66 research outputs found
The Low-Lying Dirac Spectrum of Staggered Quarks
We investigate and clarify the role of topology and the issues surrounding
the epsilon regime for staggered quarks. We study unimproved and improved
staggered quark Dirac operators on quenched lattice QCD gluon backgrounds
generated using a Symanzik-improved gluon action. For the improved Dirac
operators we find a clear separation of the spectrum into would-be zero modes
and others. The number of would-be zero modes depends on the topological charge
as predicted by the continuum Index Theorem, and the expectation values of
their chirality are large for the most improved actions (approx 0.7). The
remaining modes have low chirality and show clear signs of clustering into
quartets that become degenerate in the continuum limit. We demonstrate that the
lattice spacing and volume dependence of the eigenvalues follow expectations.
Furthermore, the non-zero modes follow the random matrix theory predictions for
all topological charge sectors. The values of the chiral condensate extracted
from fits to the theoretical distributions are consistent with each other, and
with the results obtained from the total density of eigenvalues using the
Banks-Casher relation. We conclude that staggered quarks respond correctly to
QCD topology when both fermion and gauge actions are improved.Comment: 17 pages, a few typos corrected, part of one figure change
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Bc →bs (d) form factors from lattice QCD
We present results of the first lattice QCD calculations of and
weak matrix elements. Form factors across the entire physical
range are then extracted and extrapolated to the physical-continuum limit
before combining with CKM matrix elements to predict the semileptonic decay
rates and . The lattice QCD uncertainty is
comparable to the CKM uncertainty here. Results are derived from correlation
functions computed on MILC Collaboration gauge configurations with a range of
lattice spacings including 2+1+1 flavours of dynamical sea quarks in the Highly
Improved Staggered Quark (HISQ) formalism. HISQ is also used for the
propagators of the valence light, strange, and charm quarks. Two different
formalisms are employed for the bottom quark: non-relativistic QCD (NRQCD) and
heavy-HISQ. Checking agreement between these two approaches is an important
test of our strategies for heavy quarks on the lattice. From chained fits of
NRQCD and heavy-HISQ data, we obtain the differential decay rates as well as integrated values for comparison to future experimental
results
Mass of the B_c Meson in Three-Flavor Lattice QCD
We use lattice QCD to predict the mass of the meson. We use the MILC
Collaboration's ensembles of lattice gauge fields, which have a quark sea with
two flavors much lighter than a third. Our final result is
. The first error bar is a sum in quadrature
of statistical and systematic uncertainties, and the second is an estimate of
heavy-quark discretization effects.Comment: 4 pages, 3 figures; shorten to fit in PRL; published versio
The B Meson Decay Constant from Unquenched Lattice QCD
We present determinations of the B meson decay constant f_B and of the ratio
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 b 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 much smoother
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).Comment: 4 pages, 2 figure
The Upsilon spectrum and m_b from full lattice QCD
We show results for the Upsilon spectrum calculated in lattice QCD including
for the first time vacuum polarization effects for light u and d quarks as well
as s quarks. We use gluon field configurations generated by the MILC
collaboration. The calculations compare the results for a variety of u and d
quark masses, as well as making a comparison to quenched results (in which
quark vacuum polarisation is ignored) and results with only u and d quarks. The
b quarks in the Upsilon are treated in lattice Nonrelativistic QCD through NLO
in an expansion in the velocity of the b quark. We concentrate on accurate
results for orbital and radial splittings where we see clear agreement with
experiment once u, d and s quark vacuum polarisation effects are included. This
now allows a consistent determination of the parameters of QCD. We demonstrate
this consistency through the agreement of the Upsilon and B spectrum using the
same lattice bare b quark mass. A one-loop matching to continuum QCD gives a
value for the b quark mass in full lattice QCD for the first time. We obtain
m_b^{\bar{MS}}(m_b^{\bar{MS}}) = 4.4(3) GeV. We are able to give physical
results for the heavy quark potential parameters, r_0 = 0.469(7) fm and r_1 =
0.321(5) fm. Results for the fine structure in the spectrum and the Upsilon
leptonic width are also presented. We predict the Upsilon - eta_b splitting to
be 61(14) MeV, the Upsilon^{\prime} - eta_b^{\prime} splitting as 30(19) MeV
and the splitting between the h_b and the spin-average of the chi_b states to
be less than 6 MeV. Improvements to these calculations that will be made in the
near future are discussed.Comment: 24 pages, 19 figures. Version to be published. Minor changes made and
typographical errors corrected. Experimental leptonic widths updated in
section
Improving the kinetic couplings in lattice nonrelativistic QCD
We improve the non-relativistic QCD (NRQCD) action by comparing the
dispersion relation to that of the continuum through in
perturbation theory. The one-loop matching coefficients of the
kinetic operators are determined, as well as the scale at
which to evaluate in the -scheme for each quantity. We utilise
automated lattice perturbation theory using twisted boundary conditions as an
infrared regulator. The one-loop radiative corrections to the mass
renormalisation, zero-point energy and overall energy-shift of an NRQCD
-quark are also found. We also explore how a Fat-smeared NRQCD action and
changes of the stability parameter affect the coefficients. Finally, we use
gluon field ensembles at multiple lattice spacing values, all of which include
, , and quark vacuum polarisation, to test how the improvements
affect the non-perturbatively determined and
kinetic masses, and the tuning of the quark mass
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The Hindered M1 Radiative Decay Υ(2S) → ηb(1S)γ from Lattice NRQCD
We present a calculation of the hindered M decay rate using lattice non-relativistic QCD. The calculation includes
spin-dependent relativistic corrections to the NRQCD action through
in the quark's relative velocity, relativistic corrections
to the leading order current which mediates the transition through the quark's
magnetic moment, radiative corrections to the leading spin-magnetic coupling
and for the first time a full error budget. We also use gluon field ensembles
at multiple lattice spacing values, all of which include , , and
quark vacuum polarisation. Our result for the branching fraction is
, which
agrees with the current experimental value.This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevD.92.09450
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
Neutral B-meson mixing from full lattice QCD at the physical point
We calculate the bag parameters for neutral -meson mixing in and beyond the Standard Model, in full four-flavour lattice QCD for the first time. We work on gluon field configurations that include the effect of , , and sea quarks with the Highly Improved Staggered Quark (HISQ) action at three values of the lattice spacing and with three quark masses going down to the physical value. The valence quarks use the improved NRQCD action and the valence light quarks, the HISQ action. Our analysis was blinded. Our results for the bag parameters for all five operators are the most accurate to
date. For the Standard Model operator between and mesons we find:
, . Combining our results with lattice QCD calculations of the decay constants using HISQ quarks from the Fermilab/MILC collaboration and with experimental values for and oscillation frequencies allows determination of the CKM elements and . We find , and . Our results agree well (within ) with values determined from CKM unitarity constraints based on tree-level processes (only). Using a ratio to in which CKM elements cancel in the Standard Model, we determine the branching fractions
and
. We also give results for matrix elements of the operators , and
that contribute to neutral -meson width differences.This work was funded by STFC, the Royal Society, the Wolfson Foundation and the US DOE and National Science Foundation
Resummation of transverse energy in vector boson and Higgs boson production at hadron colliders
We compute the resummed hadronic transverse energy (E_T) distribution due to
initial-state QCD radiation in vector boson and Higgs boson production at
hadron colliders. The resummed exponent, parton distributions and coefficient
functions are treated consistently to next-to-leading order. The results are
matched to fixed-order calculations at large E_T and compared with
parton-shower Monte Carlo predictions at Tevatron and LHC energies.Comment: 24 pages, 15 figure
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