arXiv subjects
Publications and source records attributed to Leonardo Giusti.
We present preliminary results for a high statistics study of the topological charge distribution in the SU(3) Yang-Mills theory obtained by using the definition of the charge suggested by Neuberger fermions. We find statistical evidence for deviations from a gaussian distribution. The large statistics required has been obtained by using PCs of the INFN-GRID.
We report on a precise computation of the topological charge distribution in the SU(3) Yang--Mills theory. It is carried out on the lattice with high statistics Monte Carlo simulations by employing the definition of the topological charge suggested by Neuberger's fermions. We observe significant deviations from a Gaussian distribution. Our results disfavour the theta behaviour of the vacuum energy predicted by instanton models, while they are compatible with the expectation from the large Nc expansion.
Algorithmic and technical progress achieved over the last few years makes QCD simulations with light dynamical quarks much faster than before. As a result lattices with pions as light as 250--300 MeV can be simulated with the present generation of computers. I review recent conceptual and numerical progress in this field, with particular emphasis on results obtained and difficulties encountered in simulations with significantly smaller quark masses with respect to previous computations. I also attempt to compare physical results for pion masses and decay constants available to date in the two-flavour theory with expectations from chiral perturbation theory.
Spontaneous chiral symmetry breaking in QCD with massless quarks at infinite volume can be seen in a finite box by studying, for instance, the dependence of the chiral condensate from the volume and the quark mass. We perform a feasibility study of this program by computing the quark condensate on the lattice in the quenched approximation of QCD at small quark masses. We carry out simulations in various topological sectors of the theory at several volumes, quark masses and lattice spacings by employing fermions with an exact chiral symmetry, and we focus on observables which are infrared stable and free from mass-dependent ultraviolet divergences. The numerical calculation is carried out with an exact variance-reduction technique, which is designed to be particularly efficient when spontaneous symmetry breaking is at work in generating a few very small low-lying eigenvalues of the Dirac operator. The finite-size scaling behaviour of the condensate in the topological sectors considered agrees, within our statistical accuracy, with the expectations of the chiral effective theory. Close to the chiral limit we observe a detailed agreement with the first Leutwyler-Smilga sum rule. By comparing the mass, the volume and the topology dependence of our results with the predictions of the chiral effective theory, we extract the corresponding low-energy constant.
The low-mode averaging technique is a powerful tool for reducing large fluctuations in correlation functions due to low-mode eigenvalues of the Dirac operator. In this work we propose a generalization to baryons and test our method on two-point correlation functions of left-handed nucleons, computed with quenched Neuberger fermions on a lattice with extension L=1.5 fm. We show that the statistical fluctuations can be reduced and the baryon signal significantly improved.
We compute the topological susceptibility for the SU(3) Yang--Mills theory by employing the expression of the topological charge density operator suggested by Neuberger's fermions. In the continuum limit we find r_0^4 chi = 0.059(3), which corresponds to chi=(191 +/- 5 MeV)^4 if F_K is used to set the scale. Our result supports the Witten--Veneziano explanation for the large mass of the eta'.
We present the results of a computation of the topological susceptibility in the SU(3) Yang--Mills theory performed by employing the expression of the topological charge density operator suggested by Neuberger's fermions. In the continuum limit we find r_0^4 chi = 0.059(3), which corresponds to chi=(191 \pm 5 MeV)^4 if F_K is used to set the scale. Our result supports the Witten--Veneziano explanation for the large mass of the eta'.
I review recent lattice computations of the matrix element relevant for K0-K0bar mixing and discuss the advantages of fermions with an exact chiral symmetry to compute K->pipi amplitudes.
In the epsilon-regime of QCD the main features of the spectrum of the low-lying eigenvalues of the (euclidean) Dirac operator are expected to be described by a certain universality class of random matrix models. In particular, the latter predict the joint statistical distribution of the individual eigenvalues in any topological sector of the theory. We compare some of these predictions with high-precision numerical data obtained from lattice QCD for a range of lattice spacings and volumes. While no complete matching is observed, the results agree with theoretical expectations at volumes larger than about 5 fm^4.
We present a calculation of the standard model ΔS=2 matrix element relevant to indirect CP violation in K->ππdecays which uses Neuberger's chiral formulation of lattice fermions. The computation is performed in the quenched approximation on a 16^3x32 lattice that has a lattice spacing a\sim 0.1 fm. The resulting bare matrix element is renormalized non-perturbatively. Our main result is B_K^{RGI}=0.87(8)^{+2+14}_{-1-14}, where the first error is statistical, the second is systematic and the third is an estimate of the uncertainty associated with the quenched approximation and with the fact that our kaons are composed of degenerate s and d quarks with masses \sim m_s/2.
We propose new techniques for the numerical implementation of the overlap-Dirac operator, which exploit the physical properties of the underlying theory to avoid nested algorithms. We test these procedures in the two-dimensional Schwinger model and the results are very promising. These techniques can be directly applied to QCD simulations. We also present a detailed computation of the spectrum and the chiral properties of the Schwinger model in the overlap lattice formulation.
We show how a specific sequential breaking pattern of a U(2) flavour symmetry occurs automatically in a broad framework. The relative orientation in U(2) space of the spurion fields that breaks the U(2) symmetry is uniquely fixed, thus determining the form of the fermion mass matrices in a predictive way.
We show that the two scenarios able to explain the HERA anomaly --- a new leptoquark coupling or a new contact interaction --- predict new contributions to atomic parity violation. These corrections are sufficiently large and different that a feasible reduction in the dominant atomic theory uncertainty could give some hint in favour of one of the two scenarios.