1,721,016 research outputs found
Baryonic thermal fluctuations in finite temperature QCD
No full textWe show how, contrary to physical intuition, thermal fluctuations of physical states having a non-vanishing baryonic
number can be fully neglected in the thermodynamics of QCD at any physical temperature. We also discuss on the
consistency between our results and the CPT theorem. The last part of this article is devoted to comment some interesting
physical features which follow from this result
No-go theorem on spontaneous parity breaking revisited
No full textAn 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 the Vafa and Witten's conjecture is plausible, actually a theorem is still lacking
Parity and CT realization in QCD
No full textWe show that an essential assumption in the Vafa-Witten theorem on P and CT realization in
vectorlike theories, the existence of a free energy density in Euclidean space in the presence of any
external Hermitian symmetry breaking source, does not apply if the symmetry is spontaneously broken.
The assumption that the free energy density is well defined 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
theta-vacuum - Phase transitions and/or symmetry breaking at theta = pi
No full textAssuming 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
Critical behavior of CP1 at theta=pi, Haldane's conjecture, and the relevant universality class
No full textUsing an approach to analyze the dependence of systems with a term we recently proposed, the
critical behavior of CP^1 at 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
Strong coupling analysis of diquark condensation
No full textThe phenomenon of diquark condensation at non-zero baryon density and zero temperature is analyzed in the
strong coupling limit of lattice QCD. The results indicate that there is attraction in the quark-quark channel also
at strong coupling, and that the attraction is more effective at high baryon density, but for infinite coupling it
is not enough to produce diquark condensation. It is argued that the absence of diquark condensation is not a
peculiarity of the strong coupling limit, but persists at sufficiently large finite couplings
MICROCANONICAL FERMIONIC AVERAGE METHOD IN THE SCHWINGER MODEL - A REALISTIC COMPUTATION OF THE CHIRAL CONDENSATE
No full textThe 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 two-dimensional QED, i.e., the Schwinger model. We exploit the possibility,
intrinsic to this method, of studying the whole plane without extra computer cost, to follow
constant physics trajectories and measure the limit of the chiral condensate. We recover the
continuum result within three decimal places. Moreover, the possibility, intrinsic to the method, of
performing simulations directly in the chiral limit allows us to compute the average plaquette energy
at m = 0, the result being in perfect agreement with the expected value
Diquark condensation at strong coupling
No full textThe possibility of diquark condensation at sufficiently large baryon chemical
potential and zero temperature is analyzed in QCD at strong coupling. In agreement with
other strong coupling analysis, it is found that a first order phase transition separates a
low density phase with chiral symmetry spontaneously broken from a high density phase
where chiral symmetry is restored. In none of the phases diquark condensation takes place
as an equilibrium state, but, for any value of the chemical potential, there is a metastable
state characterized by a non-vanishing diquark condensate. The energy difference between
this metastable state and the equilibrium state decreases with the chemical potential and
is minimum in the high density phase. The results indicate that there is attraction in the
quark-quark sector also at strong coupling, and that the attraction is more effective at high
baryon density, but for in finite coupling it is not enough to produce diquark condensation.
It is argued that the absence of diquark condensation is not a peculiarity of the strong
coupling limit, but persists at su±ciently large finite couplings
THE MICROCANONICAL FERMIONIC AVERAGE METHOD FOR ASYMPTOTICALLY FREE THEORIES - A TEST IN THE SCHWINGER MODEL
No full textWe have applied the Microcanonical Fermionic Average method to QED_2, i.e. the Schwinger Model, to test its applicability to Asymptotically Free Theories. We present here the results of the simulations, compared to the continuum results. Since the M.F.A. method allows the study of the whole plane at very small computer cost, we are able to verify the scaling of the chiral condensate at high degree, and obtain the continuum result within 3 decimal places. We present also results for the plaquette energy
theta-vacuum systems via real action simulations
No full textInspired by the results of the Ising model within an imaginary external magnetic field, we introduce a transformation in
quantum systems with a θ-vacuum term that amounts to a rescaling of z = cos θ/2. Making use of this transformation we are
able to determine the order parameter as a function of θ. The approach is successfully tested in models with both broken and unbroken CP symmetry at θ = π
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