Junko Shigemitsu

High-precision determination of the pi, K, D, and D-s decay constants from lattice QCD

We determine D and D(s) decay constants from lattice QCD with 2% errors, 4 times better than experiment and previous theory: f(D(s))=241(3) MeV, f(D)=207(4) MeV, and fD(s))/f(D)=1.164(11). We also obtain f(K)/f(pi)=1.189(7) and (f(D(s))/f(D))/(f(K)/f(pi))=0.979(11). Combining with experiment gives V(us)=0.2262(14) and V(cs)/V(cd) of 4.43(41). We use a highly improved quark discretization on MILC gluon fields that include realistic sea quarks, fixing the u/d, s, and c masses from the pi, K, and eta(c) meson masses. This allows a stringent test against experiment for D and D(s) masses for the first time (to within 7 MeV).

Highly improved staggered quarks on the lattice, with applications to charm physics

We use perturbative Symanzik improvement to create a new staggered-quark action (HISQ) that has greatly reduced one-loop taste-exchange errors, no tree-level order a{sup 2} errors, and no tree-level order (am){sup 4} errors to leading order in the quarks velocity v/c. We demonstrate with simulations that the resulting action has taste-exchange interactions that are 3-4 times smaller than the widely used ASQTAD action. We show how to bound errors due to taste exchange by comparing ASQTAD and HISQ simulations, and demonstrate with simulations that such errors are likely no more than 1% when HISQ is used for light quarks at lattice spacings of 1/10 fm or less. The suppression of (am){sup 4} errors also makes HISQ the most accurate discretization currently available for simulating c quarks. We demonstrate this in a new analysis of the {psi}-{eta}{sub c} mass splitting using the HISQ action on lattices where am{sub c}=0.43 and 0.66, with full-QCD gluon configurations (from MILC). We obtain a result of 111(5) MeV which compares well with the experiment. We discuss applications of this formalism to D physics and present our first high-precision results for D{sub s} mesons.

Neutral B meson mixing in unquenched lattice QCD

We study B{sub d} and B{sub s} mixing in unquenched lattice QCD employing the MILC Collaboration gauge configurations that include u, d, and s sea quarks based on the improved staggered quark (AsqTad) action and a highly improved gluon action. We implement the valence light quarks also with the AsqTad action and use the nonrelativistic NRQCD action for the valence b quark. We calculate hadronic matrix elements necessary for extracting Cabibbo-Kobayashi-Maskawa matrix elements from experimental measurements of mass differences {delta}M{sub d} and {delta}M{sub s}. We find {xi}{identical_to}f{sub B{sub s}}{radical}(B-circumflex{sub B{sub s}})/f{sub B{sub d}}{radical}(B-circumflex{sub B{sub d}})=1.258(33), f{sub B{sub d}}{radical}(B-circumflex{sub B{sub d}})=216(15) MeV, and f{sub B{sub s}}{radical}(B-circumflex{sub B{sub s}})=266(18) MeV. We also update previous results for decay constants and obtain f{sub B{sub d}}=190(13) MeV, f{sub B{sub s}}=231(15) MeV, and f{sub B{sub s}}/f{sub B{sub d}}=1.226(26). The new lattice results lead to updated values for the ratio of Cabibbo-Kobayashi-Maskawa matrix elements |V{sub td}|/|V{sub ts}| and for the standard model prediction for Br(B{sub s}{yields}{mu}{sup +}{mu}{sup -}) with reduced errors. We determine |V{sub td}|/|V{sub ts}|=0.214(1)(5) and Br(B{sub s}{yields}{mu}{sup +}{mu}{sup -})=3.19(19)x10{sup -9}.

B-meson decay constant from unquenched lattice QCD.

We present determinations of the -meson decay constant f(B) and f(B)(s)/f(B) using the MILC Collaboration unquenched gauge configurations, which include three flavors of light sea quarks. The mass of one of the sea quarks is kept around the strange quark mass, and we explore a range in masses for the two lighter sea quarks down to m(s)/8. The heavy quark is simulated using nonrelativistic QCD, and both the valence and sea light quarks are represented by the highly improved (AsqTad) staggered quark action. The good chiral properties of the latter action allow for a more accurate chiral extrapolation to physical up and down quarks than has been possible in the past. We find f(B)=216(9)(19)(4)(6) MeV and f(B)(s)/f(B)=1.20(3)(1).

Update: Precision D_s decay constant from full lattice QCD using very fine lattices

We update our previous determination of both the decay constant and the mass of the D s meson using the highly improved staggered quark formalism. We include additional results at two finer values of the lattice spacing along with improved determinations of the lattice spacing and improved tuning of the charm and strange quark masses. We obtain m Ds = 1.9691(32) GeV, in good agreement with experiment, and f Ds = 0.2480(25) GeV. Our result for f Ds is 1.6σ lower than the most recent experimental average determined from the D s leptonic decay rate and using V cs from Cabibbo-Kobayashi-Maskawa unitarity. Combining our f Ds with the experimental rate we obtain a direct determination of V cs = 1.010(22), or alternatively 0.990 +0.013 -0.016 using a probability distribution for statistical errors for this quantity which vanishes above 1. We also include an accurate prediction of the decay constant of the η c , f ηc = 0.3947(24) GeV, as a calibration point for other lattice calculations.

Chiral symmetry restoration in baryon rich environments

Abstract Chiral symmetry restoration in an environment rich in baryons is studied by computer simulation methods in SU(2) and SU(3) gauge theories in the quenched approximation. The basic theory of symmetry restoration as a function of chemical potential is illusstrated and the implementation of the ideas on a lattice is made explicit. A simple mean field model is presented to guide ones expectations. The second order conjugate-gradient iterative method and the pseudo-fermion Monte Carlo procedure are convergent methods of calculating the fermion propagator in an environment rich in baryons. Computer simulations of SU(3) gauge theory show an abrupt chiral symmetry restoring transition and the critical chemical potential and induced baryon density are estimated crudely. A smoother transition is observed for the color group SU(2).

B -Meson Decay Constants from Improved Lattice Nonrelativistic QCD with Physical u , d , s , and c Quarks

R. J. Dowdall, 2, ∗ C. T. H. Davies, † R. R. Horgan, C. J. Monahan, and J. Shigemitsu (HPQCD collaboration) ‡ DAMTP, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK Physics Department, College of William and Mary, Williamsburg, Virginia 23187, USA Physics Department, The Ohio State University, Columbus, Ohio 43210, USA (Dated: February 13, 2013)

Studies of chiral symmetry breaking in SU(2) lattice gauge theory

We study chiral symmetry breaking (χSB) in SU(2) lattice gauge theory with quarks in the l = 12, l = 1, l = 32, and l = 2 representations of the color group. We perform Monte Carlo evaluations of 〈ψψ〉 in the quenched approximation and extract the relevant length scales for χSB. We revise a previous estimate for the ratio between the chiral symmetry restoration temperatures for fundamental and adjoint quarks and obtain Tl = 1/Tl = 12 ∼ 8. Our results for the higher representations, l = 32and l = 2, are consistent with Casimir scaling and give C2gmom2 ∼ 4. Many aspects of our calculational method are explained in detail. The issues discussed include the relation between χSB in the quenched approximation and the spectrum of the Dirac operator, the flavor symmetries of euclidean staggered fermions, estimates of finite-size effects and the reliability of m → 0 extrapolations on finite lattices.

The

We present a new determination of the B and Bs meson decay constants using nonrelativistic quantum chromodynamics (NRQCD) b-quarks, highly improved staggered quark (HISQ) light and strange valence quarks and the MILC collaboration Nf=2+1 lattices. The new calculations improve on HPQCD’s earlier work with NRQCD b-quarks by replacing AsqTad with HISQ valence quarks, by including a more chiral MILC fine ensemble in the analysis, and by employing better tuned quark masses and overall scale. We find fB=0.191(9)  GeV, fBs=0.228(10)  GeV and fBs/fB=1.188(18). Combining the new value for fBs/fB with a recent very precise determination of the Bs meson decay constant based on HISQ b-quarks, fBs=0.225(4)  GeV, leads to fB=0.189(4)  GeV. With errors of just 2.1% this represents the most precise fB available today.

B

We use lattice QCD simulations, with MILC configurations and HISQ