arXiv subjects

C. Bernard

Publications and source records attributed to C. Bernard.

At least 73 records · Page 4Linked to original sources

arXiv2012

Staggered Chiral Perturbation Theory for Neutral B Mixing

I describe a calculation of B meson mixing at one-loop in staggered chiral perturbation theory, for the complete set of Standard Model and beyond-the-Standard Model operators. The particular lattice representation of the continuum operators used by the Fermilab Lattice/MILC collaborations (and earlier by the HPQCD collaboration) turns out to be important, and results in the presence of "wrong-spin" operators, whose contributions however vanish in the continuum limit. The relation between staggered and naive fermions also plays a key role.

hep-lat↗

arXiv2012

Pseudoscalar meson physics with four dynamical quarks

We present preliminary results for light, strange and charmed pseudoscalar meson physics from simulations using four flavors of dynamical quarks with the highly improved staggered quark (HISQ) action. These simulations include lattice spacings ranging from 0.15 to 0.06 fm, and sea-quark masses both above and at their physical value. The major results are charm meson decay constants f_D, f_{D_s} and f_{D_s}/f_D and ratios of quark masses. This talk will focus on our procedures for finding the decay constants on each ensemble, the continuum extrapolation, and estimates of systematic error.

hep-lat↗

arXiv2012

Refining new-physics searches in B -> D tau nu decay with lattice QCD

The semileptonic decay channel B -> D tau nu is sensitive to the presence of a scalar current, such as that mediated by a charged-Higgs boson. Recently the BaBar experiment reported the first observation of the exclusive semileptonic decay B -> D tau nu, finding an approximately 2-sigma disagreement with the Standard-Model prediction for the ratio R(D)=BR(B->D tau nu)/BR(B->D l nu), where l=e,mu. We compute this ratio of branching fractions using hadronic form factors computed in unquenched lattice QCD and obtain R(D) = 0.316(12)(7), where the errors are statistical and total systematic, respectively. This result is the first Standard-Model calculation of R(D) from ab initio full QCD. Its error is smaller than that of previous estimates, primarily due to the reduced uncertainty in the scalar form factor f_0(q^2). Our determination of R(D) is approximately 1-sigma higher than previous estimates and, thus, reduces the tension with experiment. We also compute R(D) in models with electrically charged scalar exchange, such as the type II two-Higgs doublet model. Once again, our result is consistent with, but approximately 1-sigma higher than, previous estimates for phenomenologically relevant values of the scalar coupling in the type II model. As a byproduct of our calculation, we also present the Standard-Model prediction for the longitudinal polarization ratio P_L (D)= 0.325(4)(3).

hep-ph↗

arXiv2012

B_s->D_s/B->D Semileptonic Form-Factor Ratios and Their Application to BR(B^0_s->μ^+μ^-)

We calculate form-factor ratios between the semileptonic decays \bar{B}->D^+\ell^-\barν and \bar{B}_s->D_s^+\ell^-\barν with lattice QCD. These ratios are a key theoretical input in a new strategy to determine the fragmentation fractions of the neutral B decays, which are needed for measurements of BR(B^0_s-> μ^+μ^-). We use the MILC ensembles of gauge configurations with 2+1 flavors of sea quarks at two lattice spacings of approximately 0.12 fm and 0.09 fm. We use the model-independent z-parametrization to extrapolate our simulation results at small recoil toward maximum recoil. Our results for the form-factor ratios are $f_0^{(s)}(M^2_π)/f_0^{(d)}(M^2_K) =1.046(44)_{stat.}(15)_{syst.}$ and $f_0^{(s)}(M^2_π)/f_0^{(d)}(M^2_π)=1.054(47)_{stat.}(17)_{syst.}$. In contrast to a QCD sum-rule calculation, no significant departure from U-spin (d<->s) symmetry is observed.

hep-lat↗

arXiv2012

Neutral B-meson mixing from three-flavor lattice QCD: Determination of the SU(3)-breaking ratio ξ

We study SU(3)-breaking effects in the neutral B_d-\bar B_d and B_s-\bar B_s systems with unquenched N_f=2+1 lattice QCD. We calculate the relevant matrix elements on the MILC collaboration's gauge configurations with asqtad-improved staggered sea quarks. For the valence light-quarks (u, d, and s) we use the asqtad action, while for b quarks we use the Fermilab action. We obtain ξ=f_{B_s}\sqrt{B_{B_s}}/f_{B_d}\sqrt{B_{B_d}}=1.268+-0.063. We also present results for the ratio of bag parameters B_{B_s}/B_{B_d} and the ratio of CKM matrix elements |V_{td}|/|V_{ts}|. Although we focus on the calculation of ξ, the strategy and techniques described here will be employed in future extended studies of the B mixing parameters ΔM_{d,s} and ΔΓ_{d,s} in the Standard Model and beyond.

hep-lat↗

arXiv2012

Neutral $B$ mixing from 2+1 flavor lattice-QCD: the Standard Model and beyond

We report on the status of our lattice-QCD calculation of the hadronic contribution to $B_d^0$ and $B^0_s$ mixing, with 2+1 flavors of dynamical sea quarks. Preliminary results for hadronic mixing matrix elements are given for a basis of five four-quark, dimension-six, $ΔB=2$ mixing operators that spans the space of all possible hadronic mixing contributions in the Standard Model and beyond. At the intermediate stage of analysis reported on in this work, our errors are competitive with published Standard Model matrix element results. For beyond the Standard Model matrix elements, this is the first unquenched calculation and the first new lattice-QCD calculation in ten years.

hep-lat↗

arXiv2011

B and D meson decay constants from 2+1 flavor improved staggered simulations

We give an update on simulation results for the decay constants f_B, f_{B_s}, f_D and f_{D_s}. These decay constants are important for precision tests of the standard model, in particular entering as inputs to the global CKM unitarity triangle fit. The results presented here make use of the MILC (2+1)-flavor asqtad ensembles, with heavy quarks incorporated using the clover action with the Fermilab method. Partially quenched, staggered chiral perturbation theory is used to extract the decay constants at the physical point. In addition, we give error projections for a new analysis in progress, based on an extended data set.

hep-lat↗

arXiv2011

B- and D-meson decay constants from three-flavor lattice QCD

We calculate the leptonic decay constants of B_{(s)} and D_{(s)} mesons in lattice QCD using staggered light quarks and Fermilab bottom and charm quarks. We compute the heavy-light meson correlation functions on the MILC asqtad-improved staggered gauge configurations which include the effects of three light dynamical sea quarks. We simulate with several values of the light valence- and sea-quark masses (down to ~m_s/10) and at three lattice spacings (a ~ 0.15, 0.12, and 0.09 fm) and extrapolate to the physical up and down quark masses and the continuum using expressions derived in heavy-light meson staggered chiral perturbation theory. We renormalize the heavy-light axial current using a mostly nonperturbative method such that only a small correction to unity must be computed in lattice perturbation theory and higher-order terms are expected to be small. We obtain f_{B^+} = 196.9(8.9) MeV, f_{B_s} = 242.0(9.5) MeV, f_{D^+} = 218.9(11.3) MeV, f_{D_s} = 260.1(10.8) MeV, and the SU(3) flavor-breaking ratios f_{B_s}/f_{B} = 1.229(26) and f_{D_s}/f_{D} = 1.188(25), where the numbers in parentheses are the total statistical and systematic uncertainties added in quadrature.

hep-lat↗

arXiv2011

Properties of light pseudoscalars from lattice QCD with HISQ ensembles

We fit lattice-QCD data for light-pseudoscalar masses and decay constants, from HISQ configurations generated by MILC, to SU(3) staggered chiral perturbation theory. At present such fits have rather high values of chi^2/d.o.f., possibly due to the lack of ensembles with lighter-than-physical sea strange-quark masses. We propose solutions to this problem for future work. We also perform simple linear interpolations near the physical point on two ensembles with different lattice spacings, and obtain the preliminary result (f_K / f_pi)^phys = 1.1872(41) in the continuum limit.

hep-lat↗

arXiv2011

B Mixing in the Standard Model and Beyond: Lattice QCD

We give a brief overview and progress report on our lattice QCD calculation of neutral B mixing hadronic matrix elements needed for Standard Model and Beyond the Standard Model physics. Reference [1] contains more details and results.

hep-lat↗

arXiv2011

Tuning Fermilab Heavy Quarks in 2+1 Flavor Lattice QCD with Application to Hyperfine Splittings

We report the non-perturbative tuning of parameters--- kappa_c, kappa_b, and kappa_crit ---that determine the heavy-quark mass in the Fermilab action. This requires the computation of the masses of Ds^(*) and Bs^(*) mesons comprised of a Fermilab heavy quark and a staggered light quark. Additionally, we report the hyperfine splittings for Ds and Bs mesons as a cross-check of our simulation and analysis methods. We find a splitting of 145 +/- 15 MeV for the Ds system and 40 +/- 9 MeV for the Bs system. These are in good agreement with the Particle Data Group average values of 143.9 +/- 0.4 MeV and 46.1 +/- 1.5 MeV, respectively. The calculations are carried out with the MILC 2+1 flavor gauge configurations at three lattice spacings $a$ approximately 0.15, 0.12, and 0.09 fm.

hep-lat↗

arXiv2010

Simulations with dynamical HISQ quarks

We report on the status of a program of generating and using configurations with four flavors of dynamical quarks, using the HISQ action. We study the lattice spacing dependence of physical quantities in these simulations, using runs at several lattice spacings, but with the light quark mass held fixed at two tenths of the strange quark mass. We find that the lattice artifacts in the HISQ simulations are much smaller than those in the asqtad simulations at the same lattice spacings and quark masses. We also discuss methods for setting the scale, or assigning a lattice spacing to ensembles run at unphysical parameters.

hep-lat↗

arXiv2010

Results for light pseudoscalar mesons

We present the current status of the MILC collaboration's calculations of the properties of the light pseudoscalar meson sector. We use asqtad staggered ensembles with 2+1 dynamical flavors down to $a \approx 0.045$ fm and light quark mass down to 0.05 $m_s$. Here we describe fits to the data using chiral forms from SU(3) chiral perturbation theory, including all staggered taste violations at NLO and the continuum NNLO chiral logarithms. We emphasize issues of convergence of the chiral expansion.

hep-lat↗

arXiv2010

Electromagnetic Corrections in Staggered Chiral Perturbation Theory

To reduce errors in light-quark mass determinations, it is now necessary to consider electromagnetic contributions to light-meson masses. Calculations using staggered quarks and quenched photons are currently underway. Suitably-extended chiral perturbation theory is necessary to extrapolate the lattice data to the physical limit. Here we give (preliminary) results for light-meson masses using staggered chiral perturbation theory including electromagnetism, and discuss the extent to which quenched-photon simulations can improve quark-mass calculations.

hep-lat↗

arXiv2010

Semileptonic decays of K and D mesons in 2+1 flavor QCD

The experimentally measured rates of the semileptonic decays K -> pi l nu and D -> K(pi) l nu can be combined with lattice calculations of the associated form factors to precisely extract the CKM matrix elements |V_{us}| and |V_{cs(d)}|. We report on the status of form factor calculations with Fermilab charm quarks and staggered light quarks on the 2+1 flavor asqtad staggered MILC ensembles. Analysis of data for the D -> pi l nu form factor provides a nontrivial test of our methods via comparison with CLEO data. We discuss the use of HISQ valence quarks to calculate the K -> pi l nu form factor f_+^{K pi}(0) and describe tests of our method.

hep-lat↗

arXiv2010

B -> D* l nu at zero recoil: an update

We present an update of our calculation of the form factor for B -> D* l nu at zero recoil, with higher statistics and finer lattices. As before, we use the Fermilab action for b and c quarks, the asqtad staggered action for light valence quarks, and the MILC ensembles for gluons and light quarks (Lüscher-Weisz married to 2+1 rooted staggered sea quarks). In this update, we have reduced the total uncertainty on F(1) from 2.6% to 1.7%. At Lattice2010 we presented a still-blinded result, but this writeup includes the unblinded result from the September 2010 CKM workshop.

hep-lat↗

arXiv2010

Staggered chiral perturbation theory in the two-flavor case and SU(2) analysis of the MILC data

In the light pseudoscalar sector, we study rooted staggered chiral perturbation theory in the two-flavor case. The pion mass and decay constant are calculated through NLO for a partially-quenched theory. In the limit where the strange quark mass is large compared to the light quark masses and the taste splittings, we show that the SU(2) staggered chiral theory emerges from the SU(3) staggered chiral theory, as expected. Explicit relations between SU(2) and SU(3) low energy constants and taste-violating parameters are given. A brief summary of updated SU(2) chiral fits to the MILC lattice data is presented.

hep-lat↗

arXiv2010

Scaling studies of QCD with the dynamical HISQ action

We study the lattice spacing dependence, or scaling, of physical quantities using the highly improved staggered quark (HISQ) action introduced by the HPQCD/UKQCD collaboration, comparing our results to similar simulations with the asqtad fermion action. Results are based on calculations with lattice spacings approximately 0.15, 0.12 and 0.09 fm, using four flavors of dynamical HISQ quarks. The strange and charm quark masses are near their physical values, and the light-quark mass is set to 0.2 times the strange-quark mass. We look at the lattice spacing dependence of hadron masses, pseudoscalar meson decay constants, and the topological susceptibility. In addition to the commonly used determination of the lattice spacing through the static quark potential, we examine a determination proposed by the HPQCD collaboration that uses the decay constant of a fictitious "unmixed s bar s" pseudoscalar meson. We find that the lattice artifacts in the HISQ simulations are much smaller than those in the asqtad simulations at the same lattice spacings and quark masses.

hep-lat↗