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M. Guagnelli

Publications and source records attributed to M. Guagnelli.

36 records · Page 2Linked to original sources
arXiv2001

K0-K0bar mixing from the Schroedinger functional and twisted mass QCD

We describe how the Schroedinger functional and twisted mass QCD can be used to compute the kaon B-parameter B_K on the lattice with Wilson fermions. This new approach is expected to reduce the systematic uncertainties on B_K through a better control of its (nonperturbative) renormalisation. Preliminary results for the bare matrix element in a physical volume of (1.5^3 x 4.5) fm^4 are presented. The renormalisation of the matrix element in a Schroedinger functional scheme is also discussed.

hep-lat↗
arXiv2000

Lattice hadron matrix elements with the Schroedinger functional: the case of the first moment of non-singlet quark density

We present the results of a non-perturbative determination of the pion matrix element of the twist-2 operator corresponding to the average momentum of non-singlet quark densities. The calculation is made within the Schroedinger functional scheme. We report the results of simulations done with the standard Wilson action and with the non-perturbatively improved clover action and we show that their ratio correctly extrapolates, in the continuum limit, to a value compatible with the residual correction factor expected from perturbation theory.

hep-lat↗
arXiv1999

Renormalization group invariant average momentum of non-singlet parton densities

We compute, within the Schrödinger functional scheme, a renormalization group invariant renormalization constant for the first moment of the non-singlet parton distribution function. The matching of the results of our non-perturbative calculation with the ones from hadronic matrix elements allows us to obtain eventually a renormalization group invariant average momentum of non-singlet parton densities, which can be translated into a preferred scheme at a specific scale.

hep-lat↗
arXiv1999

Universal continuum limit of non-perturbative lattice non-singlet moment evolution

We present evidence for the universality of the continuum limit of the scale dependence of the renormalization constant associated with the operator corresponding to the average momentum of non-singlet parton densities. The evidence is provided by a non-perturbative computation in quenched lattice QCD using the Schrödinger Functional scheme. In particular, we show that the continuum limit is independent of the form of the fermion action used, i.e. the Wilson action and the non-perturbatively improved clover action.

hep-lat↗
arXiv1998

Non-perturbative running of the average momentum of non-singlet parton densities

We determine non-perturbatively the anomalous dimensions of the second moment of non-singlet parton densities from a continuum extrapolation of results computed in quenched lattice simulations at different lattice spacings. We use a Schrödinger functional scheme for the definition of the renormalization constant of the relevant twist-2 operator. In the region of renormalized couplings explored, we obtain a good description of our data in terms of a three-loop expression for the anomalous dimensions. The calculation can be used for exploring values of the coupling where a perturbative expansion of the anomalous dimensions is not valid a priori. Moreover, our results provide the non-perturbative renormalization constant that connects hadron matrix elements on the lattice, renormalized at a low scale, with the experimental results, renormalized at much higher energy scales.

hep-lat↗
arXiv1998

Precision computation of a low-energy reference scale in quenched lattice QCD

We present results for the reference scale r_0 in SU(3) Lattice Gauge Theory for beta = 6/g_0^2 in the range 5.7 <= beta <= 6.57. The high relative accuracy of 0.3-0.6% in r_0/a was achieved through good statistics, the application of a multi-hit procedure and a variational approach in the computation of Wilson loops. A precise definition of the force used to extract r_0 has been used throughout the calculation which guarantees that r_0/a is a smooth function of the bare coupling and that subsequent continuum extrapolations are possible. The results are applied to the continuum extrapolations of the energy gap Delta in the static quark potential and the scale L_max/r_0 used in the calculation of the running coupling constant.

hep-lat↗
arXiv1997

Towards N=1 Super-Yang-Mills on the Lattice

We consider the lattice regularization of N=1 supersymmetric Yang--Mills theory with Wilson fermions. This formulation breaks supersymmetry at any finite lattice spacing; we discuss how Ward identities can be used to define a supersymmetric continuum limit, which coincides with the point where the gluino becomes massless. As a first step towards the understanding of the zero gluino-mass limit, we present results on the quenched low-lying spectrum of SU(2) N=1 Super-Yang--Mills, at $β=2.6$ on a $V=16^3 \times 32$ lattice, in the OZI approximation. Our results, in spite of the quenched and OZI approximations, are in remarkable agreement with theoretical predictions in the supersymmetric theory, for the states with masses which are not expected to get a large contribution from fermion loops.

hep-lat↗
arXiv1997

Non-perturbative quark mass renormalization

We show that the renormalization factor relating the renormalization group invariant quark masses to the bare quark masses computed in lattice QCD can be determined non-perturbatively. The calculation is based on an extension of a finite-size technique previously employed to compute the running coupling in quenched QCD. As a by-product we obtain the $Λ$--parameter in this theory with completely controlled errors.

hep-lat↗
arXiv1997

Non-perturbative O(a) improvement of the vector current

We discuss non-perturbative improvement of the vector current, using the Schroedinger Functional formalism. By considering a suitable Ward identity, we compute the improvement coefficient which gives the O(a) mixing of the tensor current with the vector one.

hep-lat↗
arXiv1997

Quenched Spectroscopy for the N=1 Super-Yang-Mills Theory

We present results for the Quenched SU(2) N=1 Super-Yang-Mills spectrum at $β=2.6$, on a $V=16^3 \times 32$ lattice, in the OZI approximation. This is a first step towards the understanding of the chiral limit of lattice N=1 SUSY.

hep-lat↗
arXiv1996

Hybrid Molecular Dynamics for Lattice Supersymmetry

We present the first results obtained with a Hybrid Molecular Dynamics algorithm applied to an $N=1$ SU(2) Super-Yang--Mills on the lattice. We derive the Hamilton equations of motion for the system with Wilson gluinos and present preliminary results on small lattices.

hep-lat↗
arXiv1995

Light meson decay constants beyond the quenched approximation

We calculate the effects of including dynamical fermion loops in the lattice QCD estimates of meson decay constants, by extrapolating the results from negative flavour numbers after a suitable matching of the pion and rho mass. For moderately light quarks, the values of the decay constants not corrected for the renormalization constants increase with respect to their quenched values.

hep-lat↗
arXiv1995

Smeared propagators for lattice hadron spectroscopy

We propose to replace ordinary propagators in lattice operator correlations entering the determination of hadron masses with space-time smeared propagators. These are defined as the inverse of the quadratic operator in the fermion action times its hermitian conjugate. We obtain a cleaner determination of hadron masses, comparable to the effect of using smeared operators.

hep-lat↗
arXiv1994

Universality and the approach to the continuum limit in lattice gauge theory

The universality of the continuum limit and the applicability of renormalized perturbation theory are tested in the SU(2) lattice gauge theory by computing two different non-perturbatively defined running couplings over a large range of energies. The lattice data (which were generated on the powerful APE computers at Rome II and DESY) are extrapolated to the continuum limit by simulating sequences of lattices with decreasing spacings. Our results confirm the expected universality at all energies to a precision of a few percent. We find, however, that perturbation theory must be used with care when matching different renormalized couplings at high energies.

hep-lat↗
arXiv1994

Non perturbative determination of the running coupling constant in quenched SU(2)

Through a finite size renormalization group technique we calculate the running coupling constant for quenched SU(2) with a few percent error over a range of energy varying by a factor thirty. The definition is based on ratio of correlations of Polyakov loops with twisted boundary conditions. The extrapolation to the continuum limit is governed by corrections due to lattice artifacts which are proportional to the square of the lattice spacing and appears rather smooth.

hep-lat↗
arXiv1993

A definition of the running coupling constant in a twisted SU(2) lattice gauge theory

We propose a definition of the running coupling constant in a SU(2) lattice gauge theory with twisted boundary conditions. It is based on the correlation of Polyakov loops extended in a twisted direction at a distance which is a fixed fraction of the totale lattice size. We make the perturbative calculation which connects this definition to standard regularization schemes. We find Lambda_Twisted-Polyakov/Lambda_MSbar = 1.6136(2).

hep-lat↗
arXiv1993

Maximal Mean Field Solutions in the Random Field Ising Model: the Pattern of the Symmetry Breaking

In this note we study the mean field equations for the $3d$ Random Field Ising Model. We discuss the phase diagram of the model, and we address the problem of finding if such equations admit more than one solution. We find two different critical values of $β$: one where the magnetization takes a non-zero expectation value, and one where we start to have more than one solution to the mean field equation. We find that, inside a given solution, there are no divergent correlation lengths.

cond-mat↗