1,721,006 research outputs found
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
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
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
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
Testing new strategies in finite density
No full textA new approach for non zero chemical potential simulations is tested in the Gross-Neveu model for infinite
flavor number, where the critical line is reconstructed in a large μ/T interval. A comparison with results from
standard imaginary chemical potential approach as well as first results for Nf = 4 QCD are presented
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
New ideas in finite density QCD
No full textWe introduce a new approach to analyze the phase diagram of QCD at finite chemical potential and temperature,
based on the definition of a generalized QCD action. Several details of the method will be discussed, with particular
emphasis on the advantages respect to the imaginary chemical potential approach
Finite density QCD: a new approach
No full textWe introduce a new approach to analyze the phase diagram of QCD at finite
chemical potential and temperature, test it in the Gross-Neveu model at finite baryon density, and apply it to the study of the chemical potential-temperature phase diagram of QCD with four degenerate flavors of Kogut-Susskind type
theta dependence of the CP9 model
No full textWe apply to the CP^9 model two recently proposed numerical techniques for the 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 the % level. The results scale well with the
renormalization group equation and show that, for CP^9 in the presence of a theta-term, CP symmetry is spontaneously
broken at in the continuum limit
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