Chiral phase transition in lattice QCD with Wilson quarks

The nature of the chiral phase transition in lattice QCD is studied for the cases of 2, 3 and 6 flavors with degenerate Wilson quarks, mainly on a lattice with the temporal direction extensionN _t=4. We find that the chiral phase transition is continuous for the case of 2 flavors, while it is of first order for 3 and 6 flavors.     

Chiral phase transition in lattice QCD with Wilson quarks

The nature of the chiral phase transition in lattice QCD is studied for the cases of 2, 3 and 6 flavors with degenerate Wilson quarks, mainly on a lattice with the temporal direction extensionN _t=4. We find that the chiral phase transition is continuous for the case of 2 flavors, while it is of first order for 3 and 6 flavors.     

A lattice Dirac operator for QCD with light dynamical quarks

In QCD chiral symmetry is explicitly broken by quark masses, the effect of which can be described reliably by chiral perturbation theory. Effects of explicit chiral symmetry breaking by the lattice regularisation of the Dirac operator, typically parametrised by the residual mass, should be negligible for almost all observables if the residual mass of the Dirac operator is much smaller than the quark mass. However, maintaining a small residual mass becomes increasingly expensive as the quark mass decreases towards the physical value and the continuum limit is approached. We investigate the feasibility of using a new approximately chiral Dirac operator with a small residual mass as an alternative to overlap and domain wall fermions for lattice simulations. Our Dirac operator is constructed from a Zolotarev rational approximation for the matrix sign function that is optimal for bulk modes of the hermitian kernel Dirac operator but not for the low-lying parts of its spectrum. We test our operator on various ³²³×64${32}^3\times 64$ lattices, comparing the residual mass and the performance of the Hybrid Monte Carlo algorithm at a similar lattice spacing and pion mass with a hyperbolic tangent operator as used by domain wall fermions. We find that our approximations have a significantly smaller residual mass than domain wall fermions at a similar computational cost, and still admit topological charge change.     

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 of the lattice gauge theory, one has to perform expensive simulations at multiple bare quark masses, and employ some modeled functions 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 indicate that this might be a promising and efficient method for investigating the spontaneous chiral-symmetry breaking in lattice QCD, which deserves further investigation.     

Deconfinement phase transition in QCD with heavy quarks

Using the pseudo-fermion method to simulate QCD with dynamical quarks we investigate the effects of heavy dynamical quarks of 2 flavours on the deconfinement phase transition in the quenched QCD. As the mass of the quark is decreased the phase transition weakens as expected. Compared to the earlier results with leading order hopping parameter expansion, however, the weakening is less rapid. Our estimated upper bound on the critical mass where the transition becomes continuous is 1.5–2 times lower than earlier results.     

Incorporating dynamical light quarks in lattice QCD at finite temperature

Using the pseudo-fermion Monte Carlo method we investigate the chiral and the deconfinement phase transitions in the SU(3) gauge theory with dynamical staggered fermions on a 6³ × 2 lattice. The energy density of the gluonic sector ε_G, the average thermal Wilson loop |L| and the order parameter are found to have a rapid variation in the same range of β (=6/g²). The variation is similar to that observed in the quenched theory but is at a smaller value of β.     

The calculation of the hadron spectrum in lattice QCD with dynamical quarks

Hadron masses are calculated on an 8³×16 lattice using four flavors of staggered fermions to generate the gauge configurations, but using Wilson fermions to calculate the hadron propagators. The identification of a value of the Wilson hopping parameter with the value of the bare quark mass used in the simulations is discussed.     

Chiral QCD phase transition in nuclear collisions

The characteristic aspects of multiparticle states generated at , as a result of chiral QCD phase transition, are studied in the framework of the O(4) theory. Predictions concerning critical events, in connection with current and future experiments with ultrarelativistic heavy ions, are presented. Received: 9 June 1997 / Revised version: 18 July 1997 / Published online: 26 February 1998     

First-order chiral phase transition in lattice QCD

The chiral phase transition in lattice QCD has been studied for light fermions of mass ma=0.025 on lattices of size 4⁴ and 8³×4 using the hybrid algorithm. We find evidence for a first-order chiral phase transition with a large latent heat. A comparison with 10³×6 data shows violations of asymptotic scaling for T_ch which are similar in magnitude to those observed in the pure gauge sector.     

Twisted mass quarks and the phase structure of lattice QCD

The phase structure of zero temperature twisted mass lattice QCD is investigated. We find strong metastabilities in the plaquette observable in correspondence of which the untwisted quark mass assumes positive or negative values. We provide interpretations of this phenomenon in terms of chiral symmetry breaking and the effective potential model of Sharpe and Singleton.Received: 24 August 2004, Revised: 29 October 2004, Published online: 25 January 2005     

QCD potentials with dynamical quarks at finite temperatures

We investigate the potential between a static quark-antiquark pair in the full QCD vacuum. The dynamical quark sea is considered both by Kogut-Susskind and Wilson fermions. Two temperatures T below and above the phase transition T_c are studied. The behavior of the interquark potential in the full quantum field is driven by the explicitly broken Z₃ symmetry in the fermionic action for T<T_c and by an additional spontaneous Z₃ symmetry breaking for T<T_c.     

A numerical study of Langevin QCD with dynamical quarks

QCD with four flavours of dynamical staggered quarks of mass ma=0.1 is simulated on a 4⁴ lattice, using two versions of the second order Langevin algorithm with bilinear noise: the naive version and one which compensates for the non-integrable term which appears to first order in the discrete Langevin time step size. Comparison with the results of an exact numerical computation of the fermion determinant reveals that these algorithms yield accurate results only at rather small values of the step size. The correction due to the non-integrable term is quantitatively unimportant for such step sizes.     

Up quark mass in lattice QCD with three light dynamical quarks and implications for strong CP invariance

A standing mystery in the standard model is the unnatural smallness of the strong CP violating phase. A massless up quark has long been proposed as one potential solution. A lattice calculation of the constants of the chiral Lagrangian essential for the determination of the up quark mass, 2alpha(8)-alpha(5), is presented. We find 2alpha(8)-alpha(5)=0.29+/-0.18, which corresponds to m(u)/m(d)=0.410+/-0.036. This is the first such calculation using a physical number of dynamical light quarks, N(f)=3.