1,664 research outputs found
New methods for B meson decay constants and form factors from lattice NRQCD
We determine the normalisation of scalar and pseudoscalar current operators
made from non-relativistic quarks and Highly Improved Staggered light
quarks in lattice Quantum Chromodynamics (QCD) through
and . We use matrix elements of these operators to
extract meson decay constants and form factors, then compare to those
obtained using the standard vector and axial-vector operators. This provides a
test of systematic errors in the lattice QCD determination of the meson
decay constants and form factors. We provide a new value for the and
meson decay constants from lattice QCD calculations on ensembles that include
, , and quarks in the sea and those which have the quark
mass going down to its physical value. Our results are GeV,
GeV and , agreeing well with earlier
results using the temporal axial current. By combining with these previous
results, we provide updated values of GeV,
GeV and .Comment: 14 pages, 10 figure
The mass of the b-quark from lattice NRQCD and lattice perturbation theory
We present a determination of the b-quark mass accurate through O(\alpha_s^2)
in perturbation theory and including partial contributions at O(\alpha_s^3).
Nonperturbative input comes from the calculation of the Upsilon and B_s
energies in lattice QCD including the effect of u, d and s sea quarks. We use
an improved NRQCD action for the b-quark. This is combined with the heavy quark
energy shift in NRQCD determined using a mixed approach of high-beta simulation
and automated lattice perturbation theory. Comparison with experiment enables
the quark mass to be extracted: in the MS bar scheme we find m_b(m_b) =
4.166(43) GeV.Comment: v2 - corrected some typos and an error in the summary plo
Role of the Euclidean Signature in Lattice Calculations of Quasidistributions and Other Nonlocal Matrix Elements
Lattice quantum chromodynamics (QCD) provides the only known systematic, nonperturbative method for first-principles calculations of nucleon structure. However, for quantities such as light-front parton distribution functions (PDFs) and generalized parton distributions (GPDs), the restriction to Euclidean time prevents direct calculation of the desired observable. Recently, progress has been made in relating these quantities to matrix elements of spatially nonlocal, zero-time operators, referred to as quasidistributions. Still, even for these time-independent matrix elements, potential subtleties have been identified in the role of the Euclidean signature. In this work, we investigate the analytic behavior of spatially nonlocal correlation functions and demonstrate that the matrix elements obtained from Euclidean lattice QCD are identical to those obtained using the Lehmann-Symanzik-Zimmermann reduction formula in Minkowski space. After arguing the equivalence on general grounds, we also show that it holds in a perturbative calculation, where special care is needed to identify the lattice prediction. Finally we present a proof of the uniqueness of the matrix elements obtained from Minkowski and Euclidean correlation functions to all order in perturbation theory
B and B-s meson decay constants from lattice QCD
We present a new determination of the B and B-s meson decay constants using nonrelativistic quantum chromodynamics (NRQCD) b-quarks, highly improved staggered quark (HISQ) light and strange valence quarks and the MILC collaboration N-f = 2 + 1 lattices. The new calculations improve on HPQCD\u27s 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 f (B) = 0.191(9) GeV, f (Bs) = 0.228(10) GeV and f (Bs)/f (B) = 1.188(18). Combining the new value for f (Bs)/f (B) with a recent very precise determination of the B-s meson decay constant based on HISQ b-quarks, f (Bs) = 0.225(4) GeV, leads to f (B) = 0.189(4) GeV. With errors of just 2.1% this represents the most precise f (B) available today
B-Meson Decay Constants from Improved Lattice Nonrelativistic QCD with Physical u,d,s, and c Quarks
We present the first lattice QCD calculation of the decay constants f(B) and f(Bs) with physical light quark masses. We use configurations generated by the MILC Collaboration including the effect of u, d, s, and c highly improved staggered quarks in the sea at three lattice spacings and with three u/d quark mass values going down to the physical value. We use improved nonrelativistic QCD (NRQCD) for the valence b quarks. Our results are f(B) = 0.186(4) GeV, f(Bs) = 0.224(4) GeV, f(Bs) / f(B) = 1.205(7), and M-Bs - M-B = 85(2) MeV, superseding earlier results with NRQCD b quarks. We discuss the implications of our results for the standard model rates for B-(s) -\u3emu(+)mu(-) and B -\u3etau n
PDFs in small boxes
PDFs can be studied directly using lattice QCD by evaluating matrix elements
of non-local operators. A number of groups are pursuing numerical calculations
and investigating possible systematic uncertainties. One systematic that has
received less attention is the effect of calculating in a finite spacetime
volume. Here we present first attempts to assess the role of the finite volume
for spatially non-local operators. We find that these matrix elements may
suffer from large finite-volume artifacts and more careful investigation is
needed.Comment: 6 pages, 3 figures, Conference: The 36th Annual International
Symposium on Lattice Field Theory - LATTICE2018, 22-28 July, 2018, Michigan
State University, East Lansing, Michigan, US
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