946 research outputs found
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
Parity realization in Vector-like theories from Fermion Bilinears
We reconsider in this paper the old aim of trying to understand if the
observed realization of discrete symmetries as Parity or CP in the QCD vacuum
can be satisfied from first principles. We show how under the appropriate
assumptions implicitely done by Vafa and Witten in their old paper on parity
realization in vector-like theories, all parity and CP odd operators
constructed from fermion bilinears of the form should
take a vanishing vacuum expectation value in a vector-like theory with N
degenerate flavours (N>1). In our analysis the Vafa-Witten theorem on the
impossibility to break spontaneously the flavour symmetry in a vector-like
theory plays a fundamental role.Comment: 12 pages, no figures To be published in JHE
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