Papers by Vicente Azcoiti
Proceedings of The XXVIII International Symposium on Lattice Field Theory — PoS(Lattice 2010), Jun 6, 2011
In order to elucidate the vacuum structure of the Aoki phase, we carried out a numerical investig... more In order to elucidate the vacuum structure of the Aoki phase, we carried out a numerical investigation of QCD with two flavours of Wilson fermions, within the p.d.f. framework and in the absence of external sources. The simulations performed at V = 4 4 suggest a rich vacuum structure, where the observable iψγ 5 ψ is allowed to take non-zero values of the same order of magnitude than the order parameter of the Aoki phase iψγ 5 τ 3 ψ. However, the simulation at higher volumes V = 6 4 suffers from large statistical errors.
arXiv (Cornell University), Oct 6, 2009
The vacuum structure of QCD with two degenerated flavours of Wilson fermions is discussed by mean... more The vacuum structure of QCD with two degenerated flavours of Wilson fermions is discussed by means of the p.d.f. (probability distribution function) formalism. Under certain assumptions, either new phases related to Aoki's appear, characterized by a non-vanishing expectation value of the condensate iψ u γ 5 ψ u + iψ d γ 5 ψ d , enriching the standard picture of the QCD vacuum structure with Wilson fermions; or the eigenvalues of the Dirac-Wilson operator must comply with a nontrivial, infinite set of sum rules, enforcing the currently accepted structure of the Aoki vacuum. The same scenario is analyzed using the Ginsparg-Wilson regularization. In this case, the absence of any parity and/or flavour breaking phase is proved for a non-zero value of the fermion mass.
Proceedings of the 34th annual International Symposium on Lattice Field Theory (LATTICE2016). 24-30 July 2016. University of Southampton, 2016
Physical Review D, 1990
We propose a lattice regularization of quantum electrodynamics whose dynamical variables are comp... more We propose a lattice regularization of quantum electrodynamics whose dynamical variables are compact gauge fields (i.e., belonging to the algebra of U(1) but defined on a compact manifold), which is free from (lattice) topological excitations and nonconfining. Exact gauge invariance is lost on the lattice; nevertheless, the model preserves Hermiticity and unitarity for all values of the coupling constant. Gauge invariance is recovered in the weak-coupling limit.
We review a method for numerical simulations of lattice gauge theories at non-zero baryonic chemi... more We review a method for numerical simulations of lattice gauge theories at non-zero baryonic chemical potential we recently proposed. We first report on a test of the method using a solvable model and then present results for the phase structure of four flavour QCD. For the first time the region of chemical potential up to 1.4 T_C is explored, finding a first order transition line.
We simulate the compact U ( 1)-Higgs model in a four-dimensional l ttice. We present a numerical ... more We simulate the compact U ( 1)-Higgs model in a four-dimensional l ttice. We present a numerical study for the behaviour around the region where the transition between the confining and Higgs phases disappears. The transition line is found to be first order and to end in a second-order point. We measure the critical exponents ofthe endpoint obtaining the mean field ones within errors. The study of the critical behaviour of lattice gauge theories in four dimensions is crucial to formulate nonperturbatively the underlying continuum quan-tum field theory. Even if a continuum limit is possi-ble, the question remains whether the obtained the-ory is trivial or not. For the selfcoupled scalar field with a quartic term in the action (the 204 theory) no other critical point in d = 4 has been found than an infrared stable gaus-sian fixed point at the origin 2 = 0, m = 0. This means that the only fixed point with infinite correlation
Symmetry, 2021
Prior to the establishment of QCD as the correct theory describing hadronic physics, it was reali... more Prior to the establishment of QCD as the correct theory describing hadronic physics, it was realized that the essential ingredients of the hadronic world at low energies are chiral symmetry and its spontaneous breaking. Spontaneous symmetry breaking is a non-perturbative phenomenon, and, thanks to massive QCD simulations on the lattice, we have at present a good understanding of the vacuum realization of the non-abelian chiral symmetry as a function of the physical temperature. As far as the UA(1) anomaly is concerned, and especially in the high temperature phase, the current situation is however far from satisfactory. The first part of this article is devoted to reviewing the present status of lattice calculations, in the high temperature phase of QCD, of quantities directly related to the UA(1) axial anomaly. In the second part, some recently suggested interesting physical implications of the UA(1) anomaly in systems where the non-abelian axial symmetry is fulfilled in the vacuum ...
Nuclear Physics B - Proceedings Supplements, 1994
Nuclear Physics B - Proceedings Supplements, 1993
We simulate the U(1)-Higgs lattice model with fixed Higgs modulus, in the region where the transi... more We simulate the U(1)-Higgs lattice model with fixed Higgs modulus, in the region where the transition between the Higgs and Confining phases disappears. We find a first order transition line that ends in a second order point. Thermodynamic magnitudes and their associated critical exponents are introduced. The ineasured data allow us to fit the critical exponents finding values in agreement, with the mean field prediction. The location of the critical point and the slope of the first order line are accurately given.
Physics Letters B, 1993
We study SU (2) lattice gauge theory in small volumes and with twist m = (1, 1, 1). We investigat... more We study SU (2) lattice gauge theory in small volumes and with twist m = (1, 1, 1). We investigate the presence of the periodic instantons of Q = 1 2 and determine their free energy and their contribution to the splitting of energy flux sectors E(e = (1, 1, 1)) − E(e = (0, 0, 0)).
Physics Letters B, 1998
We report results of simulations of strong coupling, finite density QCD obtained within a MFA ins... more 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.
Physical Review D, 1982
ABSTRACT A new scenario is suggested for the discussion of the old problem of generations. We sha... more ABSTRACT A new scenario is suggested for the discussion of the old problem of generations. We shall assume that the rest mass m of a particle described by the Lagrangian L(x) = -m(1-x/sup 2/)/sup 1/2/ has its origin in the momentum p/sub theta/, canonical conjugate of a supplementary dimension of space, beyond the usual four dimensions. More precisely, we shall contemplate the possibility that the different generations, or for definiteness, the charged leptons e,..mu..,tau,. . . are states, with different p/sub theta/, of a unique physical system, whose free Lagrangian will depend on theta-dot in addition to x: L(x,theta-dot). The requirement that the relativistic relationship between the momentum p/sub x/ and velocity x is maintained in five dimensions leads to the simplest Lagrangian L(x,theta-dot) = -..lambda..(1-x/sup 2/) (-theta-dot / (1-x/sup 2/)/sup 1/2/), ..lambda.. being a constant with dimension (theta)/sup -1/. With this Lagrangian, the function of p/sub theta/ that plays the role of the mass in the usual relativistic relation between p/sub x/ and x is m(p/sub theta/) = p/sub theta/(1-..beta..lnp/sub theta//..lambda..). The quantization of momentum p/sub theta/ with periodic conditions leads to a mass spectrum compatible with experimental data only if the number of generations is three. In the present work we consider that the mass differences within each generation should be explained in the context of grand unified theories (GUT's). In the last section, though, the complementary information that GUT's might supply in our context is suggested, and a value for the mass of the top quark and a bound for the mass of the tau neutrino are obtained.
Nuclear Physics B - Proceedings Supplements, 1994
Nuclear Physics B, 1993
We study numerically the critical properties of the U(1)-Higgs lattice model, with fixed Higgs mo... more We study numerically the critical properties of the U(1)-Higgs lattice model, with fixed Higgs modulus, in the region of small gauge coupling where the Higgs and Confining phases merge. We find evidence of a first order transition line that ends in a second order point. By means of a rotation in parameter space we introduce thermodynamic magnitudes and critical exponents in close resemblance with simple models that show analogous critical behaviour. The measured data allow us to fit the critical exponents finding values in agreement with the mean field prediction. The location of the critical point and the slope of the first order line are accurately given.
Nuclear Physics B, 2014
In this work we study the two and three-dimensional antiferromagnetic Ising model with an imagina... more In this work we study the two and three-dimensional antiferromagnetic Ising model with an imaginary magnetic field iθ at θ = π. In order to perform numerical simulations of the system we introduce a new geometric algorithm not affected by the sign problem. Our results for the 2D model are in agreement with the analytical solution. We also present new results for the 3D model which are qualitatively in agreement with mean-field predictions.
Physics Letters B, 1995
We report the results of a measurement of susceptibilities in noncompact QED 4 in 8 4 , 10 4 and ... more 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 MF A 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.
Physics Letters B, 1995
We analyse the phase diagram of the lattice gauged Nambu-Jona Lasinio model with the help of a me... more 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.
Physics Letters B, 1995
We analyse the phase diagram of the lattice gauged Nambu-Jona Lasinio model with the help of a me... more 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.
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Papers by Vicente Azcoiti