R. Aloisio

Probability distribution function of the diquark condensate in two colours QCD

Abstract We consider diquark condensation in finite density lattice SU(2). We first present an extension of Vafa–Witten result, on spontaneous breaking of vector-like global symmetries, that allows us to formulate a no-go theorem for diquark condensation in a region of the chemical potential–mass parameter space. We then describe a new technique to calculate diquark condensate in unquenched lattice simulations with any number of flavours directly at zero external source, without using any potentially dangerous extrapolation procedure. We apply it to the strong coupling limit and find compelling evidences for a second order transition to a phase with nonzero diquark condensate. Our results are in quantitative agreement with low-energy effective Lagrangian calculations.

Fermion condensates in two colours finite density QCD at strong coupling

Abstract We study unquenched lattice SU (2) at nonzero chemical potential at strong coupling and with eight flavours of Kogut–Susskind fermions. Introducing a diquark source term we analyze the behaviour of different types of fermion condensates. Using a non standard approach we can obtain results at zero external source without extrapolations. We find strong evidences for a (high density) second order phase transition where a diquark condensate appears. The corresponding critical chemical potential is in good agreement with half the pion mass.

Three and two colours finite density QCD at strong coupling: A new look

Abstract Simulations in finite density, β=0 lattice QCD by means of the Monomer–Dimer–Polymer algorithm show a signal of first-order transition at finite temporal size. This behaviour agrees with predictions of the mean field approximation, but is difficult to reconcile with infinite mass analytical solution. The MDP simulations are considered in detail and severe convergence problems are found for the SU(3) gauge group, in a wide region of chemical potential. Simulations of SU(2) model show discrepancies with MDP results as well.

A Note on DSR-like approach to space-time

Abstract In this Letter we discuss the possibility to define a space–time with a DSR based approach. We show that the strategy of defining a non-linear realization of the Lorentz symmetry with a consistent vector composition law cannot be reconciled with the extra request of an invariant length (time) scale. The latter request forces to abandon the group structure of the translations and leaves a space–time structure where points with relative distances smaller or equal to the invariant scale cannot be unambiguously defined.

Rigorous arguments against current wisdoms in finite density QCD

Abstract QCD at finite chemical potential is analytically investigated in the region of large bare fermion masses. We show that, contrary to the general wisdom, the phase of the fermion determinant is irrelevant at zero temperature. However if the system is put at finite temperature, the contribution of the phase is finite. We also discuss on the quenched approximation and suggest that the origin of the failure of this approximation in finite density QCD could relay on the fundamental role that Pauli exclusion principle plays in this case.

First order phase transition in finite density QCD using the modulus of the Dirac determinant

Abstract We report results of simulations of strong coupling, finite density QCD obtained within a MFA inspired approach where the fermion determinant in the integration measure is replaced by its absolute value. Contrary to the standard wisdom, we show that within this approach a clear signal for a first order phase transition appears with a critical chemical potential in extremely good agreement with the results obtained with the Glasgow algorithm. The modulus of the fermion determinant seems therefore to preserve some of the relevant physical properties of the system. We also analyze the dependence of our results on the quark mass, including both the chiral and large mass limit, and the theory in the quenched approximation.

FRUSTRATION IN FINITE DENSITY QCD

Abstract We present a detailed analysis of the QCD partition function in the Grand Canonical formalism. Using the fugacity expansion we find evidence for numerical instabilities in the standard evaluation of its coefficients. We discuss the origin of this problem and propose an issue to it. The correct analysis shows no evidence for a discontinuity in the baryonic density in the strong coupling limit. The moderate optimism that was inspired by the Grand Canonical Partition Function calculations in the last years has to be considered ill-founded.

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

Finite density QCD in the chiral limit

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DIQUARK CONDENSATION IN TWO COLOUR QCD

and for a transition line that, starting from the temperature critical point at