37 research outputs found
Zero-momentum modes and chiral limit in compact lattice QED
The influence of zero-momentum gauge modes on physical observables is
investigated for compact lattice QED with dynamical and quenched Wilson
fermions. Within the Coulomb phase, zero-momentum modes are shown to hide the
critical behaviour of gauge invariant fermion observables near the chiral
limit. Methods for eliminating zero-momentum modes are discussed.Comment: Talk at Lattice 2000, Bangalore, 4 pages, 3 figures, LaTe
Lattice gluodynamics computation of Landau-gauge Green's functions in the deep infrared
We present recent results for the Landau-gauge gluon and ghost propagators in
SU(3) lattice gluodynamics obtained on a sequence of lattices with linear
extension ranging from L=64 to L=96 at , thus reaching "deep
infrared" momenta down to 75 MeV. Our gauge-fixing procedure essentially uses a
simulated annealing technique which allows us to reach gauge-functional values
closer to the global maxima than standard approaches do. Our results are
consistent with the so-called decoupling solutions found for Dyson-Schwinger
and functional renormalization group equations.Comment: 6 pages, 5 figures. References added, minor changes to match
published versio
Performance studies of the two-step multiboson algorithm in compact lattice QED
The performance of the two-step multiboson (TSMB) algorithm is investigated
in comparison with the hybrid Monte Carlo (HMC) method for compact lattice QED
with standard Wilson fermions both in the Coulomb and confinement phases. The
restriction to QED allows for extensive measurements of autocorrelation times.
Preliminary results show that the TSMB algorithm is at least competitive with
standard HMC.Comment: Talk at Lattice2001(algorithms), Berlin, 3 pages, LaTe
The photon propagator in compact QED_{2+1}: the effect of wrapping Dirac strings
We discuss the influence of closed Dirac strings on the photon propagator in
the Landau gauge emerging from a study of the compact U(1) gauge model in 2+1
dimensions. This gauge also minimizes the total length of the Dirac strings.
Closed Dirac strings are stable against local gauge-fixing algorithms only due
to the torus boundary conditions of the lattice. We demonstrate that these
left-over Dirac strings are responsible for the previously observed unphysical
behavior of the propagator of space-like photons (D_T) in the deconfinement
(high temperature) phase. We show how one can monitor the number N_3 of thermal
Dirac strings which allows to separate the propagator measurements into N_3
sectors. The propagator in N_3 \neq 0 sectors is characterized by a non--zero
mass and an anomalous dimension similarly to the confinement phase. Both mass
squared and anomalous dimension are found to be proportional to N_3.
Consequently, in the N_3=0 sector the unphysical behavior of the D_T photon
propagator is cured and the deviation from the free massless propagator
disappears.Comment: 13 pages, 13 figures, 1 tabl
Indirect determination of the Kugo-Ojima function from lattice data
We study the structure and non-perturbative properties of a special Green's
function, u(q), whose infrared behavior has traditionally served as the
standard criterion for the realization of the Kugo-Ojima confinement mechanism.
It turns out that, in the Landau gauge, u(q) can be determined from a dynamical
equation, whose main ingredients are the gluon propagator and the ghost
dressing function, integrated over all physical momenta. Using as input for
these two (infrared finite) quantities recent lattice data, we obtain an
indirect determination of u(q). The results of this mixed procedure are in
excellent agreement with those found previously on the lattice, through a
direct simulation of this function. Most importantly, in the deep infrared the
function deviates considerably from the value associated with the realization
of the aforementioned confinement scenario. In addition, the dependence of
u(q), and especially of its value at the origin, on the renormalization point
is clearly established. Some of the possible implications of these results are
briefly discussed.Comment: 25 pages, 10 figures; v2: typos corrected, expanded version that
matches the published articl
A renormalization group invariant scalar glueball operator in the (Refined) Gribov-Zwanziger framework
This paper presents a complete algebraic analysis of the renormalizability of
the operator in the Gribov-Zwanziger (GZ) formalism as
well as in the Refined Gribov-Zwanziger (RGZ) version. The GZ formalism offers
a way to deal with gauge copies in the Landau gauge. We explicitly show that
mixes with other gauge variant operators, and we determine
the mixing matrix to all orders, thereby only using algebraic arguments.
The mixing matrix allows us to uncover a renormalization group invariant
including the operator . With this renormalization group
invariant, we have paved the way for the study of the lightest scalar glueball
in the GZ formalism. We discuss how the soft breaking of the BRST symmetry of
the GZ action can influence the glueball correlation function. We expect
non-trivial mass scales, inherent to the GZ approach, to enter the pole
structure of this correlation function.Comment: 27 page
Constraints on the nonuniversal Z^\prime couplings from B\to\pi K, \pi K^{\ast} and \rho K Decays
Motivated by the large difference between the direct CP asymmetries
and , we
combine the up-to-date experimental information on ,
and decays to pursue possible solutions with the nonuniversal
model. Detailed analyses of the relative impacts of different
types of couplings are presented in four specific cases. Numerically, we find
that the new coupling parameters, and with a common
nontrivial new weak phase , which are relevant to the
contributions to the electroweak penguin sector
and , are crucial to the observed " puzzle". Furthermore,
they are found to be definitely unequal and opposite in sign. We also find that
can put a strong constraint on the new
couplings, which implies the contributions to the coefficient of
QCD penguins operator involving the parameter required.Comment: 27 pages, 6 figures. References and a note adde
IR finiteness of the ghost dressing function from numerical resolution of the ghost SD equation
We solve numerically the Schwinger-Dyson (SD hereafter) ghost equation in the
Landau gauge for a given gluon propagator finite at k=0 (alpha_gluon=1) and
with the usual assumption of constancy of the ghost-gluon vertex ; we show that
there exist two possible types of ghost dressing function solutions, as we have
previously inferred from analytical considerations : one singular at zero
momentum, satisfying the familiar relation alpha_gluon+2 alpha_ghost=0 between
the infrared exponents of the gluon and ghost dressing functions(in short,
respectively alpha_G and alpha_F) and having therefore alpha_ghost=-1/2, and
another which is finite at the origin (alpha_ghost=0), which violates the
relation. It is most important that the type of solution which is realized
depends on the value of the coupling constant. There are regular ones for any
coupling below some value, while there is only one singular solution, obtained
only at a critical value of the coupling. For all momenta k<1.5 GeV where they
can be trusted, our lattice data exclude neatly the singular one, and agree
very well with the regular solution we obtain at a coupling constant compatible
with the bare lattice value.Comment: 17 pages, 3 figures (one new figure and a short paragraph added
Strong-coupling study of the Gribov ambiguity in lattice Landau gauge
We study the strong-coupling limit beta=0 of lattice SU(2) Landau gauge
Yang-Mills theory. In this limit the lattice spacing is infinite, and thus all
momenta in physical units are infinitesimally small. Hence, the infrared
behavior can be assessed at sufficiently large lattice momenta. Our results
show that at the lattice volumes used here, the Gribov ambiguity has an
enormous effect on the ghost propagator in all dimensions. This underlines the
severity of the Gribov problem and calls for refined studies also at finite
beta. In turn, the gluon propagator only mildly depends on the Gribov
ambiguity.Comment: 14 pages, 22 figures; minor changes, matches version to appear in
Eur. Phys. J.
Chiral phase boundary of QCD at finite temperature
We analyze the approach to chiral symmetry breaking in QCD at finite
temperature, using the functional renormalization group. We compute the running
gauge coupling in QCD for all temperatures and scales within a simple truncated
renormalization flow. At finite temperature, the coupling is governed by a
fixed point of the 3-dimensional theory for scales smaller than the
corresponding temperature. Chiral symmetry breaking is approached if the
running coupling drives the quark sector to criticality. We quantitatively
determine the phase boundary in the plane of temperature and number of flavors
and find good agreement with lattice results. As a generic and testable
prediction, we observe that our underlying IR fixed-point scenario leaves its
imprint in the shape of the phase boundary near the critical flavor number:
here, the scaling of the critical temperature is determined by the
zero-temperature IR critical exponent of the running coupling.Comment: 39 pages, 8 figure