Presence Of Dynamical Quarks Research Articles

Strong coupling model for string breaking on the lattice

A model for SU( n) string breaking on the lattice is formulated using strong coupling ideas. Although necessarily rather crude, the model gives an explicit picture of string breaking in the presence of dynamical quarks as a mixing phenomenon that involves the string state and a two-meson state. A careful analysis, within the model, of the Wilson loop shows that the evolution of the mixing angle as a function of separation may obscure the expected crossover effect. If a sufficiently extensive mixing region exists then an appropriate combination of transition amplitudes can help in revealing the effect. The sensitivity of the mixing region to the values of the meson energy and the dynamical quark mass is explored and an assessment made of the detectibility of string breaking in a practical lattice simulation.

Coulomb-Gauge in QCD: Renormalization and Confinement

We review the Coulomb gauge in QCD and report some recent results. The minimal Coulomb gauge is defined and the fundamental modular region, a region without Gribov copies, is described. The Coulomb gauge action is expressed in terms of the dynamical degrees of freedom with an instantaneous Coulomb interaction, and its physical meaning is discussed. The local Coulomb gauge action with phase-space and ghost variables is derived, and its BRS-invariance and renormalizability are reviewed. It is shown that the instantanteous part $V(R)$ of $g^2 D_{00}(R, t)$, the time-time component of the gluon propagator, is a renormalization-group invariant $V(R) = f(R\Lambda_{QCD})/R$, and that the contribution of $V(R)$ to the Wilson loop exponentiates. It is conjectured that $V(R) \sim \kappa_{coul}R$ at large $R$, and that $\kappa_{coul}$ provides an order parameter for confinement of color even in the presence of dynamical quarks.

ϒ and J/Ψ spectroscopy using clover fermions in the presence of dynamical quarks

We calculate spin averaged splittings in the bottomonium and charmonium systems using the clover action. We compare static and kinetic masses of heavy-heavy and heavy-light mesons and discuss the consistency of the results.

A comparison of Clover and Wilson spectroscopy in the presence of dynamical quarks

We present preliminary results of light hadron spectroscopy using valence, tadpole-improved, Clover fermions on an ensemble of gauge configurations generated with 2 flavors of staggered fermions at a beta of 5.6. We compare the slope and intercept of the curve M_V vs. M_PS^2 for Clover and Wilson fermions. We show that a higher order chiral perturbation theory ansatz works very well for chiral extrapolations.

Topological susceptibility in lattice QCD with dynamical quarks

Topological susceptibility is studied in lattice QCD with two flavors of dynamical Kogut-Susskind quarks on a 20 4 and an 8 3 × 16 lattice employing the cooling method. The topological susceptibility is strongly suppressed by the presence of dynamical quarks, and it is consistent with zero for the zero quark mass limit. Furthermore, we find a linear m q dependence of the topological susceptibility in the chirally broken phase, and even stronger suppression of topological fluctuations in the chirally symmetric phase.

Interface tension and chiral order parameter profile with dynamical quarks.

We investigate the free energy distribution across the interface of coexisting quark and hadron matter in the framework of lattice QCD. We calculate the interface tension [alpha] with the differential method'' for pure SU(3) gauge theory and in the presence of dynamical quarks with four flavors. Using lattices with a spatial volume 8[sup 2][times]16 we discuss the agreement between the differential and integral method in the pure gluonic case. With dynamical fermions it turns out that the interface tension is very small and we estimate an upper bound of [alpha]/[ital T][sub [ital c]][sup 3][lt]0.1. The chiral condensate indicates the same width of the domain wall as the Polyakov loop distribution and the other thermodynamical observables under consideration.

CHIRAL INTERFACE FOR QCD WITH DYNAMICAL FERMIONS

The free energy distribution across the interface of coexisting quark and hadron mat-ter is studied in the framework of lattice QCD. We calculate the interface tension α with the “differential method” for pure SU(3) gauge theory and in the presence of dynamical quarks with four flavors. Using lattices with spatial volume 82×16 it turns out that α is very small and compatible with zero. The chiral condensate indicates the same width of the domain wall as the Polyakov loop distribution and the other thermodynamical observables.

Glueballs and topological charge in the presence of dynamical quarks

We have estimated the string tension and the masses of the 0 ++ and 2 ++ glueballs in lattice QCD with 2 light flavours of staggered quarks. In addition we have measured the topological charge and hence susceptibility of our gauge field configurations. The simulations were performed at 6/ g 2 = 5.6 and quark masses m = 0.01 and 0.025 (lattice units) on lattice sizes ranging from 12 4 to 16 4.

Vacuum structure around quark systems

The vacuum structure of QCD with dynamical fermions around a static quark-antiquark pair is explored by calculating the correlation functions < L( r 1) L †( r 2) O( r)> between a static meson and several characteristic observables for which we choose the chiral condensate, the charge density, and the energy density. We find a restoration of chiral symmetry around static color sources as a consequence of gluon string creation and a screening of static quarks by virtual antiquarks in analogy to QED. Our computations explicitly exhibit the formation of a chromoelectric flux tube and its breaking in the presence of dynamical quarks.

Order of the deconfining transition for finite-temperature QCD in the presence of dynamical quarks.

We present a high-statistics computation of finite-temperature QCD: The effects of dynamical quarks are included by the pseudofermion method. We concentrate only on one value of the bare quark mass and try to determine the order of the transition. The metastability observed in a previ- ous preliminary study of ours is confirmed also by this new set of data. Ordered start. did not allo~ us to study the properties of the transi- tion in detail. We decided, therefore, to consolidate our computation by improving the statistics and the density of the measurements around P, . The outcome of this work is presented here. Let us briefly review the results of Ref. 1 to justify our choice of parameters. We performed Monte Carlo simulations including dynamical quarks by the pseudo- fermion method. We used two different lattice volumes, 63&&4 and 83x4. For all the values of the bare quark masses that we studied (m = 0.1, 0.2, 0.5, 1, 2) we always observed a sharp crossover until values of m for which P, was compatible with the pure gauge value. The quantities we measured were 8'and The results were pretty much independent of the dimension of the lattice and also stable with respect to

Chiral-symmetry restoration transition in the presence of dynamical quarks.

By Monte Carlo methods, we study the restoration of chiral symmetry at finite temperature for gauge theories with color group SU(3). The effect of dynamical quarks is taken into account using the pseudofermion method. Chiral symmetry is indeed restored at finite temperature for values of the coupling \ensuremath{\beta}=6/${g}^{2}$ smaller than in the pure gauge case. We find signs of metastability in the neighborhood of the transition; this could suggest that the transition is discontinuous. The ratio ${T}_{\mathrm{ch}/{T}_{D}\mathrm{\ensuremath{\approxeq}}}$1, where ${T}_{\mathrm{ch}({T}_{D})\mathrm{\ensuremath{\approxeq}}}$390 MeV is the physical temperature at which chiral symmetry (confinement) is lost. The internal fermionic energy is measured also.

On the phase diagram of finite-temperature QCD in the presence of dynamical quarks

We study finite-temperature QCD at intermediate values of the bare quark mass m by pseudofermions Monte Carlo for three fermionic flavors on 6 3 × 4 and 8 3 × 2 lattices. On the 6 3 × 4 lattice we study the finite-temperature deconfining and chiral restoration transitions up to values of m for which the critical coupling β c is the same as the pure gauge case. On the 8 3 × 2 lattice we carry on a direct comparison between the pseudofermions method and the hopping parameter expansion for intermediate values of m: we conclude that in this region of values of m the two methods give different results.

Finite temperature QCD in the presence of dynamical quarks

By using Monte Carlo (MC) methods we study the deconfining transition of QCD with color group SU(3) at finite temperature. The fermion polarization effects are kept in account by implementing the pseudo-fermion method. We use two different lattice volumes (6 3 × 4, 8 3 × 4) to check finite size effects. Even in the presence of light quarks we find a discontinuous transition. The existence of this transition is observed both creating metastable states and using mixed phase runs.

Mean field analysis of SU( N) deconfining transitions in the presence of dynamical quarks

The deconfining transitions of SU( N) lattice gauge theories, both with and without quarks, are studied using strong coupling techniques combined with a mean field analysis. In the pure gauge sector our analysis suggests first-order transitions for N ⩾ 3 and a second-order transition only for N = 2. Quarks are incorporated via an effective external field h related to the Wilson hopping parameter. For N = 2 the transition disappears for arbitrarily small h, whereas for finite N ⩾ 3 it disappears above non-zero critical field h c. h c approaches zero as N → ∞ even though the pure gauge sector transition remains first order. Our results for h c in the SU(3) case agree well with recent Monte Carlo simulations.