37 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
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
2+1 flavor simulations of QCD with improved staggered quarks
The MILC collaboration has been performing realistic simulations of full QCD
with 2+1 flavors of improved staggered quarks. Our simulations allow for
controlled continuum and chiral extrapolations. I present results for the light
pseudoscalar sector: masses and decay constants, quark masses and
Gasser-Leutwyler low-energy constants. In addition I will present some results
for heavy-light mesons, decay constants and semileptonic form factors, obtained
in collaboration with the HPQCD and Fermilab lattice collaborations. Such
calculations will help in the extraction of CKM matrix elements from
experimental measurements.Comment: To appear in the proceedings of QNP06, IVth International Conference
on Quarks and Nuclear Physics, Madrid, June 200
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
Heavy-light current-current correlators
The current-current correlator method has been used successfully to obtain
very accurate results for quark masses and the coupling alpha_s. The
calculations were done using Highly Improved Staggered Quarks (HISQ) and
heavy-heavy meson correlators. We now extend this work to the significantly
more challenging heavy-light case, reporting the first results here. The aim is
to determine nonperturbative Z factors for NRQCD heavy-light currents, but
first we test the method in the HISQ case where Z=1.Comment: 7 pages. Presented at the XXVIII International Symposium on Lattice
Field Theory (Lattice 2010), June 14-19 2010, Villasimius, Ital
The D to K and D to pi semileptonic decay form factors from Lattice QCD
We present a new and very high statistics study of D and D_s semileptonic
decay form factors on the lattice. We work with MILC N_f=2+1 lattices and use
the Highly Improved Staggered Action (HISQ) for both the charm and the light
valence quarks. We use both scalar and vector currents to determine the form
factors f_0(q^2) and f_+(q^2) for a range of D and D_s form factors including
those for D to pi and D to K semileptonic decays. By using a phased boundary
condition we are able to tune accurately to q^2=0. We also compare the shape in
q^2 to that from experiment. We show that the form factors are very insensitive
to the spectator quark: D to K and D_s to eta_s form factors are essentially
the same, and the same is true for D to pi and D_s to K. This has important
implications when considering the corresponding B/B_s processes.Comment: 7 pages, 9 figures, talk given at the XXIX International Symposium on
Lattice Field Theory, July 10-16, 2011, Squaw Valley, Lake Tahoe, C
Light hadrons in 2+1 flavor lattice QCD
This talk will focus on recent results by the MILC collaboration from
simulations of light hadrons in 2+1 flavor lattice QCD. We have achieved high
precision results in the pseudoscalar sector, including masses and decay
constants, plus quark masses and Gasser-Leutwyler parameters from well
controlled chiral perturbation theory fits to our data. We also show
spectroscopy results for vector mesons and baryons.Comment: To appear in the proceedings of the First Meeting of the APS Topical
Group on Hadronic Physics, Fermilab, Batavia, Illinois, Oct. 24-26, 200
Light-quark connected intermediate-window contributions to the muon g â 2 hadronic vacuum polarization from lattice QCD
We present a lattice-QCD calculation of the light-quark connected contribution to window observables
associated with the leading-order hadronic vacuum polarization contribution to the anomalous magnetic
moment of the muon, aHVP;LO
ÎŒ . We employ the MILC Collaborationâs isospin-symmetric QCD gauge-field
ensembles, which contain four flavors of dynamical highly improved staggered quarks with four lattice
spacings between a â 0.06â0.15 fm and close-to-physical quark masses. We consider several effectivefield-
theory-based schemes for finite volume and other lattice corrections and combine the results via
Bayesian model averaging to obtain robust estimates of the associated systematic uncertainties. After
unblinding, our final results for the intermediate and âW2â windows are all;W
ÎŒ Ă°connĂ ÂŒ 206.6Ă°1.0Ă Ă 10â10
and all;W2
ÎŒ Ă°connĂ ÂŒ 100.7Ă°3.2Ă Ă 10â10, respectivelyUnited States Department of Energy (DOE)National Science Foundation (NSF) ACI-1548562
CNS-0521433
ACI-1532235
ACI-1532236
OCI-0725070
ACI-1238993
PHY17-19626
PHY20-13064
DGE 2040434Lilly Endowment, Inc.University of Colorado BoulderColorado State UniversityState of IllinoisBEISUK Research & Innovation (UKRI)
Science & Technology Facilities Council (STFC) ST/P002307/1
ST/R002452/1
ST/R00689X/1United States Department of Energy (DOE) DE-SC0010005
DE-SC0010120
DE-SC0011090
DE-SC0021006
DE-SC0015655Funding Opportunity Announcement Scientific Discovery through Advanced Computing: High Energy Physics LAB 22-2580Simons Foundation under their Simons Fellows in Theoretical Physics programUniversities Research Association Visiting Scholarship Awards 20-S-12
21-S-05SRA (Spain) PID2019-106087 GB-C21Junta de Andalucia FQM-101
A-FQM-467-UGR18
P18-FR-4314Spanish Government RYC2020-030244-IUK Research & Innovation (UKRI)
Science & Technology Facilities Council (STFC) ST/T000945/1Fermi Research Alliance, LLC (FRA) DE-AC02-07CH1135
Operator Relations for SU(3) Breaking Contributions to K and K* Distribution Amplitudes
We derive constraints on the asymmetry a1 of the momentum fractions carried
by quark and antiquark in K and K* mesons in leading twist. These constraints
follow from exact operator identities and relate a1 to SU(3) breaking
quark-antiquark-gluon matrix elements which we determine from QCD sum rules.
Comparing our results to determinations of a1 from QCD sum rules based on
correlation functions of quark currents, we find that, for a1^\parallel(K*) the
central values agree well and come with moderate errors, whereas for a1(K) and
a1^\perp(K*) the results from operator relations are consistent with those from
quark current sum rules, but come with larger uncertainties. The consistency of
results confirms that the QCD sum rule method is indeed suitable for the
calculation of a1. We conclude that the presently most accurate predictions for
a1 come from the direct determination from QCD sum rules based on correlation
functions of quark currents and are given by: a1(K) = 0.06\pm 0.03,
a1^\parallel(K*) = 0.03\pm 0.02, a1^\perp(K*) = 0.04\pm 0.03.Comment: 21 page