermal condition previously imposed a priori. This framework can be extended to particles with arbitrary spin.

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arXiv2026

Vector-Meson Spin Alignment from Anisotropic Quark or Hadron Coalescence

The distribution of particles is highly anisotropic in the initial stage of a heavy-ion collision. In this paper we demonstrate that this anisotropy induces a sizable effect on the spin alignment of vector mesons. We study two different production mechanisms for $ϕ$ and $K^{*0}$ mesons, on one hand the coalescence of quarks and on the other that of pseudoscalar mesons. In the quark-coalescence picture where $ϕ$ and $K^{*0}$ are produced via a bare vector coupling to quarks, a negative $δρ_{00}^y$ of order $10^{-3}$ is observed. In contrast, when $ϕ$ and $K^{*0}$ are produced via quark coalescence with a vertex with spin-orbit coupling, or when they are produced via pseudoscalar-meson coalescence, a positive $δρ_{00}^y$ emerges. In all cases, the magnitude of the spin alignment is directly proportional to the degree of anisotropy. The sign difference between the cases provides a possibility to clarify the production mechanism for vector mesons.

hep-ph↗
arXiv2025

Dissipative corrections to the particle momentum spectrum of a decoupling fluid

We present an \emph{ab initio} calculation within quantum statistical field theory and linear response theory, of the dissipative correction to the momentum spectrum of scalar particles emitted at decoupling (freeze-out) from a relativistic fluid assuming the initial state to be in local thermodynamic equilibrium. We obtain an expansion of the Wigner function of the interacting quantum field in terms of the gradients of the classical thermo-hydrodynamic fields - four-temperature vector and reduced chemical potential - evaluated on the initial local-equilibrium hypersurface, rather than on the decoupling (freeze-out) hypersurface as usual in kinetic theory. The gradient expansion includes an unexpected zeroth order term depending on the differences between thermo-hydrodynamic fields at the decoupling and the initial hypersurface. This term encodes a memory of the initial state which is related to the long-distance persistence of the correlation function between Wigner operator and stress-energy tensor and charged current that is discussed in detail. We address the phenomenological implications of these corrections for the momentum spectra measured in relativistic nuclear collisions.

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arXiv2025

Hyperon spin correlation in high-energy heavy-ion collisions

Recent experimental data show an unexpectedly large spin alignment of $ϕ$ mesons in high-energy heavy-ion collisions, which can be explained by short-distance fluctuations of strong-force fields (vector $ϕ$ fields) within the constituent-quark model. We calculate the hyperon spin correlations within the same model, taking into account hydrodynamic effects and a $ϕ$ field fluctuating in space-time according to a Gaussian distribution. The $Λ\barΛ$ spin correlation induced by the $ϕ$ field is shown to be negative as opposed to that of $ΛΛ$ or $\barΛ\barΛ$. We thus propose a new net spin-correlation observable as a sensitive probe to separate strong-force effects from hydrodynamic ones. With the strength of the field fluctuations extracted from the observed $ϕ$ spin alignment, we predict the collision-energy dependence of the hyperon spin correlations and also investigate the dependence of the net spin correlation on azimuthal-angle and rapidity difference.

hep-ph↗
arXiv2025

Dissipative currents and transport coefficients in relativistic spin hydrodynamics

We determine the form of dissipative currents at the first order in relativistic spin hydrodynamics with finite chemical potential including gradients of the spin potential. Taking advantage of isotropy in the hydrodynamic local rest frame, using a suitable matching condition for the flow velocity and enforcing the semi-positivity of entropy production, we find 23 dissipative transport coefficients relating dissipative currents to gradients of the thermo-hydrodynamic fields: 4 for the symmetric part of the energy-momentum tensor, 5 for the antisymmetric part, 3 for the conserved vector current, and 11 for the spin tensor. We compare our finding with previous results in literature.

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arXiv2025

Production of $K^+K^-$ Pairs Through Decay of $ϕ$ Meson

We develop a theoretical framework for the production of $K^+K^-$ pairs through the decay of $ϕ$ mesons produced from a thermal background, based on the Nambu-Jona-Lasinio (NJL) model. The differential production rate of $K^+K^-$ is related to the self-energy of the $ϕ$ meson, incorporating the contributions of the quark loop at leading order and the kaon loop at next-to-leading order in the $1/N_c$ expansion. We numerically evaluate the invariant mass spectrum of the $K^+K^-$ pair both in vacuum and at finite temperature. The inclusion of the kaon loop results in a finite width of the spectrum, improving agreement with experimental data. We also investigate the spin alignment of the $ϕ$ meson induced by its motion relative to the thermal background. In the NJL model with only chiral condensates, we find that deviations from the unpolarized limit of 1/3 are negligible.

hep-ph↗
arXiv2025

Vector and Tensor Spin Polarization for Vector Bosons at Local Equilibrium

We derive expressions for the vector and tensor components of the spin polarization of massive vector bosons at local thermodynamic equilibrium up to second order in the space-time gradients of the thermodynamic fields pertaining to the canonical stress-energy tensor and spin tensor of the free Proca field. A set of Feynman rules is devised to calculate the Wigner function and the matrix-valued spin-dependent distribution (MVSD) functions order by order in space-time gradients. Due to constraints imposed by time-reversal symmetry, the leading contribution to spin alignment - defined as the 00-component of the tensor polarization - arises from second-order terms in MVSD, for which we provide an analytic formula. We discuss the physical meaning of different contributions to vector and tensor polarization. These formulae provide a prediction of a contribution to the spin alignment which can be compared with the observations in relativistic heavy-ion collisions.

hep-ph↗
arXiv2025

Spin alignment of vector mesons in heavy-ion collision

We give a brief review on the spin alignment induced by the strong field and the shear-stress tensor. In experiments, a significant positive deviation from 1/3 is observed for the $ϕ$ meson, which can be explained by the anisotropy of the strong field fluctuation in the meson's rest frame, while the anisotropy is mainly a consequence of the motion of meson relative to the quark-gluon plasma. On the other hand, the shear-induced spin alignment is of the order $10^{-4}\sim10^{-5}$ if the magnitude of thermal shear tensor is $10^{-2}$.

hep-ph↗
arXiv2025

Spin polarization of fermions at local equilibrium: Second-order gradient expansion

We present a calculation of the spin polarization of spin-1/2 fermions in a relativistic fluid at local thermodynamic equilibrium at the second order in the gradient expansion, including second-order derivatives. The second-order derivative terms vanish if the local equilibrium hypersurface is the hyperplane $t=const$ in the collision center-of-mass frame. However, since the freeze-out hypersurface has a non-trivial space-time structure, these terms may result in a non-vanishing contribution to the spin polarization, whose magnitude needs to be assessed with numerical computations.

hep-th↗
arXiv2024

Vector meson's spin alignments in high energy reactions

The global spin alignment of vector mesons has been observed by the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL). It provides a unique opportunity to probe the correlation between the polarized quark and antiquark in the strongly coupled quark-gluon plasma (sQGP) produced in relativistic heavy ion collisions, opening a new window to explore the properties of sQGP. In addition, spin alignments of vector mesons have also been observed in other high-energy particle collisions such as $e^+e^-$ annihilations at high energies where hadron production is dominated by quark fragmentation mechanism. The results obtained are quite different from those obtained in heavy ion collisions where quark coalescence/combination mechanism dominates suggesting strong dependence on hadronization mechanisms. So comprehensive studies in different hadronization processes are needed. In this article, we present a brief review of theoretical and experimental advances in the study of vector meson's spin alignments in a variety of high-energy particle collisions, with emphasis on hadronization mechanisms.

hep-ph↗
arXiv2024

Holographic spin alignment of $J/ψ$ meson in magnetized plasma

We study the mass spectra and spin alignment of vector meson $J/ψ$ in a thermal magnetized background using a generalized theoretical framework based on gauge/gravity duality. Utilizing a soft wall model for the QGP background and a massive vector field for the $J/ψ$ meson, we delve into the meson's spectral function and spin parameters $(λ_θ,\, λ_φ,\,λ_{θφ})$ for different cases, assessing their response to variations in magnetic field strength, momentum, and temperature. We initially examine scenarios where a meson's momentum aligns parallel to the magnetic field in helicity frame. Our results reveal a magnetic field-induced positive $λ_θ^\text{H}$ for low meson momentum, transitioning to negative with increased momentum. As a comparison, we also study the case of momentum perpendicular to the magnetic field and find the direction of magnetic field does not affect the qualitative behavior for the $eB$-dependence of $λ_θ^\text{H}$. Moreover, we apply our model to real heavy-ion collisions for three different spin quantization directions. Further comparisons with experimental data show qualitative agreement for spin parameters $λ_θ$ and $λ_φ$ in the helicity and Collins-Soper frames.

hep-ph↗
arXiv2024

Holographic spin alignment for vector mesons

We develop a general framework for studying the spin alignment $ρ_{00}$ for flavorless vector mesons by using the gauge/gravity duality. Focusing on the dilepton production through vector meson decay, we derive the relation between production rates at each spin channel and meson's spectral function, which can be evaluated by holographic models for a strongly coupled system. As examples, we study $ρ_{00}$ for $J/ψ$ and $ϕ$ mesons, induced by the relative motion to a thermal background, within the soft-wall model. We show that $ρ_{00}$ in the helicity frame for $J/ψ$ and $ϕ$ mesons have positive and negative deviations from 1/3 at $T=150$ MeV, respectively, which consequently leads to different properties for their global spin alignments.

hep-ph↗
arXiv2024

Linear response theory for spin alignment of vector mesons in thermal media

We present a calculation of the spin alignment for unflavored vector mesons in thermalized quark-gluon plasma based on the Kubo formula in linear response theory. This is achieved by expanding the system to the first order of the coupling constant and the spatial gradient. The effect strongly relies on the vector meson's spectral functions which are determined by the interaction and medium properties. The spectral functions are calculated for the one-quark-loop self-energy with meson-quark interaction. The numerical results show that the correction to the spin alignment from the thermal shear tensor is of the order $10^{-4}\sim10^{-5}$ for the chosen values of quark-meson coupling constant, if the magnitude of thermal shear tensor is $10^{-2}$.

hep-ph↗
arXiv2024

Entropy current and entropy production in relativistic spin hydrodynamics

We use a first-principle quantum-statistical method to derive the expression of the entropy production rate in relativistic spin hydrodynamics. We show that the entropy current is not uniquely defined and can be changed by means of entropy-gauge transformations, much the same way as the stress-energy tensor and the spin tensor can be changed with pseudo-gauge transformations. We show that the local thermodynamic relations, which are admittedly educated guesses in relativistic spin hydrodynamics inspired by those at global thermodynamic equilibrium, do not hold in general and they are also non-invariant under entropy-gauge transformations. Notwithstanding, we show that the entropy production rate is independent of those transformations and we provide a universally applicable expression, extending that known in literature, from which one can infer the dissipative parts of the energy momentum and spin tensors.

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arXiv2024

Mass splitting and spin alignment for $ϕ$ mesons in a magnetic field in NJL model

Based on the Nambu-Jona-Lasinio (NJL) model, we develop a framework for calculating the spin alignment of vector mesons and applied it to study $ϕ$ mesons in a magnetic field. We calculate mass spectra for $ϕ$ mesons and observe mass splitting between the longitudinally polarized state and transversely polarized states. The $ϕ$ meson in a thermal equilibrium system is preferred to occupy the state with spin $λ=0$ than those with spin $λ=\pm1$, because the former state has a smaller energy. As a consequence, we conclude that the spin alignment will be larger than 1/3 if one measures along the direction of the magnetic field, which is qualitatively consistent with the recent STAR data. Around the critical temperature $T_{C}=150$ MeV, the positive deviation from 1/3 is proportional to the square of the magnetic field strength, which agrees with the result from the non-relativistic coalescence model. Including the anomalous magnetic moments for quarks will modify the dynamical masses of quarks and thus affect the mass spectra and spin alignment of $ϕ$ mesons. The discussion of spin alignment in the NJL model may help us better understand the formation of hadron's spin structure during the chiral phase transition.

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arXiv2023

Momentum dependence of $ϕ$ meson's spin alignment

We study the rapidity and azimuthal angle dependences of the global spin alignment $ρ_{00}$ for $ϕ$ mesons 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 $ρ_{00}<1/3$ at the rapidity $Y=0$, and then it increases with rapidity and becomes $ρ_{00}>1/3$ 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 $ρ_{00}$ at different rapidities.

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