arXiv subjects

A. F. Grillo

Publications and source records attributed to A. F. Grillo.

At least 37 records · Page 2Linked to original sources

arXiv2002

Space Time Fluctuations and Ultra High Energy Cosmic Ray Interactions

The intimate geometry of space-time is expected to suffer stochastic fluctuations as a result of quantum gravitational effects. These fluctuations may induce observable consequences on the propagation of high energy particles over large distances, so that the strength and the characteristics of these fluctuations may be constrained, mainly in the range of energies of interest for cosmic ray physics. While invoked as a possible explanation for the detection of the puzzling cosmic rays with energies in excess of the threshold for photopion production (the so-called super-GZK particles), we demonstrate here that lower energy observations may provide strong constraints on the role of a fluctuating space-time structure.

astro-ph↗

arXiv2000

Probability Distribution Function of the Diquark Condensate in Two Colours QCD

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 condensation at 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 phase transition, where a diquark condensate appears, as well as quantitative agreement between lattice results and low-energy effective Lagrangian calculations.

hep-lat↗

arXiv2000

Fermion Condensates in Two Colours Finite Density QCD at Strong Coupling

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.

hep-lat↗

arXiv2000

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.

hep-lat↗

arXiv2000

Probing The Structure of Space-Time with Cosmic Rays

The study of the interactions of Cosmic Rays (CR's) with universal diffuse background radiation can provide very stringent tests of the validity of Special Relativity. The interactions we consider are the ones characterized by well defined energy thresholds whose energy position can be predicted on the basis of Special Relativity. We argue that the experimental confirmation of the existence of these thresholds can in principle put very stringent limits on the scale where Special Relativity and/or continuity of space-time may possibly break down.

astro-ph↗

arXiv1999

Three and Two Colours Finite Density QCD at Strong Coupling: A New Look

Simulations in finite density, beta=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.

hep-lat↗

arXiv1999

Strongly Coupled QCD at Finite Baryon Density

The analytical results obtained in the infinite mass and strong coupling limits of QCD are difficult to reconcile with the predictions of the Monomer Dimer Polymer algorithm. We have reconsidered in detail the results obtained with this simulation scheme and evidences of severe convergence problems are presented for the SU(3) and SU(2) gauge group.

hep-lat↗

arXiv1999

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 $β$ and for a transition line that, starting from the temperature critical point at $μ=0$, moves towards smaller $β$ with increasing $μ$ as expected from simplified phenomenological arguments.

hep-lat↗

arXiv1998

Rigorous arguments against current wisdoms in finite density QCD

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.

hep-lat↗

arXiv1998

Finite density QCD with heavy quarks

In the large fermion mass limit of QCD at finite density the structure of the partition function greatly simplifies and can be studied analytically. We show that, contrary to general wisdom, the phase of the Dirac determinant is relevant only at finite temperature and can be neglected for zero temperature fields.

hep-lat↗

arXiv1998

Phase transition(s) in finite density QCD

The Grand Canonical formalism is generally used in numerical simulations of finite density QCD since it allows free mobility in the chemical potential $μ$. We show that special care has to be used in extracting numerical results to avoid dramatic rounding effects and spurious transition signals. If we analyze data correctly, with reasonable statistics, no signal of first order phase transition is present and results using the Glasgow prescription are practically coincident with the ones obtained using the modulus of the fermionic determinant.

hep-lat↗

arXiv1998

Frustration in Finite Density QCD

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.

hep-lat↗

arXiv1998

First Order Phase Transition in Finite Density QCD using the modulus of the Dirac Determinant

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 of a 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.

hep-lat↗

arXiv1997

Finite Density QCD in the Chiral Limit

We present the first results of an exact simulation of full QCD at finite density in the chiral limit. We have used a MFA (Microcanonical Fermionic Average) inspired approach for the reconstruction of the Grand Canonical Partition Function of the theory; using the fugacity expansion of the fermionic determinant we are able to move continuously in the ($β-μ$) plane with $m=0$.

hep-lat↗

arXiv1996

Chiral Susceptibilities in noncompact QED: a new determination of the $γ$ exponent and the critical couplings

We report the results of a measurement of susceptibilities in noncompact $QED_4$ in $8^4, 10^4$ and $12^4$ lattices. Due to the potentialities of the $MFA$ approach, we have done simulations in the chiral limit which are therefore free from arbitrary mass extrapolations. Our results in the Coulomb phase show unambiguously that the susceptibility critical exponent $γ=1$ independently of the flavour symmetry group. The critical couplings extracted from these calculations are in perfect agreement with previous determinations based on the fermion effective action and plaquette energy, and outside the predictions of a logarithmically improved scalar mean field theory by eight standard deviations.

hep-lat↗

arXiv1996

The gauged Nambu - Jona Lasinio model: a mean field calculation with non mean field exponents

We analyse the phase diagram of the lattice gauged Nambu-Jona Lasinio model with the help of a mean field approximation plus numerical simulations. We find a phase transition line in the coupling parameters space separating the chirally broken phase from the symmetric phase, which is in good qualitative agreement with results obtained in the quenched-ladder approximation. The mean field approximation relates the critical exponents along the continuous phase transition line with the mass dependence of the chiral condensate in the Coulomb phase of standard noncompact $QED$. Our numerical results for noncompact $QED$ strongly suggest non mean field exponents along the critical line.

hep-lat↗

arXiv1996

Testing logarithmic violations to scaling in strongly coupled QED

Using very precise measurements of the critical couplings for the chiral transition of non compact $QED_4$ with up to 8 flavours, we analyse the behaviour of the order parameter at the critical point using the equation of state of a logarithmically improved scalar mean field theory, that of the Nambu-Jona Lasinio theory and a pure power law. The first case is definitively excluded by the numerical data. The stability of the fits for the last two cases, as well as the behaviour with the number of flavours of the exponent of the logarithmic violations to the scaling favour clearly a pure power law scaling with non mean field exponents.

hep-lat↗

arXiv1996

The Continuum Limit of Non Compact QED

Since four-fermion operators in strongly coupled $QED$ are nonperturbatively renormalizable, we analyze here the phase diagram and critical behaviour of the Gauged Nambu-Jona Lasinio model. Our mean field approximation relates the critical exponents along the continuous phase transition line with the mass dependence of the chiral condensate in the Coulomb phase of standard noncompact $QED$. The numerical results for noncompact $QED$ strongly suggest non mean field exponents along the critical line.

hep-lat↗