563 research outputs found

### Masses and decay constants of $D_{(s)}^*$ and $B_{(s)}^*$ mesons in Lattice QCD with $N_f = 2 + 1 + 1$ twisted-mass fermions

We present a lattice calculation of the decay constants and masses of
$D_{(s)}^*$ and $B_{(s)}^*$ mesons using the gauge configurations produced by
the European Twisted Mass Collaboration (ETMC) with $N_f = 2 + 1 + 1$ dynamical
quarks and at three values of the lattice spacing $a \sim 0.06 - 0.09$ fm. Pion
masses are simulated in the range $m_{\pi} \sim 210 - 450$ MeV, while the
strange and charm quark masses are close to their physical values. We computed
the ratios of vector to pseudoscalar decay constants or masses for various
values of the heavy-quark mass $m_h$ in the range $0.7 m_c^{phys} \lesssim m_h
\lesssim 3 m_c^{phys}$. In order to reach the physical b-quark mass, we
exploited the HQET prediction that, in the static limit of infinite heavy-quark
mass, all the considered ratios are equal to one. We obtain: $f_{D^*}/f_{D} =
1.078(36),$ $m_{D^*}/m_{D} = 1.0769(79)$, $f_{D^*_{s}}/f_{D_{s}} = 1.087(20)$,
$m_{D^*_{s}}m_{D_{s}} = 1.0751(56)$, $f_{B^*}/f_{B} = 0.958(22)$,
$m_{B^*}/m_{B} = 1.0078(15)$, $f_{B^*_{s}}/f_{B_{s}} = 0.974(10)$ and
$m_{B^*_{s}}/m_{B_{s}} = 1.0083(10)$. Combining them with the corresponding
experimental masses from the PDG and the pseudoscalar decay constants
calculated by ETMC, we get: $f_{D^*} = 223.5(8.4)~\mathrm{MeV}$, $m_{D^*} =
2013(14)~\mathrm{MeV}$, $f_{D^*_{s}} = 268.8(6.6)~\mathrm{MeV}$, $m_{D^*_{s}}
= 2116(11)~\mathrm{MeV}$, $f_{B^*} = 185.9(7.2)~\mathrm{MeV}$, $m_{B^*} =
5320.5(7.6)~\mathrm{MeV}$, $f_{B^*_{s}} = 223.1(5.4)~\mathrm{MeV}$ and
$m_{B^*_{s}}= 5411.36(5.3)~\mathrm{MeV}$.Comment: 7 pages, 4 figures, in proceedings of 34th annual International
Symposium on Lattice Field Theory, 24-30 July 2016, University of Southampton
(UK). In version v2 the quality of the figures is improve

### Chirally enhanced corrections to FCNC processes in the generic MSSM

Chirally enhanced supersymmetric QCD corrections to FCNC processes are
investigated in the framework of the MSSM with generic sources of flavor
violation. These corrections arise from flavor-changing self-energy diagrams
and can be absorbed into a finite renormalization of the squark-quark-gluino
vertex. In this way enhanced two-loop and even three-loop diagrams can be
efficiently included into a leading-order (LO) calculation. Our corrections
substantially change the values of the parameters delta^{d,LL}_{23},
delta^{d,LR}_{23}, delta^{d,RL}_{23}, and delta^{d,RR}_{23} extracted from
Br[B->X_s gamma] if tan(beta) is large. We find stronger (weaker) constraints
compared to the LO result for negative (positive) values of mu. The constraints
on delta^{d,LR,RL}_{13} and delta^{d,LR,RL}_{23} from B_d mixing and B_s mixing
change drastically if the third-generation squark masses differ from those of
the first two generations. K mixing is more strongly affected by the chirally
enhanced loop diagrams and even sub-percent deviations from degenerate down and
strange squark masses lead to profoundly stronger constraints on
delta^{d,LR,RL}_{12}.Comment: 19 pages, 10 figure

### Improved Renormalization of Lattice Operators: A Critical Reappraisal

We systematically examine various proposals which aim at increasing the
accuracy in the determination of the renormalization of two-fermion lattice
operators. We concentrate on three finite quantities which are particularly
suitable for our study: the renormalization constants of the vector and axial
currents and the ratio of the renormalization constants of the scalar and
pseudoscalar densities. We calculate these quantities in boosted perturbation
theory, with several running boosted couplings, at the "optimal" scale q*. We
find that the results of boosted perturbation theory are usually (but not
always) in better agreement with non-perturbative determinations of the
renormalization constants than those obtained with standard perturbation
theory. The finite renormalization constants of two-fermion lattice operators
are also obtained non-perturbatively, using Ward Identities, both with the
Wilson and the tree-level Clover improved actions, at fixed cutoff ($\beta$=6.4
and 6.0 respectively). In order to amplify finite cutoff effects, the quark
masses (in lattice units) are varied in a large interval 0<am<1. We find that
discretization effects are always large with the Wilson action, despite our
relatively small value of the lattice spacing ($a^{-1} \simeq 3.7$ GeV). With
the Clover action discretization errors are significantly reduced at small
quark mass, even though our lattice spacing is larger ($a^{-1} \simeq 2$ GeV).
However, these errors remain substantial in the heavy quark region. We have
implemented a proposal for reducing O(am) effects, which consists in matching
the lattice quantities to their continuum counterparts in the free theory. We
find that this approach still leaves appreciable, mass dependent,
discretization effects.Comment: 54 pages, Latex, 5 figures. Minor changes in text between eqs.(86)
and (88

### Phenomenology of the Standard Model from Lattice QCD

Some recent results of lattice QCD calculations which are relevant for the phenomenology of the Standard Model are reviewed. They concern the lattice determinations of quark masses, studies of K-Kbar and B-Bbar mixings, and a prediction of the B_s-mesons lifetime difference. The results of a recent analysis of the CKM unitarity triangle, which is mostly based on the lattice calculations of the relevant hadronic matrix elements, are also presented

### Quark Masses and Renormalization Constants from Quark Propagator and 3-point Functions

We have computed the light and strange quark masses and the renormalization
constants of the quark bilinear operators, by studying the large-p^2 behaviour
of the lattice quark propagator and 3-point functions. The calculation is
non-perturbatively improved, at O(a), in the chiral limit. The method used to
compute the quark masses has never been applied so far, and it does not require
an explicit determination of the quark mass renormalization constant.Comment: LATTICE99 (Improvement and Renormalization) - 3 pages, 2 figure

### Hypercubic effects in semileptonic decays of heavy mesons, toward $B \to \pi \ell \nu$, with $N_f=2+1+1$ Twisted fermions

We present a preliminary study toward a lattice determination of the vector
and scalar form factors of the $B \to \pi \ell \nu$ semileptonic decays. We
compute the form factors relative to the transition between heavy-light
pseudoscalar mesons, with masses above the physical D-mass, and the pion. We
simulate heavy-quark masses in the range $m_c^{phys} < m_h < 2m_c^{phys}$.
Lorentz symmetry breaking due to hypercubic effects is clearly observed in the
data, and included in the decomposition of the current matrix elements in terms
of additional form factors. We discuss the size of this breaking as the
parent-meson mass increases. Our analysis is based on the gauge configurations
produced by the European Twisted Mass Collaboration with $N_f = 2 + 1 + 1$
flavors of dynamical quarks at three different values of the lattice spacing
and with pion masses as small as $210$ MeV.Comment: 7 pages, 5 figures; contribution to the XXXVI International Symposium
on Lattice Field Theory (LATTICE2018), East Lansing (Michigan State
University, USA), July 22-28, 201

### First Lattice QCD Study of the Sigma -> n Axial and Vector Form Factors with SU(3) Breaking Corrections

We present the first quenched lattice QCD study of the form factors relevant
for the hyperon semileptonic decay Sigma -> n l nu. The momentum dependence of
both axial and vector form factors is investigated and the values of all the
form factors at zero-momentum transfer are presented. Following the same
strategy already applied to the decay K0 -> pi- l nu, the SU(3)-breaking
corrections to the vector form factor at zero-momentum transfer, f1(0), are
determined with great statistical accuracy in the regime of the simulated quark
masses, which correspond to pion masses above ~ 0.7 GeV. Besides f1(0) also the
axial to vector ratio g1(0) / f1(0), which is relevant for the extraction of
the CKM matrix element Vus, is determined with significant accuracy. Due to the
heavy masses involved, a polynomial extrapolation, which does not include the
effects of meson loops, is performed down to the physical quark masses,
obtaining f1(0) = -0.948 +/- 0.029 and g1(0) / f1(0) = -0.287 +/- 0.052, where
the uncertainties do not include the quenching effect. Adding a recent
next-to-leading order determination of chiral loops, calculated within the
Heavy Baryon Chiral Perturbation Theory in the approximation of neglecting the
decuplet contribution, we obtain f1(0) = -0.988 +/- 0.029(lattice) +/-
0.040(HBChPT). Our findings indicate that SU(3)-breaking corrections are
moderate on both f1(0) and g1(0). They also favor the experimental scenario in
which the weak electricity form factor, g2(0), is large and positive, and
correspondingly the value of |g1(0) / f1(0)| is reduced with respect to the one
obtained with the conventional assumption g2(q**2) = 0 based on exact SU(3)
symmetry.Comment: final version to appear in Nucl. Phys.

### QCDF90: Lattice QCD with Fortran 90

We have used Fortran 90 to implement lattice QCD. We have designed a set of
machine independent modules that define fields (gauge, fermions, scalars,
etc...) and overloaded operators for all possible operations between fields,
matrices and numbers. With these modules it is very simple to write high-level
efficient programs for QCD simulations. To increase performances our modules
also implements assignments that do not require temporaries, and a machine
independent precision definition. We have also created a useful compression
procedure for storing the lattice configurations, and a parallel implementation
of the random generators. We have widely tested our program and modules on
several parallel and single processor supercomputers obtaining excellent
performances.Comment: LaTeX file, 8 pages, no figures. More information available at:
http://hep.bu.edu/~leviar/qcdf90.htm

### The Taming of QCD by Fortran 90

We implement lattice QCD using the Fortran 90 language. We have designed
machine independent modules that define fields (gauge, fermions, scalars,
etc...) and have defined overloaded operators for all possible operations
between fields, matrices and numbers. With these modules it is very simple to
write QCD programs. We have also created a useful compression standard for
storing the lattice configurations, a parallel implementation of the random
generators, an assignment that does not require temporaries, and a machine
independent precision definition. We have tested our program on parallel and
single processor supercomputers obtaining excellent performances.Comment: Talk presented at LATTICE96 (algorithms) 3 pages, no figures, LATEX
file with ESPCRC2 style. More information available at:
http://hep.bu.edu/~leviar/qcdf90.htm

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