arXiv subjects

R. Petronzio

Publications and source records attributed to R. Petronzio.

At least 19 recordsLinked to original sources

arXiv2013

Tau/Charm Factory Accelerator Report

The present Report concerns the current status of the Italian Tau/Charm accelerator project and in particular discusses the issues related to the lattice design, to the accelerators systems and to the associated conventional facilities. The project aims at realizing a variable energy Flavor Factory between 1 and 4.6 GeV in the center of mass, and succeeds to the SuperB project from which it inherits most of the solutions proposed in this document. The work comes from a cooperation involving the INFN Frascati National Laboratories accelerator experts, the young newcomers, mostly engineers, of the Cabibbo Lab consortium and key collaborators from external laboratories.

physics.acc-ph↗

arXiv2013

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 which can be computed on the lattice. It is based on the expansion of the path-integral in powers of the small parameters (m_d - m_u)/Lambda_{QCD} and alpha_{em}, where m_f is the renormalized quark mass and alpha_{em} 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, M_{pi+}^2-M_{pi0}^2= 1.44(13)(16) x 10^3 MeV^2, for the Dashen's theorem breaking parameter epsilon_{gamma}= 0.79(18)(18), for the light quark masses, [m_d - m_u](MSbar,2 GeV)= 2.39(8)(17) MeV, [m_u / m_d](MSbar,2 GeV)= 0.50(2)(3) and for the flavour 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.

hep-lat↗

arXiv2013

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.

hep-lat↗

arXiv2012

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

We present a new method to evaluate with high precision 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.

hep-lat↗

arXiv2012

Lattice QCD calculation of strong isospin breaking effects

We present a new method to evaluate with high precision the isospin breaking effects due to the 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 $m_d - m_u$. In this talk we discuss how to apply this method to compute the leading isospin breaking effects for several physical quantities of interest: the kaon masses, the kaon decay constants and the neutron-proton mass splitting.

hep-lat↗

arXiv2010

Distance preconditioning for lattice Dirac operators

We propose a preconditioning of the Dirac operator based on the factorisation of a predefined function related to the decay of the propagator with the distance. We show that it can improve the accuracy of correlators involving heavy quarks at large distances and accelerate the computation of light quark propagators.

hep-lat↗

arXiv2008

Anatomy and Phenomenology of the Lepton Flavor Universality in SUSY Theories

High precision electroweak tests, such as deviations from the Standard Model expectations of the Lepton Flavor Universality breaking in K->lnu (with l= e or mu), represent a powerful tool to test the Standard Model and, hence, to constrain or obtain indirect hints of New Physics beyond it. We explore such a possibility within Supersymmetric theories. Interestingly enough, a process that in itself does not need lepton flavor violation to occur, i.e. the violation of mu-e non-universality in K_l2, proves to be quite effective in constraining not only relevant regions of SUSY models where lepton flavor is conserved, but even those where specific lepton flavor violating contributions arise. Indeed, a comparison with analogous bounds coming from tau lepton flavor violating decays shows the relevance of the measurement of R_K to probe Lepton Flavor Violation in SUSY. We outline the role and the interplay of the direct New Physics searches at the LHC with the indirect searches performed by LFU tests.

hep-ph↗

arXiv2008

Quenched lattice calculation of the vector channel B --> D* l nu decay rate

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 nu. 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(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 Vcb by the experimental decay rate without model dependent extrapolations. The value of Vcb agrees with the one obtained from the B --> D l nu channel and makes us confident that the quenched approximation well applies to these transitions.

hep-lat↗

arXiv2007

Quenched lattice calculation of the B --> D l nu decay rate

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 nu. 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(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 Vcb by the experimental decay rate without model dependent extrapolations.

hep-lat↗

arXiv2007

Quenched lattice calculation of semileptonic heavy-light meson form factors

We calculate, in the continuum limit of quenched lattice QCD, the matrix elements of the heavy-heavy vector current between heavy-light pseudoscalar meson states. We present the form factors for different values of the initial and final meson masses at finite momentum transfer. In particular, we calculate the non-perturbative correction to the differential decay rate of the process B --> D l nu including the case of a non-vanishing lepton mass.

hep-lat↗

arXiv2007

QCD with light Wilson quarks on fine lattices (II): DD-HMC simulations and data analysis

In this second report on our recent numerical simulations of two-flavour QCD, we provide further technical details on the simulations and describe the methods we used to extract the meson masses and decay constants from the generated ensembles of gauge fields. Among the topics covered are the choice of the DD-HMC parameters, the issue of stability, autocorrelations and the statistical error analysis. Extensive data tables are included as well as a short discussion of the quark-mass dependence in partially quenched QCD, supplementing the physics analysis that was presented in the first paper in this series.

hep-lat↗

arXiv2006

QCD with light Wilson quarks on fine lattices (I): first experiences and physics results

Recent conceptual, algorithmic and technical advances allow numerical simulations of lattice QCD with Wilson quarks to be performed at significantly smaller quark masses than was possible before. Here we report on simulations of two-flavour QCD at sea-quark masses from slightly above to approximately 1/4 of the strange-quark mass, on lattices with up to 64x32^3 points and spacings from 0.05 to 0.08 fm. Physical sea-quark effects are clearly seen on these lattices, while the lattice effects appear to be quite small, even without O(a) improvement. A striking result is that the dependence of the pion mass on the sea-quark mass is accurately described by leading-order chiral perturbation theory up to meson masses of about 500 MeV.

hep-lat↗

arXiv2006

Probing New Physics through mu-e Universality in K->lnu

The recent NA48/2 improvement on R_K=Gamma(K->e nu_e)/Gamma(K->mu nu_mu) emphasizes the role of K_l2 decays in probing the mu-e universality. Supersymmetric (SUSY) extensions of the Standard Model can exhibit mu-e non-universal contributions. Their origin is twofold: those deriving from lepton flavor conserving couplings are subdominant with respect to those arising from lepton flavor violating (LFV) sources. We show that mu-e non-universality in K_l2 is quite effective in constraining relevant regions of SUSY models with LFV (for instance, supergravities with a see-saw mechanism for neutrino masses). A comparison with analogous bounds coming from tau LFV decays proves the relevance of the measurement of R_K to probe LFV in SUSY.

hep-ph↗

arXiv2005

Stability of lattice QCD simulations and the thermodynamic limit

We study the spectral gap of the Wilson--Dirac operator in two-flavour lattice QCD as a function of the lattice spacing $a$, the space-time volume $V$ and the current-quark mass $m$. It turns out that the median of the probability distribution of the gap scales proportionally to $m$ and that its width is practically equal to $a/\sqrt{V}$. In particular, numerical simulations are safe from accidental zero modes in the large-volume regime of QCD.

hep-lat↗

arXiv2005

Status of the APENet project

We present the current status of APENet, our custom 3-dimensional interconnect architecture for PC clusters environment. We report some micro-benchmarks on our recent large installation as well as new developments on the software and hardware side. The low level device driver has been reworked by following a custom hardware RDMA architecture, and MPICH-VMI, an implementation of the MPI library, has been ported to APENet.

hep-lat↗

arXiv2005

Non-perturbative Pion Matrix Element of a twist-2 operator from the Lattice

We give a continuum limit value of the lowest moment of a twist-2 operator in pion states from non-perturbative lattice calculations. We find that the non-perturbatively obtained renormalization group invariant matrix element is _{RGI} = 0.179(11), which corresponds to ^{MSbar}(2 GeV) = 0.246(15). In obtaining the renormalization group invariant matrix element, we have controlled important systematic errors that appear in typical lattice simulations, such as non-perturbative renormalization, finite size effects and effects of a non-vanishing lattice spacing. The crucial limitation of our calculation is the use of the quenched approximation. Another question that remains not fully clarified is the chiral extrapolation of the numerical data.

hep-lat↗

arXiv2004

APENet: LQCD clusters a la APE

Developed by the APE group, APENet is a new high speed, low latency, 3-dimensional interconnect architecture optimized for PC clusters running LQCD-like numerical applications. The hardware implementation is based on a single PCI-X 133MHz network interface card hosting six indipendent bi-directional channels with a peak bandwidth of 676 MB/s each direction. We discuss preliminary benchmark results showing exciting performances similar or better than those found in high-end commercial network systems.

hep-lat↗

arXiv2004

Finite size effects of a pion matrix element

We investigate finite size effects of the pion matrix element of the non-singlet, twist-2 operator corresponding to the average momentum of non-singlet quark densities. Using the quenched approximation, they come out to be surprisingly large when compared to the finite size effects of the pion mass. As a consequence, simulations of corresponding nucleon matrix elements could be affected by finite size effects even stronger which could lead to serious systematic uncertainties in their evaluation.

hep-lat↗