arXiv subjects

Xiao-Dan Wang

Publications and source records attributed to Xiao-Dan Wang.

6 recordsLinked to original sources

arXiv2022

Fluctuations of conserved charges in strong magnetic fields from lattice QCD

We present the first lattice QCD results of the second order fluctuations of and correlations among net baryon number, electric charge and strangeness in (2+1)-flavor lattice QCD in the presence of a background magnetic field with physical pion mass $m_π=135$ MeV. To mimic the magnetic field strength produced in the early stage of heavy-ion collision experiments we use 6 different values of the magnetic field strength up to $ \sim $10$m_π^2$. We find that the correlations between baryon number and electric charge along the transition line are substantially affected by magnetic fields in the current $eB$ window, which could be useful for probing the existence of a magnetic field in heavy-ion collision experiments.

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arXiv2021

Correlated Dirac Eigenvalues and Axial Anomaly in Chiral Symmetric QCD

We investigate the Dirac eigenvalue spectrum ($ρ(λ,m_l)$) to study the microscopic origin of axial anomaly in high temperature phase of QCD. We propose novel relations between the derivatives ($\partial^n ρ(λ,m_l)/\partial m_l^n$) of the Dirac eigenvalue spectrum with respect to the quark mass ($m_l$) and the $(n+1)$-point correlations among the eigenvalues ($λ$) of the massless Dirac operator. Based on these relations, we present lattice QCD results for $\partial^n ρ(λ,m_l)/\partial m_l^n$ ($n=1, 2, 3$) with $m_l$ corresponding to pion masses $m_π=160-55$ MeV, and at a temperature of about 1.6 times the chiral phase transition temperature. Calculations were carried out using (2+1)-flavors of highly improved staggered quarks and the tree-level Symanzik gauge action with the physical strange quark mass, three lattice spacings $a=0.12, 0.08, 0.06$ fm, and lattices having aspect ratios $4-9$. We find that $ρ(λ\to0,m_l)$ develops a peaked structure. This peaked structure, which arises due to non-Poisson correlations within the infrared part of the Dirac eigenvalue spectrum, becomes sharper as $a\to0$, and its amplitude is proportional to $m_l^2$. After continuum and chiral extrapolations, we find that the axial anomaly remains manifested in two-point correlation functions of scalar and pseudo-scalar mesons in the chiral limit. We demonstrate that the behavior of $ρ(λ\to0,m_l)$ is responsible for it.

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arXiv2020

Chiral phase structure of three flavor QCD in a background magnetic field

We investigate the chiral phase structure of three flavor QCD in a background $U(1)$ magnetic field using the standard staggered action and the Wilson plaquette gauge action. We perform simulations on lattices with a temporal extent of $N_τ=4$ and four spatial extents of $N_σ= 8,16, 20$ and 24. We choose a smaller-than-physical quark mass in lattice spacing as $am = 0.030$ such that there exists a crossover transition at vanishing magnetic fields, and adopt two values of magnetic field strength in lattice spacing $a \sqrt{ e{B}}\simeq 1.5$ and 2. We find that the transition becomes stronger in the presence of a background magnetic field, and turns into a first order as seen from the volume scaling of the order parameter susceptibility as well as the metastable states in the time history of the chiral condensate. On the other hand, the chiral condensate and transition temperature always increase with $B$ even within the regime of a first order phase transition. This suggests that the discrepancy in the behavior of chiral condensates and transition temperature as a function of $B$ between earlier lattice studies using larger-than-physical pion masses with standard staggered fermions and those using physical pions with improved staggered fermions is mainly due to lattice cutoff effects.

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arXiv2020

Meson masses in external magnetic fields with HISQ fermions

We studied the temporal correlation function of mesons in the pseudo-scalar channel in (2+1)-flavor QCD in the presence of external magnetic fields at zero temperature. The simulations were performed on $32^3 \times 96$ lattices using the Highly Improved Staggered Quarks (HISQ) action with $m_π \approx $ 230 MeV. The strength of magnetic fields $|eB|$ ranges from 0 to around 3.3 GeV$^2$ ($\sim 60 m_π^2$). We found that the masses of neutral pseudo-scalar particles, e.g. neutral pion and kaon, monotonouslly decrease as the magnetic field grows and then saturate at a nonzero value. It is observed that heavier neutral pseudo-scalars are less affected by magnetic fields. Moreover, we found a non-monotonous behavior of charged pion and kaon mass in magnetic field for the first time. In the case of small magnetic field (0 $\leq~|eB| \lesssim$ 0.3 GeV$^2~\sim 6m_π^2$ ) the mass of charged pseudo-scalar grows with magnetic field and can be well described by the Lowest Landau Level approximation, while for $|eB|$ larger than 0.3 GeV$^2$ the mass starts to decrease. The possible connection between $|eB|$ dependences of neutral pion mass and the decreasing behavior of pseudo-critical temperature in magnetic field is discussed. Due to the nonzero value of neutral pion mass our simulation indicates that the superconducting phase of QCD does not exist in the current window of magnetic field.

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arXiv2019

Phase structure of three flavor QCD in external magnetic fields using HISQ fermions

We study the phase structure of QCD with three degenerate flavors in the external magnetic fields using highly improved staggered quarks (HISQ). The simulations are performed on $16^3\times 6$ lattice. In order to investigate the quark mass dependence of the chiral transition we choose the values of the bare quark masses 0.015 and 0.0009375 in the lattice unit, corresponding to $m_π=320$ MeV and 80 MeV in the continuum limit. We found no indication of a first order phase transition in the current window of quark masses and external magnetic fields. Unlike to the case with standard staggered fermions inverse magnetic catalysis is always observed at about the critical temperature. The microscopic origin of this phenomena are further discussed by looking into the Dirac eigenvalue spectrum.

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arXiv2017

Chiral phase transition of three flavor QCD with nonzero magnetic field using standard staggered fermions

Lattice simulations for (2+1)-flavor QCD with external magnetic field demonstrated that the quark mass is one of the important parameters responsible for the (inverse) magnetic catalysis. We discuss the dependences of chiral condensates and susceptibilities, the Polyakov loop on the magnetic field and quark mass in three degenerate flavor QCD. The lattice simulations are performed using standard staggered fermions and the plaquette action with spatial sizes Ns = 16 and 24 and a fixed temporal size Nt = 4. The value of the quark masses are chosen such that the system undergoes a first order chiral phase transition and crossover with zero magnetic field. We find that in light mass regime, the quark chiral condensate undergoes magnetic catalysis in the whole temperature region and the phase transition tend to become stronger as the magnetic field increases. In crossover regime, deconfinement transition temperature is shifted by the magnetic field when quark mass ma is less than 0.4. The lattice cutoff effects are also discussed.

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