158 research outputs found
New Method for Dynamical Fermions and Chiral-Symmetry Breaking
The reasons for the feasibility of the Microcanonical Fermionic Average
() approach to lattice gauge theory with dynamical fermions are discussed.
We then present a new exact algorithm, which is free from systematic errors and
convergent even in the chiral limit.Comment: 3 pages, DFTUZ 93/20, to appear in the Proceedings of Lattice 93,
Dalla
The Schwinger Model on the lattice in the Microcanonical Fermionic Average approach
The Microcanonical Fermionic Average method has been used so far in the
context of lattice models with phase transitions at finite coupling. To test
its applicability to Asymptotically Free theories, we have implemented it in
QED, \it i.e.\rm the Schwinger Model. We exploit the possibility, intrinsic
to this method, of studying the whole plane at negligible computer
cost, to follow constant physics trajectories and measure the limit
of the chiral condensate. We recover the continuum result within 3 decimal
places.Comment: TeX file, 7 pages + 3 figures in Postscrip
Testing logarithmic violations to scaling in strongly coupled QED
Using very precise measurements of the critical couplings for the chiral
transition of non compact with up to 8 flavours, we analyse the
behaviour of the order parameter at the critical point using the equation of
state of a logarithmically improved scalar mean field theory, that of the
Nambu-Jona Lasinio theory and a pure power law. The first case is definitively
excluded by the numerical data. The stability of the fits for the last two
cases, as well as the behaviour with the number of flavours of the exponent of
the logarithmic violations to the scaling favour clearly a pure power law
scaling with non mean field exponents.Comment: 6 pages, 3 postscript figures, 2 postscript tables (tar-ed, zip-ed,
uu-encoded
Critical Behavior of the Schwinger Model with Wilson Fermions
We present a detailed analysis, in the framework of the MFA approach, of the
critical behaviour of the lattice Schwinger model with Wilson fermions on
lattices up to , through the study of the Lee-Yang zeros and the specific
heat. We find compelling evidence for a critical line ending at
at large . Finite size scaling analysis on lattices and indicates a continuous transition. The hyperscaling relation
is verified in the explored region.Comment: 12 pages LaTeX file, 10 figures in one uuencoded compressed
postscript file. Report LNF-95/049(P
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 lattice are shown
Investigation of a Toy Model for Frustration in Abelian Lattice Gauge Theory
We introduce a lattice model with local U(1) gauge symmetry which
incorporates explicit frustration in d >2. The form of the action is inspired
from the loop expansion of the fermionic determinant in standard lattice QED.
We study through numerical simulations the phase diagram of the model,
revealing the existence of a frustrated (antiferromagnetic) phase for d=3 and
d=4, once an appropriate order parameter is identified.Comment: 7 pages, 7 figure
No-go theorem on spontaneous parity breaking revisited
An essential assumption in the Vafa and Witten's theorem on P and CT
realization in vector-like theories concerns the existence of a free energy
density in Euclidean space in the presence of any external hermitian symmetry
breaking source. We show how this requires the previous assumption that the
symmetry is realized in the vacuum. Even if Vafa and Witten's conjecture is
plausible, actually a theorem is still lacking.Comment: Talk presented at LATTICE99(Theoretical Developments),3 pages. Latex
using espcrc2.st
Chiral Susceptibilities in noncompact QED: a new determination of the exponent and the critical couplings
We report the results of a measurement of susceptibilities in noncompact
in and lattices. Due to the potentialities of the
approach, we have done simulations in the chiral limit which are
therefore free from arbitrary mass extrapolations. Our results in the Coulomb
phase show unambiguously that the susceptibility critical exponent
independently of the flavour symmetry group. The critical couplings extracted
from these calculations are in perfect agreement with previous determinations
based on the fermion effective action and plaquette energy, and outside the
predictions of a logarithmically improved scalar mean field theory by eight
standard deviations.Comment: 11 pages, figures on reques
Theta-vacuum: Phase Transitions and/or Symmetry Breaking at
Assuming that a quantum field theory with a -vacuum term in the
action shows non-trivial -dependence and provided that some reasonable
properties of the probability distribution function of the order parameter
hold, we argue that the theory either breaks spontaneously CP at
or shows a singular behavior at some critical between 0 and .
This result, which applies to any model with a pure imaginary contribution to
the euclidean action consisting in a quantized charge coupled to a phase, as
QCD, is illustrated with two simple examples; one of them intimately related to
Witten's result on SU(N) in the large limit.Comment: 9 pages, 2 figures, 2 references added, final version to appear in
Progr. Theor. Phy
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