875 research outputs found
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
QCD with a theta-vacuum term: a complex system with a simple complex action
We reanalyze in the first part of this paper the old question of P and CT
realization in QCD. The second part is devoted to establish general results on
the phase structure of this model in the presence of a -vacuum term.Comment: 15 pages, to be published in the proceedings of the ``International
Workshop on Non-Perturbative Methods and Lattice QCD'', Guangzhou, China,
15-21 May 200
Critical Behavior of CP^1 at theta = pi, Haldane's Conjecture and the Universality Class
Using an approach to analyze the theta dependence of systems with a
theta-term we recently proposed, the critical behavior of CP^1 at theta=pi is
studied. We find a region outside the strong coupling regime where Haldane's
conjecture is verified. The critical line however does not belong to the
universality class of the Wess-Zumino-Novikov-Witten model at topological
coupling k=1 since it shows continuously varying critical exponents.Comment: 4 pages, 5 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
Diquark condensation in two colour QCD
Unquenched lattice SU(2) is studied at nonzero chemical potential in the
strong coupling limit. The topic of diquark condensation is addressed analyzing
the probability distribution function of the diquark condensate. We present
results at zero external source without using any potentially dangerous
extrapolation procedure. We find strong evidences for a (high density) second
order phase transition where a diquark condensate appears, and show
quantitative agreement of lattice calculations with low-energy effective
Lagrangian calculations.Comment: 9 pages, 6 figures, to be published in the proceedings of the
``International Workshop on Non-Perturbative Methods and Lattice QCD'',
Guangzhou, China, 15-21 May 200
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