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R. Petronzio

Publications and source records attributed to R. Petronzio.

54 records · Page 3Linked to original sources
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

Moments of parton evolution probabilities on the lattice within the Schroedinger functional scheme

We define, within the Schroedinger functional scheme (SF), the matrix elements of the twist-2 operators corresponding to the first two moments of non-singlet parton densities. We perform a lattice one-loop calculation that fixes the relation between the SF scheme and other common schemes and shows the main source of lattice artefacts. This calculation sets the basis for a numerical evaluation of the non-perturbative running of parton densities.

hep-lat↗
arXiv1998

Comparison of Improved and Unimproved Quenched Hadron Spectroscopy

We make a comparison between our quenched-hadron-spectroscopy results for the non-perturbatively-improved Wilson action and the corresponding unimproved case, at beta=6.2 on the same set of gauge configurations. Within our statistics, we find a sizeable improvement for the baryon spectrum and for the determination of the strange-quark mass.

hep-lat↗
arXiv1997

Non-Perturbatively Improved Quenched Hadron Spectroscopy

We make a quenched lattice simulation of hadron spectroscopy at beta=6.2 with the Wilson action non-perturbatively improved. With respect to the unimproved case, the estimate of the lattice spacing is less influenced by the choice of input hadron masses. We study also the effects of using an improved quark mass in the fits to the dependence of hadron masses upon quark masses.

hep-lat↗
arXiv1997

Non-perturbative renormalization constants on the lattice from flavour non-singlet Ward identities

By imposing axial and vector Ward identities for flavour-non-singlet currents, we estimate in the quenched approximation the non-perturbative values of combinations of improvement coefficients, which appear in the expansion around the massless case of the renormalization constants of axial, pseudoscalar, vector, scalar non-singlet currents and of the renormalized mass. These coefficients are relevant for the completion of the improvement programme to O(a) of such operators. The simulations are performed with a clover Wilson action non-perturbatively improved.

hep-lat↗
arXiv1997

Flavour changing top decays in supersymmetric extensions of the standard model

Flavour changing top decays $t \to c Z^0, t \to c g$ and $t \to c γ$ are predicted with invisible rates within the standard model and may represent a window on new physics. We consider these processes in supersymmetric extensions of the standard model and we show that observable rates can be obtained only if the SUSY breaking is non universal and flavour dependent.

hep-ph↗
arXiv1996

QCD at finite temperature and partially negative flavour numbers

We study dynamical fermion effects in lattice QCD at finite temperature. The method adopted is basically the extrapolation from negative flavour numbers already tested at zero temperature and based on the simulation of local bosonic theories, with an essential difference. With an appropriate choice of the boundary conditions on the bosonic fields, called ``bermions'', it is possible to separate the $Z_3$ breaking contribution of fermion loops to the effective action from the one conserving $Z_3$: the former is simulated exactly at a fixed positive and even flavour number, while the extrapolation from negative flavour numbers is made only on the $Z_3$ invariant part of the action. We test this approach by comparing our results on a $16^3\times 2$ lattice with those from a hopping parameter expansion and our results on a $16^3\times 4$ lattice with those of direct Monte Carlo simulations including the fermion determinant.

hep-lat↗
arXiv1996

Evidence for eta prime - pion splitting in unquenched lattice QCD

We perform an extrapolation from negative to positive flavour numbers of full QCD lattice estimates of the $η'$ mass. The extrapolations are carried out by keeping $ρ$ and $π$ masses at fixed values. We find an $η'$ -- $π$ splitting which shows a flavour dependence consistent with the Witten Veneziano formula based on the $U(1)$ anomaly. The quantitative splitting is consistent with the estimates made in the quenched approximation.

hep-lat↗
arXiv1996

Pseudofermion observables for static heavy meson decay constants on the lattice

A method based on the Monte Carlo inversion of the Dirac operator on the lattice provides low noise results for the correlations entering the definition of the heavy meson decay constant in the static limit. The method is complementary to the usual method of smeared sources, avoids the systematic error arising from optimizing the size of the smearing volume and is more efficient for the values of lattice parameters that we have explored.

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↗
arXiv1994

Quark-Resonance model

We construct an effective Lagrangian for low energy hadronic interactions through an infinite expansion in inverse powers of the low energy cutoff $Λ_χ$ of all possible chiral invariant non-renormalizable interactions between quarks and mesons degrees of freedom. We restrict our analysis to the leading terms in the $1/N_c$ expansion. The effective expansion is in $(μ^2/\cutoff^2 )^P \ln (\cutoff^2/μ^2 )^Q$. Concerning the next-to-leading order, we show that, while the pure $μ^2/\cutoff^2 $ corrections cannot be traced back to a finite number of non renormalizable interactions, those of order $(μ^2/\cutoff^2 ) \ln (\cutoff^2/μ^2 )$ receive contributions from a finite set of $1/\cutoff^2$ terms. Their presence modifies the behaviour of observable quantities in the intermediate $Q^2$ region. We explicitely discuss their relevance for the two point vector currents Green's function.

hep-ph↗
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↗