311 research outputs found
Thermodynamic instabilities in dynamical quark models with complex conjugate mass poles
We show that the CJT thermodynamic potential of dynamical quark models with a
quark propagator represented by complex conjugate mass poles inevitably
exhibits thermodynamic instabilities. We find that the minimal coupling of the
quark sector to a Polyakov loop potential can strongly suppress but not
completely remove such instabilities. This general effect is explicitly
demonstrated in the framework of a covariant, chirally symmetric, effective
quark model.Comment: Minor typos corrected, submitted versio
Scaling studies of QCD with the dynamical HISQ action
We study the lattice spacing dependence, or scaling, of physical quantities
using the highly improved staggered quark (HISQ) action introduced by the
HPQCD/UKQCD collaboration, comparing our results to similar simulations with
the asqtad fermion action. Results are based on calculations with lattice
spacings approximately 0.15, 0.12 and 0.09 fm, using four flavors of dynamical
HISQ quarks. The strange and charm quark masses are near their physical values,
and the light-quark mass is set to 0.2 times the strange-quark mass. We look at
the lattice spacing dependence of hadron masses, pseudoscalar meson decay
constants, and the topological susceptibility. In addition to the commonly used
determination of the lattice spacing through the static quark potential, we
examine a determination proposed by the HPQCD collaboration that uses the decay
constant of a fictitious "unmixed s bar s" pseudoscalar meson. We find that the
lattice artifacts in the HISQ simulations are much smaller than those in the
asqtad simulations at the same lattice spacings and quark masses.Comment: 36 pages, 11 figures, revised version to be published. Revisions
include discussion of autocorrelations and several clarification
Direct determination of the strange and light quark condensates from full lattice QCD
We determine the strange quark condensate from lattice QCD for the first time and compare its value to that of the light quark and chiral condensates. The results come from a direct calculation of the expectation value of the trace of the quark propagator followed by subtraction of the appropriate perturbative contribution, derived here, to convert the non-normal-ordered mψ̅ ψ to the MS̅ scheme at a fixed scale. This is then a well-defined physical “nonperturbative” condensate that can be used in the operator product expansion of current-current correlators. The perturbative subtraction is calculated through O(αs) and estimates of higher order terms are included through fitting results at multiple lattice spacing values. The gluon field configurations used are “second generation” ensembles from the MILC collaboration that include 2+1+1 flavors of sea quarks implemented with the highly improved staggered quark action and including u/d sea quarks down to physical masses. Our results are ⟨s̅ s⟩MS̅ (2 GeV)=-(290(15) MeV)3, ⟨l̅ l⟩MS̅ (2 GeV)=-(283(2) MeV)3, where l is a light quark with mass equal to the average of the u and d quarks. The strange to light quark condensate ratio is 1.08(16). The light quark condensate is significantly larger than the chiral condensate in line with expectations from chiral analyses. We discuss the implications of these results for other calculations
Dynamical Locking of the Chiral and the Deconfinement Phase Transition in QCD
We study the fixed-point structure of four-fermion interactions in two-flavor
QCD with Nc colors close to the finite-temperature phase boundary. In
particular, we analyze how the fixed-point structure of four-fermion
interactions is related to the confining dynamics in the gauge sector. We show
that there exists indeed a mechanism which dynamically locks the chiral phase
transition to the deconfinement phase transition. This mechanism allows us to
determine a window for the values of physical observables in which the two
phase transitions lie close to each other.Comment: 14 pages, 5 figure
Width of the QCD transition in a Polyakov-loop DSE model
We consider the pseudocritical temperatures for the chiral and deconfinement
transitions within a Polyakov-loop Dyson-Schwinger equation approach which
employs a nonlocal rank-2 separable model for the effective gluon propagator.
These pseudocritical temperatures differ by a factor of two when the quark and
gluon sectors are considered separately, but get synchronized and become
coincident when their coupling is switched on. The coupling of the
Polyakov-loop to the chiral quark dynamics narrows the temperature region of
the QCD transition in which chiral symmetry and deconfinement is established.
We investigate the effect of rescaling the parameter T_0 in the Polyakov-loop
potential on the QCD transition for both the logarithmic and polynomial forms
of the potential. While the critical temperatures vary in a similar way, the
width of the transition is stronger affected for the logarithmic potential. For
this potential the character of the transition changes from crossover to a
first order one when T_0 < 210 MeV, but it remains crossover in the whole range
of relevant T_0 values for the polynomial form.Comment: 10 pages, 6 figures, results for polynomial form of Polyakov-loop
potential included, references added, final version to appear in Phys. Rev.
Density of states and Fisher's zeros in compact U(1) pure gauge theory
We present high-accuracy calculations of the density of states using
multicanonical methods for lattice gauge theory with a compact gauge group U(1)
on 4^4, 6^4 and 8^4 lattices. We show that the results are consistent with weak
and strong coupling expansions. We present methods based on Chebyshev
interpolations and Cauchy theorem to find the (Fisher's) zeros of the partition
function in the complex beta=1/g^2 plane. The results are consistent with
reweighting methods whenever the latter are accurate. We discuss the volume
dependence of the imaginary part of the Fisher's zeros, the width and depth of
the plaquette distribution at the value of beta where the two peaks have equal
height. We discuss strategies to discriminate between first and second order
transitions and explore them with data at larger volume but lower statistics.
Higher statistics and even larger lattices are necessary to draw strong
conclusions regarding the order of the transition.Comment: 14 pages, 16 figure
Quarkonium mass splittings in three-flavor lattice QCD
We report on calculations of the charmonium and bottomonium spectrum in
lattice QCD. We use ensembles of gauge fields with three flavors of sea quarks,
simulated with the asqtad improved action for staggered fermions. For the heavy
quarks we employ the Fermilab interpretation of the clover action for Wilson
fermions. These calculations provide a test of lattice QCD, including the
theory of discretization errors for heavy quarks. We provide, therefore, a
careful discussion of the results in light of the heavy-quark effective
Lagrangian. By and large, we find that the computed results are in agreement
with experiment, once parametric and discretization errors are taken into
account.Comment: 21 pages, 17 figure
Thermalization in SU(3) gauge theory after a deconfining quench
We determine the time evolution of fluctuations of the Polyakov loop after a
quench into the deconfined phase of SU(3) gauge theory from a simple classical
relativistic Lagrangian. We compare the structure factors, which indicate
spinodal decomposition followed by relaxation, to those obtained via Markov
Chain Monte Carlo techniques in SU(3) lattice gauge theory. We find that the
time when the structure factor peaks diverges like in the
long-wavelength limit. This is due to formation of competing Z(3) domains for
configurations where the Polyakov loop exhibits non-perturbatively large
variations in space, which delay thermalization of long wavelength modes. For
realistic temperatures, and away from the extreme weak-coupling limit, we find
that even modes with on the order of experience delayed thermalization.
Relaxation times of very long wavelength modes are found to be on the order of
the size of the system; thus, the dynamics of competing domains should
accompany the hydrodynamic description of the deconfined vacuum.Comment: 7 pages, 7 figure
The intrinsic strangeness and charm of the nucleon using improved staggered fermions
We calculate the intrinsic strangeness of the nucleon, - ,
using the MILC library of improved staggered gauge configurations using the
Asqtad and HISQ actions. Additionally, we present a preliminary calculation of
the intrinsic charm of the nucleon using the HISQ action with dynamical charm.
The calculation is done with a method which incorporates features of both
commonly-used methods, the direct evaluation of the three-point function and
the application of the Feynman- Hellman theorem. We present an improvement on
this method that further reduces the statistical error, and check the result
from this hybrid method against the other two methods and find that they are
consistent. The values for and found here, together with
perturbative results for heavy quarks, show that dark matter scattering through
Higgs-like exchange receives roughly equal contributions from all heavy quark
flavors.Comment: 17 pages, 14 figure
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