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In this work we study the two and three-dimensional antiferromagnetic Ising model with an imaginary magnetic field $i��$ at $��=��$. In order to perform numerical simulations of the system we introduce a new geometric algorithm not affected by the sign problem. Our results for the $2D$ model are in agreement with the analytical solutions. We also present new results for the $3D$ model which are qualitatively in agreement with mean-field predictions.

We present a new study of the form factors for D -> K semileptonic decay from lattice QCD that allows us to compare the shape of the vector form factor to experiment and, for the first time, to extract V_cs using results from all experimental q^2 bins. The valence quarks are implemented with the Highly Improved Staggered Quark action on MILC configurations that include u, d and s sea quarks. The scalar and vector currents are nonperturbatively normalised and, using phased boundary conditions, we are able to cover the full q^2 range accessible to experiment. Our result is V_cs = 0.963(5)_{expt}(14)_{lattice}. We also demonstrate that the form factors are insensitive to whether the spectator quark is u/d or s, which has implications for other decay channels., 5 pages, 6 figures

Recently we studied the B, Bs, D and Ds meson decay constants using various treatments for the heavy quark. For B mesons, we determined fB, fBs, and fBs/fB with NRQCD bottom quarks. We then combined the ratio fBs/fB and another very precise determination from HPQCD for fBs using heavy HISQ quarks, and extracted fB with 2% total errors. We also calculated fD, fDs, and fDs/fD using HISQ charm quarks. Here we review our results and briefly discuss their implications for the determination of the CKM matrix elements |Vcd| and |Vcs|., Comment: 7 pages, 4 figures, and 2 tables. Talk given at Lattice 2012, June 24-29, 2012, Cairns, Australia

We study the two-dimensional Antiferromagnetic Ising Model with an imaginary magnetic field i��at ��=��. We use a new geometric algorithm which does not present a sign problem. This allows us to perform efficient numerical simulations of this system., 7 pages, 3 figures, submitted as a contribution to the 30th International Symposium on Lattice Field Theory, June 24-29, 2012, Cairns, Australia

We calculate the J/�� mass, leptonic width and radiative decay rate to ����_c from lattice QCD including u, d and s quarks in the sea for the first time. We use the Highly Improved Staggered Quark formalism and nonperturbatively normalised vector currents for the leptonic and radiative decay rates. Our results are: M_{J/��} -M_{��_c} = 116.5(3.2) MeV; ��(J/�� to e^+e^-) = 5.48(16) keV; ��(J/�� to ����_c) = 2.49(19) keV. The first two are in good agreement with experiment, with ��(J/�� to e^+e^-) providing a test of a decay matrix element in QCD, independent of CKM uncertainties, to 2%. At the same time results for the time moments of the correlation function can be compared to values from the charm contribution to ��(e^+e^- to hadrons), giving a 1.5% test of QCD. Our results show that an improved experimental error would enable a similarly strong test from ��(J/�� to ����_c)., 20 pages, 16 figures. Added some text plus a figure and two tables comparing the time moments of the charmonium vector correlator to results extracted from experiment for the charm contribution to e^+e^- -> hadrons. This allows a 1.5% test of QCD. Further minor amendments to the text. Version accepted for publication by Physical Review D

We present a study of $D \rightarrow K, l \nu$ semileptonic decays on the lattice which employs the HISQ action for both the charm and the light quarks. We work with MILC unquenched $N_f = 2 + 1$ lattices and determine the scalar form factor $f_0(q^2)$. This form factor is obtained from a scalar current matrix element that does not require any operator matching. We find $f^{D \rightarrow K}_0(0) \equiv f^{D \rightarrow K}_+(0) = 0.747(19)$ in the chiral plus continuum limit and hereby improve the theory error on this quantity by a factor of $\sim$4 compared to previous lattice determinations. Combining the new theory result with recent experimental measurements of the product $f^{D \rightarrow K}_+(0) * |V_{cs}| $ from BaBar and CLEO-c leads to a very precise direct determination of the CKM matrix element $|V_{cs}| $, $|V_{cs}| = 0.961(11)(24)$, where the first error comes from experiment and the second is the lattice QCD theory error., Comment: 7 pages, 6 figures, and 1 table. Talk given at Lattice 2010, June 14-19, 2010, Villasimius, Italy

We present a new determination of the $B_s$ leptonic decay constant from lattice QCD simulations that use gluon configurations from MILC and a highly improved discretization of the relativistic quark action for both valence quarks. Our result, $f_{B_s} = 0.225(4)$\,GeV, is almost three times more accurate than previous determinations. We analyze the dependence of the decay constant on the heavy quark's mass and obtain the first empirical evidence for the leading $1/\sqrt{m_h}$ dependence predicted by Heavy Quark Effective Theory (HQET). As a check, we use our analysis technique to calculate the $m_{B_s}-m_{\eta_b}/2$ mass difference. Our result agrees with experiment to within errors of $11\,\mathrm{MeV}$ (better than 2%). We discuss how to extend our analysis to other quantities in $B_s$ and $B$ physics, making 2%-precision possible for the first time., Comment: 5 pages, 3 figures

We extend our earlier lattice-QCD analysis of heavy-quark correlators to smaller lattice spacings and larger masses to obtain new values for the c mass and QCD coupling, and, for the first time, values for the b mass: m_c(3GeV,n_f=4)=0.986(6)GeV, alpha_msb(M_Z,n_f=5)=0.1183(7), and m_b(10GeV,n_f=5)=3.617(25)GeV. These are among the most accurate determinations by any method. We check our results using a nonperturbative determination of the mass ratio m_b(mu,n_f)/m_c(mu,n_f); the two methods agree to within our 1% errors and taken together imply m_b/m_c=4.51(4). We also update our previous analysis of alpha_msb from Wilson loops to account for revised values for r_1 and r_1/a, finding a new value alpha_\msb(M_Z,n_f=5)=0.1184(6); and we update our recent values for light-quark masses from the ratio m_c/m_s. Finally, in the Appendix, we derive a procedure for simplifying and accelerating complicated least-squares fits., 16 pages, 10 figures, 3 tables

We discuss the status of our ongoing efforts to improve on our calculation of the $D_s$ decay constant. We show preliminary results on the ratio of the charm to the strange quark mass. We also present preliminary results for spectroscopy, decay constants and bottom quark mass obtained by performing calculations with highly improved staggered quarks at masses above the c mass and close to the b mass., Comment: 8 pages. Contribution to 27th International Symposium on Lattice Field Theory, Beijing, China, 2009

We give improved results for B meson masses using NRQCD $b$ quarks and HISQ light valence quarks for a range of lattice spacings and sea quark masses enabling controlled extrapolation to the physical point., 6 pages, 3 figures, poster presented at the XXVI International Symposium on Lattice Field Theory, July 14-19, 2008, Williamsburg, Virginia, USA

We provide a new determination of the charm quark mass using the Highly Improved Staggered Quark (HISQ) action, finding m_c(3 GeV) = 0.983(23) GeV. Our determination makes extensive use of second order lattice perturbation theory in matching the bare lattice mass to the MSbar scheme. This matching utilises both traditional diagrammatic perturbation theory and weak coupling simulations. The second of these techniques allows us to extract perturbative coefficients from Monte-Carlo simulations and the process of doing this is laid out in some detail here., 7 pages, 4 figures, talk presented at the XXVI International Symposium on Lattice Field Theory, July 14-19, 2008, Williamsburg, Virginia, USA

We present an update of results from the HPQCD collaboration on charm physics using the Highly Improved Staggered Quark action. This includes a precise determination of m_c using moments of current-current correlators combined with high-order continuum QCD perturbation theory. We also include an update on the determination of alpha_s from lattice QCD, preliminary results on the determination of m_b and a summary plot of the status of the gold-plated meson spectrum. There is an appendix on tackling systematic errors in fitting using the Bayesian approach., Comment: 14 pages, 7 figures, combined proceedings of talks by Christine Davies and Peter Lepage at the International Symposium on Lattice Field Theory 2008, Williamsburg

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 $f_{D_s}/f_D$ = 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)., Comment: 4 pages, 2 figures. Published version - changes from original include a more extensive discussion of errors and an error budget table covering more quantities. There are very small changes in some of the values reported

We examine the gluon propagator in the Laplacian gauge in quenched lattice QCD as a function of the number of colours. We observe a weak dependence on $N_c$ over the whole momentum range. This implies an almost $N_c$-independent gluon pole mass in units of the string tension., 5 pages, 7 figures. A study of the gluon propagator in SU(4) for various lattice sizes was added. Extended version to appear in Phys. Rev. D

We extend our study of the gluon propagator in quenched lattice QCD using the Laplacian gauge to a finer lattice. We verify the existence of a pole mass as we take the continuum limit and deduce a value of $\sim 600^{+150}_{-30}$ MeV for this pole mass. We find a finite value of $(454(5){\rm MeV})^{-2}$ for the renormalized zero-momentum propagator, in agreement with results on coarser lattices., Comment: 5 pages, 5 figures. Extended version to appear in Phys. Rev. D

We calculate the critical value of the hopping parameter, $\kappa_c$, in Lattice QCD with Wilson fermions, to two loops in perturbation theory. This quantity is an additive renormalization; as such, it is characterized not only by the standard caveats regarding the asymptotic nature of perturbative results, but also by a linear divergence in the lattice spacing. Consequently, our calculation tests rather stringently the limits of applicability of perturbation theory. We compare our results to non perturbative evaluations of $\kappa_c$ coming from Monte Carlo simulations. Finally, we apply a tadpole improvement technique on our results; this shifts them quite favourably towards the non perturbative values., Comment: 9 pages, LATeX with 2 EPS figures

We study the gluon propagator in quenched lattice QCD using the Laplacian gauge which is free of lattice Gribov copies. We compare our results with those obtained in the Landau gauge on the lattice, as well as with various approximate solutions of the Dyson Schwinger equations. We find a finite value $\sim (445 \rm{MeV})^{-2}$ for the renormalized zero-momentum propagator (taking our renormalization point at 1.943 GeV), and a pole mass $\sim 640 \pm 140$ MeV., Discussion of the renormalized gluon propagator and of the Laplacian gauge fixing procedure extended. Version to appear in Phys. Rev. D. 15 pages, 8 figures

We present lattice results for the gluon propagator for SU(2) and SU(3) in the Laplacian gauge which avoids lattice Gribov copies. In SU(3) we compare with the most recent lattice calculation in Landau gauge and with various approximate solutions of the Dyson Schwinger equations (DSE)., Comment: 3 pages, 3 figures; Talk presented at Confinement IV, July 3-8, 2000, Vienna, Austria. An additional line of explanation regarding the gauge fixing procedure was added

We compute the ratio between the scale $��_L$ associated with a lattice formulation of QCD using the overlap-Dirac operator, and $��_{MS-bar}$. To this end, the one-loop relation between the lattice coupling $g_0$ and the coupling renormalized in the MS-bar scheme is calculated, using the lattice background field technique. We also compute the one-loop renormalization $Z_��$ of the two-quark operators $\bar�� ����$, where $��$ denotes a generic Dirac matrix. Furthermore, we study the renormalization of quark bilinears which are more extended and have better chiral properties. Finally, we present improved estimates of $Z_��$, coming from cactus resummation and from mean field perturbation theory., 3 pages, LATeX with 2 PS figure files. Talk presented at Confinement IV, Vienna, July 2000. Longer write-ups in: hep-lat/9909158 (NPB 571 (2000) 257), hep-lat/0002010 (NPB 580 (2000) 394). One further reference added, no other changes

We review recent results for B_d and B_s mixing parameters using MILC N_f=2+1 lattices, NRQCD b-quarks and AsqTad light quarks. Latest numbers for decay constants f_B and f_{B_s} are also presented. Combining our lattice results with experimental determinations of the mass differences \Delta M_d and \Delta M_s leads to an important ratio of elements of the CKM matrix, |V_{td}|/|V_{ts}| = 0.214(1)(5) and an updated Standard Model number for the branching fraction Br(B_s -> mu^+ mu^-) = 3.19(19) x 10^{-9}. Preliminary new results for f_{B_s} based on other actions are also described., Comment: 7 pages, 5 figures, contribution to the 27th International Symposium on Lattice Field Theory, Beijing, July 2009

We use a relativistic highly improved staggered quark action to discretize charm quarks on the lattice. We calculate the masses and the dispersion relation for heavy-heavy and heavy-light meson states, and show that for lattice spacings below .1 fm, the discretization errors are at the few percent level. We also discuss the prospects for accurate calculations at the few percent level of f_D_s, f_D, and the leptonic width of the psi and phi., 7 pages, contribution to Lattice 2006