G.M. de Divitiis

On the discretization of physical momenta in lattice QCD

Abstract The adoption of two distinct boundary conditions for two fermions species on a finite lattice allows to deal with arbitrary relative momentum between the two particle species, in spite of the momentum quantization rule due to a limited physical box size. We test the physical significance of this topological momentum by checking in the continuum limit the validity of the expected energy–momentum dispersion relations.

Flavour changing top decays in supersymmetric extensions of the standard model

Abstract The flavour changing top decays t → cZ 0 , t → cg and t → cγ 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.

Leading isospin breaking effects on the lattice

We present a method to evaluate on the lattice the leading isospin breaking effects due to both the small mass difference between the up and down quarks and the QED interaction. Our proposal is applicable in principle to any QCD+QED gauge invariant hadronic observable that can be computed on the lattice. It is based on the expansion of the path integral in powers of the small parameters (md-mu)/ΛQCD and αem, where mf is the renormalized quark mass and αem the renormalized fine structure constant. In this paper we discuss in detail the general strategy of the method and the conventional, although arbitrary, separation of QCD from QED isospin breaking corrections. We obtain results for the pion mass splitting, Mπ+2-Mπ02=1.44(13)(16)×103  MeV2, for the Dashen’s theorem breaking parameter eγ=0.79(18)(18), for the light quark masses, [md-mu](MS ,2  GeV)=2.39(8)(17)  MeV, [mu/md](MS ,2  GeV)=0.50(2)(3), and for the flavor symmetry breaking parameters R and Q. We also update our previous results for the QCD isospin breaking corrections to the Kl2 decay rate and for the QCD contribution to the neutron-proton mass splitting. The numerical results of this paper have been obtained by using the gluon field configurations produced by the ETMC Collaboration with nf=2 dynamical quarks. We treated the dynamical quarks as electrically neutral particles (electroquenched approximation) and neglected a disconnected diagram in the charged and neutral pion mass splitting. We provide all the formulas necessary to remove these approximations and discuss in detail the estimate of the associated systematic uncertainties.

Isospin breaking effects due to the up-down mass difference in lattice QCD

A bstractWe present a new method to evaluate with high precision leading isospin breaking effects due to the small mass difference between the up and down quarks using lattice QCD. Our proposal is applicable in principle to any hadronic observable which can be computed on the lattice. It is based on the expansion of the path-integral in powers of the small parameter md − mu. In this paper, we apply this method to compute the leading isospin breaking effects for several physical quantities of interest: the kaon meson masses, the kaon decay constant, the form factors of semileptonic Kℓ3 decays and the neutron-proton mass splitting.

Quenched lattice calculation of the B → Dłv decay rate

Abstract We calculate, in the continuum limit of quenched lattice QCD, the form factor that enters in the decay rate of the semileptonic decay B → D l ν . Making use of the step scaling method (SSM), previously introduced to handle two scale problems in lattice QCD, and of flavour twisted boundary conditions we extract G B → D ( w ) at finite momentum transfer and at the physical values of the heavy quark masses. Our results can be used in order to extract the CKM matrix element V c b by the experimental decay rate without model dependent extrapolations.

On the extraction of zero momentum form factors on the lattice

We propose a method to expand correlation functions with respect to the spatial components of external momenta. From the coefficients of the expansion it is possible to extract Lorentz-invariant form factors at zero spatial momentum transfer avoiding model dependent extrapolations. These objects can be profitably calculated on the lattice. We have explicitly checked the validity of the proposed procedure by considering two-point correlators with insertions of the axial current, the form factors of the semileptonic decay of pseudoscalar mesons, and the hadronic vacuum polarization tensor entering, for example, the lattice calculation of the anomalous magnetic moment of the muon.

Quenched lattice calculation of the vector channel B→D⋆ℓν decay rate

Abstract We calculate, in the continuum limit of quenched lattice QCD, the form factor that enters the decay rate of the semileptonic decay B → D ⋆ l ν . By using the step scaling method (SSM), previously introduced to handle two scale problems in lattice QCD, and by adopting flavor twisted boundary conditions we extract F B → D ⋆ ( w ) at finite momentum transfer ( w ⩾ 1 ) and at the physical values of the heavy quark masses. Our results can be used in order to extract the CKM matrix element V c b by the experimental decay rate without model dependent extrapolations. The value of V c b agrees with the one obtained from the B → D l ν channel and makes us confident that the quenched approximation well applies to these transitions.

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

Abstract 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 appear to be rather smooth and proportional to the square of the lattice spacing.

The bermions: an approach to lattice QCD dynamical fermions from negative flavour numbers

Abstract We estimate the effects of dynamical fermions by extrapolating to positive flavour numbers the results from negative values obtained by adding to the pure gauge sector a fermion action where the fields obey a Bose statistics: the bermions.

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

We propose a novel definition of the running coupling constant in a quenched SU(2) lattice gauge theory with twisted boundary conditions suitable for non-perturbative numerical simulations. It is based on the correlation of Polyakov loops extended in a twisted direction at a distance which is a fixed fraction of the total lattice size. We make the perturbative calculation which connects this definition to standard regularization schemes. We find Λtwisted Polyakov/ΛMS=1.6136(2).