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Xiang-Qian Luo

Publications and source records attributed to Xiang-Qian Luo.

At least 19 recordsLinked to original sources

H-Dibaryon from Lattice QCD with Improved Anisotropic Actions

The six quark state(uuddss) called H dibaryon($J^P=0^+$,$S=-2$) has been calculated to study its existence and stability. The simulations are performed in quenched QCD on $8^3 \times 24$ and $16^3 \times 48$ anisotropic lattices with Symanzik improved gauge action and Clover fermion action. The gauge coupling is $β=2.0$ and aspect ratio $ξ=a_s/a_t=3.0$. Preliminary results indicate that mass of H dibaryon is 2134(100)Mev on $8^3 \times 24$ lattice and 2167(59)Mev on $16^3 \times 48$ respectively. It seems that the radius of H dibaryon is very large and the finite size effect is very obvious.

hep-lat

Phase structure of lattice QCD with two flavors of Wilson quarks at finite temperature and chemical potential

We present results for phase structure of lattice QCD with two degenerate flavors ($N_f=2$) of Wilson quarks at finite temperature $T$ and small baryon chemical potential $μ_B$. Using the imaginary chemical potential for which the fermion determinant is positive, we perform simulations at points where the ratios of pseudo-scalar meson mass to the vector meson mass $m_π/m_ρ$ are between $0.943(3)$ and $0.899(4)$ as well as in the quenched limit. By analytic continuation to real quark chemical potential $μ$, we obtain the transition temperature as a function of small $μ_B$. We attempt to determine the nature of transition at imaginary chemical potential by histogram, MC history, and finite size scaling. In the infinite heavy quark limit, the transition is of first order. At intermediate values of quark mass $m_q$ corresponding to the ratio of $m_π/m_ρ$ in the range from $0.943(3)$ to $0.899(4)$ at $aμ_I=0.24$, the MC simulations show absence of phase transition.

hep-lat

Phase diagram of QCD with 2+1 flavors of Wilson quarks at finite temperature and chemical potential

The first results on lattice at finite temperature $T$ and chemical potential $μ$ with 2+1 flavors of Wilson quarks are presented. We have studied the dependence of chiral phase transition and deconfinement phase transition on quark mass. Finite volume size analysis and Binder cumulants are used to determine the properties of phase transition. Phase diagram of QCD with 2+1 flavors of Wilson quarks are presented.

hep-lat

Estimate of the charmed 0-- hybrid meson spectrum from quenched lattice QCD

We compute from quenched lattice QCD the ground state masses of the charmed hybrid mesons cbar c g, with exotic quantum numbers JPC=1-+, 0+- and 0--. The 0-- hybrid meson spectrum has never been provided by lattice simulations due to the difficulties to extract high gluonic excitations from noise. We employ improved gauge and fermion actions on the anisotropic lattice, which reduce greatly the lattice artifacts, and lead to very good signals. The data are extrapolated to the continuum limit, with finite size effects under well control. For 1-+ and 0+- hybrid mesons, the ground state masses are 4.405(38) GeV and 4.714(52) GeV. We predict for the first time from lattice QCD, the ground state mass of 0-- to be 5.883(146) GeV.

hep-lat

Gluonic excitation of non-exotic hybrid charmonium from lattice QCD

The ground and first excited states of the hybrid charmonium ${\bar c} c g$, with non-exotic quantum numbers $J^{PC}=0^{-+}$, $1^{--}$ and $1^{++}$ are investigated using quenched lattice QCD. They are completely ignored in the literature, only because their ground states are degenerate with $η_c$, $J/ψ$, and $χ_{c1}$, and are difficult to be distinguished from these conventional charmonium mesons in experiment. However, we observe strong gluonic radial excitations in the first excited states; We predict that their masses are 4.352(225)GeV, 4.379(149)GeV and 7.315(257)GeV, completely different from the first excited states of the corresponding conventional charmonium. Their relevance to the recent discovery of the Y(4260) state and future experimental search for other states are also discussed.

hep-lat

Phase structures of strong coupling lattice QCD with overlap fermions at finite temperature and chemical potential

We perform the first study of lattice QCD with overlap fermions at finite temperature $T$ and chemical potential $μ$. We start from the Taylor expanded overlap fermion action, and derive in the strong coupling limit the effective free energy by mean field approximation. On the ($μ,T$) plane and in the chiral limit, there is a tricritical point, separating the second order chiral phase transition line at small $μ$ and large $T$, and first order chiral phase transition line at large $μ$ and small $T$.

hep-lat

Spontaneous chiral-symmetry breaking of lattice QCD with massless dynamical quarks

One of the most challenging issues in QCD is the investigation of spontaneous chiral-symmetry breaking, which is characterized by the non-vanishing chiral condensate when the bare fermion mass is zero. In standard methods, one has to perform expensive lattice simulations at multiple bare quark masses, and employ some modeled function to extrapolate the data to the chiral limit. This paper applies the probability distribution function method to computing the chiral condensate in lattice QCD with massless dynamical quarks, without any ambiguous mass extrapolation. The results for staggered quarks indicates that the method might be a more efficient alternative for investigating the spontaneous chiral-symmetry breaking in lattice QCD.

hep-lat

Phase diagram of QCD at finite temperature and chemical potential from lattice simulations with dynamical Wilson quarks

We present the first results for lattice QCD at finite temperature $T$ and chemical potential $μ$ with four flavors of Wilson quarks. The calculations are performed using the imaginary chemical potential method at $κ=0$, 0.001, 0.15, 0.165, 0.17 and 0.25, where $κ$ is the hopping parameter, related to the bare quark mass $m$ and lattice spacing $a$ by $κ=1/(2ma+8)$. Such a method allows us to do large scale Monte Carlo simulations at imaginary chemical potential $μ=i μ_I$. By analytic continuation of the data with $μ_I < πT/3$ to real values of the chemical potential, we expect at each $κ\in [0,κ_{chiral}]$, a transition line on the $(μ, T)$ plane, in a region relevant to the search for quark gluon plasma in heavy-ion collision experiments. The transition is first order at small or large quark mass, and becomes a crossover at intermediate quark mass.

hep-lat

Bound States and Critical Behavior of the Yukawa Potential

We investigate the bound states of the Yukawa potential $V(r)=-λ\exp(-αr)/ r$, using different algorithms: solving the Schrödinger equation numerically and our Monte Carlo Hamiltonian approach. There is a critical $α=α_C$, above which no bound state exists. We study the relation between $α_C$ and $λ$ for various angular momentum quantum number $l$, and find in atomic units, $α_{C}(l)= λ[A_{1} \exp(-l/ B_{1})+ A_{2} \exp(-l/ B_{2})]$, with $A_1=1.020(18)$, $B_1=0.443(14)$, $A_2=0.170(17)$, and $B_2=2.490(180)$.

hep-ph

Monte Carlo study of glueball masses in the Hamiltonian limit of SU(3) lattice gauge theory

Using Standard Euclidean Monte Carlo techniques, we discuss in detail the extraction of the glueball masses of 4-dimensional SU(3) lattice gauge theory in the Hamiltonian limit, where the temporal lattice spacing is zero. By taking into account the renormalization of both the anisotropy and the Euclidean coupling, we calculate the string tension and masses of the scalar, axial vector and tensor states using standard Wilson action on increasingly anisotropic lattices, and make an extrapolation to the Hamiltonian limit. The results are compared with estimates from various other Hamiltonian and Euclidean studies. We find that more accurate determination of the glueball masses and the mass ratios has been achieved and the results are a significant improvement upon previous Hamiltonian estimates. The continuum predictions are then found by extrapolation of results obtained from smallest values of spatial lattice spacing. For the lightest scalar, tensor and axial vector states we obtain masses of $m_{0^{++}}=1654 \pm 83$ MeV, $m_{2^{++}}=2272\pm 115$ MeV and $m_{1^{+-}}=2940\pm 165$ MeV, respectively. These are consistent with the estimates obtained in the previous studies in the Euclidean limit. The consistency is a clear evidence of universality between Euclidean and Hamiltonian formulations. From the accuracy of our estimates, we conclude that the standard Euclidean Monte Carlo method is a reliable technique for obtaining results in the Hamiltonian version of the theory, just as in Euclidean case.

hep-lat

Tricritical point of lattice QCD with Wilson quarks at finite temperature and density

First principle study of QCD at finite temperature $T$ and chemical potential $μ$ is essential for understanding a wide range of phenomena from heavy-ion collisions to cosmology and neutron stars. However, in the presence of finite density, the critical behavior lattice gauge theory without species doubling, is unknown. At strong coupling, we examine the phase structure on the $(μ,T)$ plane, using Hamiltonian lattice QCD with Wilson fermions. A tricritical point is found, separating the first and second order chiral phase transitions. Such a tricritical point at finite $T$ has not been found in previous work in the Hamiltonian formalism with Kogut-Susskind fermions or naive fermions.

hep-lat

QCD at Finite temperature and density with staggered and Wilson quarks

One of the most challenging issues in particle physics is to study QCD in extreme conditions. Precise determination of the QCD phase diagram on temperature $T$ and chemical potential $μ$ plane will provide valuable information for quark-gluon plasma (QGP) and neutron star physics. We present results for phase structure on the $(μ, T)$ plane for lattice QCD with Wilson fermions from strong coupling Hamiltonian analysis and Kogut-Susskind Fermions from Lagrangian Monte Carlo simulations at intermediate coupling.

hep-lat

Chiral condensate of lattice QCD with massless quarks from the probability distribution function method

We apply the probability distribution function method to the study of chiral properties of QCD with quarks in the exact massless limit. A relation among the chiral condensate, zeros of the Bessel function and eigenvalue of Dirac operator is also given. The chiral condensate in this limit can be measured with small number of eigenvalues of the massless Dirac operator and without any ambiguous mass extrapolation. Results for SU(3) gauge theory with quenched Kogut-Susskind quarks on the $10^4$ lattice are shown.

hep-lat

Exotic mesons from quantum chromodynamics with improved gluon and quark actions on the anisotropic lattice

Hybrid (exotic) mesons, which are important predictions of quantum chromodynamics (QCD), are states of quarks and anti-quarks bound by excited gluons. First principle lattice study of such states would help us understand the role of ``dynamical'' color in low energy QCD and provide valuable information for experimental search for these new particles. In this paper, we apply both improved gluon and quark actions to the hybrid mesons, which might be much more efficient than the previous works in reducing lattice spacing error and finite volume effect. Quenched simulations were done at $β=2.6$ and on a $ξ=3$ anisotropic $12^3\times36$ lattice using our PC cluster. We obtain $2013 \pm 26 \pm 71$ MeV for the mass of the $1^{-+}$ hybrid meson ${\bar q}qg$ in the light quark sector, and $4369 \pm 37 \pm 99$Mev in the charm quark sector; the mass splitting between the $1^{-+}$ hybrid meson ${\bar c}c g$ in the charm quark sector and the spin averaged S-wave charmonium mass is estimated to be $1302 \pm 37 \pm 99$ MeV. As a byproduct, we obtain $1438 \pm 32 \pm 57$ MeV for the mass of a P-wave $1^{++}$ ${\bar u}u$ or ${\bar d}d$ meson and $1499 \pm 28 \pm 65$ MeV for the mass of a P-wave $1^{++}$ ${\bar s}s$ meson, which are comparable to their experimental value 1426 MeV for the $f_1(1420)$ meson. The first error is statistical, and the second one is systematical. The mixing of the hybrid meson with a four quark state is also discussed.

hep-lat

Hamiltonian lattice quantum chromodynamics at finite density with Wilson fermions

Quantum chromodynamics (QCD) at sufficiently high density is expected to undergo a chiral phase transition. Understanding such a transition is of particular importance for neutron star or quark star physics. In Lagrangian SU(3) lattice gauge theory, the standard approach breaks down at large chemical potential $μ$, due to the complex action problem. The Hamiltonian formulation of lattice QCD doesn't encounter such a problem. In a previous work, we developed a Hamiltonian approach at finite chemical potential $μ$ and obtained reasonable results in the strong coupling regime. In this paper, we extend the previous work to Wilson fermions. We study the chiral behavior and calculate the vacuum energy, chiral condensate and quark number density, as well as the masses of light hadrons. There is a first order chiral phase transition at zero temperature.

hep-lat

Monte Carlo Hamiltonian: Inverse Potential

The Monte Carlo Hamiltonian method developed recently allows to investigate ground state and low-lying excited states of a quantum system, using Monte Carlo algorithm with importance sampling. However, conventional MC algorithm has some difficulties when applying to inverse potentials. We propose to use effective potential and extrapolation method to solve the problem. We present examples from the hydrogen system.

hep-lat