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G. P. Lepage

Publications and source records attributed to G. P. Lepage.

At least 55 records · Page 3Linked to original sources
arXiv2013

V_cs from D_s to ϕlν semileptonic decay and full lattice QCD

We determine the complete set of axial and vector form factors for the Ds to ϕlν decay from full lattice QCD for the first time. The valence quarks are implemented using the Highly Improved Staggered Quark action and we normalise the appropriate axial and vector currents fully nonperturbatively. The q^2 and angular distributions we obtain for the differential rate agree well with those from the BaBar experiment and, from the total branching fraction, we obtain Vcs = 1.017(63), in good agreement with that from D to Klν semileptonic decay. We also find the mass and decay constant of the ϕ meson in good agreement with experiment, showing that its decay to K{\bar{K}} (which we do not include here) has at most a small effect. We include an Appendix on nonperturbative renormalisation of the complete set of staggered vector and axial vector bilinears needed for this calculation.

hep-lat↗
arXiv2013

B, Bs, K and pi weak matrix elements with physical light quarks

Calculations of pseudoscalar decay constants of B, Bs, K and pi mesons with physical light quarks are presented. We use HISQ ensembles that include u,d,s and c sea quarks at three lattice spacings. HISQ is used for the valence light quarks and a radiatively improved NRQCD action for the heavy quarks. The key results are f_{B^+}=0.184(4)$ GeV, f_{B_s}=0.224(4) GeV, f_{B_s}/f_{B^+}=1.217(8), f_{K^+}/f_{pi^+}=1.1916(21), f_{K^+}=155.37(34) MeV, giving a significant improvement over previous results that required chiral extrapolation. We also calculate the Wilson flow scale w_0, finding w_0=0.1715(9) fm.

hep-lat↗
arXiv2013

The shape of the D -> K semileptonic form factor from full lattice QCD and V_cs

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.

hep-lat↗
arXiv2013

Direct determination of strange and light quark condensates from full lattice QCD

We determine the strange and light quark condensates in full lattice QCD for the first time. This is done by direct calculation of the expectation value of the trace of the quark propagator followed by subtraction of the appropriate perturbative contribution to convert to a value for the condensate in the MS-bar scheme at 2 GeV. We use lattice QCD configurations including u, d, s and c quarks in the sea with u/d quark masses going down to the physical value. We find the ratio of the strange to the light quark condensate to be 1.08(16).

hep-lat↗
arXiv2012

Direct determination of the strange and light quark condensates from full lattice QCD

We determine the strange quark condensate from lattice QCD for the first time and compare its value to that of the light quark and chiral condensates. The results come from a direct calculation of the expectation value of the trace of the quark propagator followed by subtraction of the appropriate perturbative contribution, derived here, to convert the non-normal-ordered $m\barψψ$ to the $\bar{MS}$ scheme at a fixed scale. This is then a well-defined physical `nonperturbative' condensate that can be used in the Operator Product Expansion of current-current correlators. The perturbative subtraction is calculated through $\mathcal{O}(α_s)$ and estimates of higher order terms are included through fitting results at multiple lattice spacing values. The gluon field configurations used are `second generation' ensembles from the MILC collaboration that include 2+1+1 flavors of sea quarks implemented with the Highly Improved Staggered Quark action and including $u/d$ sea quarks down to physical masses. Our results are : $<\bar{s}{s}>^{\bar{MS}}(2 \mathrm{GeV})= -(290(15) \mathrm{MeV})^3$, $<\bar{l}{l}>^{\bar{MS}}(2\, \mathrm{GeV})= -(283(2) \mathrm{MeV})^3$, where $l$ is a light quark with mass equal to the average of the $u$ and $d$ quarks. The strange to light quark condensate ratio is 1.08(16). The light quark condensate is significantly larger than the chiral condensate in line with expectations from chiral analyses. We discuss the implications of these results for other calculations.

hep-lat↗
arXiv2012

Precision tests of the J/ψ from full lattice QCD: mass, leptonic width and radiative decay rate to η_c

We calculate the J/ψ mass, leptonic width and radiative decay rate to γη_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/ψ} -M_{η_c} = 116.5(3.2) MeV; Γ(J/ψ to e^+e^-) = 5.48(16) keV; Γ(J/ψ to γη_c) = 2.49(19) keV. The first two are in good agreement with experiment, with Γ(J/ψ 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 Γ(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 Γ(J/ψ to γη_c).

hep-lat↗
arXiv2012

Heavy meson masses and decay constants from relativistic heavy quarks in full lattice QCD

We determine masses and decay constants of heavy-heavy and heavy-charm pseudoscalar mesons as a function of heavy quark mass using a fully relativistic formalism known as Highly Improved Staggered Quarks for the heavy quark. We are able to cover the region from the charm quark mass to the bottom quark mass using MILC ensembles with lattice spacing values from 0.15 fm down to 0.044 fm. We obtain f_{B_c} = 0.427(6) GeV; m_{B_c} = 6.285(10) GeV and f_{η_b} = 0.667(6) GeV. Our value for f_{η_b} is within a few percent of f_Υ confirming that spin effects are surprisingly small for heavyonium decay constants. Our value for f_{B_c} is significantly lower than potential model values being used to estimate production rates at the LHC. We discuss the changing physical heavy-quark mass dependence of decay constants from heavy-heavy through heavy-charm to heavy-strange mesons. A comparison between the three different systems confirms that the B_c system behaves in some ways more like a heavy-light system than a heavy-heavy one. Finally we summarise current results on decay constants of gold-plated mesons.

hep-lat↗
arXiv2011

Fast Fits for Lattice QCD Correlators

We illustrate a technique for fitting lattice QCD correlators to sums of exponentials that is significantly faster than traditional fitting methods --- 10--40 times faster for the realistic examples we present. Our examples are drawn from a recent analysis of the Upsilon spectrum, and another recent analysis of the D -> pi semileptonic form factor. For single correlators, we show how to simplify traditional effective-mass analyses.

hep-lat↗
arXiv2011

The Upsilon spectrum and the determination of the lattice spacing from lattice QCD including charm quarks in the sea

We give results for the Upsilon spectrum from lattice QCD using an improved version of the NRQCD action for b quarks which includes radiative corrections to kinetic terms at O(v^4) in the velocity expansion. We also include for the first time the effect of up, down, strange and charm quarks in the sea using 'second generation' gluon field configurations from the MILC collaboration. Using the Υ 2S - 1S splitting to determine the lattice spacing, we are able to obtain the 1P - 1S splitting to 1.4% and the 3S - 1S splitting to 2.4%. Our improved result for M (Upsilon) - M (eta_b) is 70(9) MeV and we predict M(Upsilon) - M(eta_b') = 35(3) MeV. We also calculate pi, K and eta_s correlators using the Highly Improved Staggered Quark action and perform a chiral and continuum extrapolation to give values for M(eta_s) (0.6893(12) GeV) and f_{eta_s} (0.1819(5) GeV) that allow us to tune the strange quark mass as well as providing an independent and consistent determination of the lattice spacing. Combining the NRQCD and HISQ analyses gives mb/ms = 54.7(2.5) and a value for the heavy quark potential parameter of r_1 = 0.3209(26) fm.

hep-lat↗
arXiv2011

High-Precision $f_{B_s}$ and HQET from Relativistic Lattice QCD

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 $m_{B_s}-m_{η_b}/2$ 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.

hep-lat↗
arXiv2010

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 $m_{D_s}$ = 1.9691(32) GeV, in good agreement with experiment, and $f_{D_s}$ = 0.2480(25) GeV. Our result for $f_{D_s}$ is 1.6$σ$ lower than the most recent experimental average determined from the $D_s$ leptonic decay rate and using $V_{cs}$ from CKM unitarity. Combining our $f_{D_s}$ with the experimental rate we obtain a direct determination of $V_{cs} = 1.010(22)$, or alternatively $0.990 {+0.013 \atop -0.016}$ 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 $η_c$, $f_{η_c}$ = 0.3947(24) GeV, as a calibration point for other lattice calculations.

hep-lat↗
arXiv2010

High-Precision c and b Masses, and QCD Coupling from Current-Current Correlators in Lattice and Continuum QCD

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.

hep-lat↗
arXiv2010

Precise charm to strange mass ratio and light quark masses from full lattice QCD

By using a single formalism to handle charm, strange and light valence quarks in full lattice QCD for the first time, we are able to determine ratios of quark masses to 1%. For $m_c/m_s$ we obtain 11.85(16), an order of magnitude more precise than the current PDG average. Combined with 1% determinations of the charm quark mass now possible this gives $\bar{m}_s(2{\rm GeV}) =$ 92.4(1.5) MeV. The MILC result for $m_s/m_l = 27.2(3) $ yields $\bar{m}_l(2{\rm GeV})$ = 3.40(7) MeV for the average of $u$ and $d$ quark masses.

hep-ph↗
arXiv2009

Recent results on B mixing and decay constants from HPQCD

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 ΔM_d and ΔM_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.

hep-lat↗
arXiv2009

Towards precise relativistic b quarks on the lattice

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.

hep-lat↗
arXiv2009

Moving NRQCD for heavy-to-light form factors on the lattice

We formulate Non-Relativistic Quantum Chromodynamics (NRQCD) on a lattice which is boosted relative to the usual discretization frame. Moving NRQCD (mNRQCD) allows us to treat the momentum for the heavy quark arising from the frame choice exactly. We derive mNRQCD through O(1/m^2,v^4), as accurate as the NRQCD action in present use, both in the continuum and on the lattice with O(a^4) improvements. We have carried out extensive tests of the formalism through calculations of two-point correlators for both heavy-heavy (bottomonium) and heavy-light (B_s) mesons in 2+1 flavor lattice QCD and obtained nonperturbative determinations of energy shift and external momentum renormalization. Comparison to perturbation theory at O(α_s) is also made. The results demonstrate the effectiveness of mNRQCD. In particular we show that the decay constants of heavy-light and heavy-heavy mesons can be calculated with small systematic errors up to much larger momenta than with standard NRQCD.

hep-lat↗
arXiv2009

A Prediction of the B*_c mass in full lattice QCD

By using the Highly Improved Staggered Quark formalism to handle charm, strange and light valence quarks in full lattice QCD, and NRQCD to handle bottom valence quarks we are able to determine accurately ratios of the B meson vector-pseudoscalar mass splittings, in particular, (m(B*_c)-m(B_c))/(m(B*_s)-m(B_s)). We find this ratio to be 1.15(15), showing the `light' quark mass dependence of this splitting to be very small. Hence we predict m(B_c*) = 6.330(7)(2)(6) GeV where the first two errors are from the lattice calculation and the third from existing experiment. This is the most accurate prediction of a gold-plated hadron mass from lattice QCD to date.

hep-lat↗