90 research outputs found
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
Theta dependence of CP^9 model
We apply to the model two recently proposed numerical techniques for
simulation of systems with a theta term. The algorithms, successfully tested in
the strong coupling limit, are applied to the weak coupling region. The results
agree and errors have been evaluated and are at % level. The results scale well
with the renormalization group equation and show that, for in presence
of a theta term, CP symmetry is spontaneously broken at in the
continuum limit.Comment: 4 pages, 4 figure
A Geometric Monte Carlo Algorithm for the Antiferromagnetic Ising model with "Topological" Term at
In this work we study the two and three-dimensional antiferromagnetic Ising
model with an imaginary magnetic field at . In order to
perform numerical simulations of the system we introduce a new geometric
algorithm not affected by the sign problem. Our results for the model are
in agreement with the analytical solutions. We also present new results for the
model which are qualitatively in agreement with mean-field predictions
Seeking the Equation of State of Non-Compact Lattice QED
We perform a high statistics calculation of the equation of state for
non-compact QED on large lattices. The calculation extends to fermionic
correlation lengths of , and it is combined with a finite size
scaling analysis of the lattice data.Comment: 32 pages, uuencoded Z-compressed postscript file. Also available from
http://www.desy.de/pub/preprints/desy/1996
Spontaneous Parity Violation
We disprove the Vafa-Witten theorem on the impossibility of spontaneously
breaking parity in vector-like gauge field theories, identifying a mechanism
driven by quantum fluctuations. With the introduction of a meromorphic Lattice
formulation, defined over 5 dimensions, we demonstrate that the minima of the
free energy can be distinct from the maxima of the partition function :
identifying and evaluating a suitable contour for the partition function
defined such that asymptotic behaviour of the complex action is
non-oscillatory.Comment: 6 page
Finite Density Fat QCD
Lattice formulation of Finite Baryon Density QCD is problematic from computer
simulation point of view; it is well known that for light quark masses the
reconstructed partition function fails to be positive in a wide region of
parameter space. For large bare quark masses, instead, it is possible to obtain
more sensible results; problems are still present but restricted to a small
region. We present evidence for a saturation transition independent from the
gauge coupling and for a transition line that, starting from the
temperature critical point at , moves towards smaller with
increasing as expected from simplified phenomenological arguments.Comment: 14 pages, 10 figure
Phase structure of a generalized Nambu Jona-Lasinio model with Wilson fermions in the mean field or large -expansion
We analyze the vacuum structure of a generalized lattice Nambu--Jona-Lasinio
model with two flavors of Wilson fermions, such that its continuum action is
the most general four-fermion action with 'trivial' color interactions, and
having a symmetry in the chiral limit. The phase structure
of this model in the space of the two four-fermion couplings shows, in addition
to the standard Aoki phases, new phases with , in
close analogy to similar results recently suggested by some of us for lattice
QCD with two degenerate Wilson fermions. This result shows how the phase
structure of an effective model for low energy QCD cannot be entirely
understood from Wilson Chiral Perturbation Theory, based on the standard QCD
chiral effective Lagrangian approach.Comment: 24 pages, 3 figures, sent to Nuclear Physics
Efficient Cluster Algorithm for CP(N-1) Models
Despite several attempts, no efficient cluster algorithm has been constructed
for CP(N-1) models in the standard Wilson formulation of lattice field theory.
In fact, there is a no-go theorem that prevents the construction of an
efficient Wolff-type embedding algorithm. In this paper, we construct an
efficient cluster algorithm for ferromagnetic SU(N)-symmetric quantum spin
systems. Such systems provide a regularization for CP(N-1) models in the
framework of D-theory. We present detailed studies of the autocorrelations and
find a dynamical critical exponent that is consistent with z = 0.Comment: 14 pages, 3 figure
Non-compact Lattice QED with Two Charges: Phase Diagram and Renormalization Group Flow
The phase diagram of non-compact lattice QED in four dimensions with
staggered fermions of charges 1 and is investigated. The renormalized
charges are determined and found to be in agreement with perturbation theory.
This is an indication that there is no continuum limit with non-vanishing
renormalized gauge coupling, and that the theory has a validity bound for every
finite value of the renormalized coupling. The renormalization group flow of
the charges is investigated and an estimate for the validity bound as a
function of the cut-off is obtained. Generalizing this estimate to all fermions
in the Standard Model,it is found that a cut-off at the Planck scale implies
that has to be less than . Due to spontaneous chiral symmetry
breaking, strongly bound fermion-antifermion composite states are generated.
Their spectrum is discussed.Comment: 35 pages, LATEX, 26 PostScript figures (uuencoded
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