1,533 research outputs found
QCD equation of state at non-zero chemical potential
We present our new results for the QCD equation of state at nonzero chemical
potential at N_t=6 and compare them with N_t=4. We use the Taylor expansion
method with terms up to sixth order in simulations with 2+1 flavors of improved
asqtad quarks along a line of constant physics with m_l=0.1 m_s and
approximately physical strange quark mass m_s.Comment: 7 pages, 10 figures, presented at Lattice 2008 (Nonzero Temperature
and Density), College of William and Mary, Williamsburg, V
Electromagnetic contributions to pseudoscalar masses
We report on the calculation by the MILC Collaboration of the electromagnetic effects on kaon
and pion masses. These masses are computed in QCD with dynamical (asqtad staggered) quarks
plus quenched photons at three lattice spacings varying from 0.12 to 0.06 fm. The masses are fit
to staggered chiral perturbation theory with NLO electromagnetic terms, as well as analytic terms
at higher order. We extrapolate the results to physical light-quark masses and to the continuum
limit. At the current stage of the analysis, most, but not all, of the systematic errors have been
estimated. The main goal is the comparison of kaon electromagnetic splittings to those of the
pion, i.e., an evaluation of the corrections to “Dashen’s theorem.” This in turn will allow us to
significantly reduce the systematic errors in our determination of m<sub>u</sub>/m<sub>d</sub>
Lattice QCD ensembles with four flavors of highly improved staggered quarks
We present results from our simulations of quantum chromodynamics (QCD) with
four flavors of quarks: u, d, s, and c. These simulations are performed with a
one-loop Symanzik improved gauge action, and the highly improved staggered
quark (HISQ) action. We are generating gauge configurations with four values of
the lattice spacing ranging from 0.06 fm to 0.15 fm, and three values of the
light quark mass, including the value for which the Goldstone pion mass is
equal to the physical pion mass. We discuss simulation algorithms, scale
setting, taste symmetry breaking, and the autocorrelations of various
quantities. We also present results for the topological susceptibility which
demonstrate the improvement of the HISQ configurations relative to those
generated earlier with the asqtad improved staggered action.Comment: 43 pages, 11 postscript figures, 15 tables, minor changes in text,
version published in Phys. Rev.
Leptonic decay-constant ratio f_{K^+}/f_{pi^+} from lattice QCD with physical light quarks
A calculation of the ratio of leptonic decay constants f_{K^+}/f_{\pi^+}
makes possible a precise determination of the ratio of CKM matrix elements
|V_{us}|/|V_{ud}| in the Standard Model, and places a stringent constraint on
the scale of new physics that would lead to deviations from unitarity in the
first row of the CKM matrix. We compute f_{K^+}/f_{\pi^+} numerically in
unquenched lattice QCD using gauge-field ensembles recently generated that
include four flavors of dynamical quarks: up, down, strange, and charm. We
analyze data at four lattice spacings a ~ 0.06, 0.09, 0.12, and 0.15 fm with
simulated pion masses down to the physical value 135 MeV. We obtain
f_{K^+}/f_{\pi^+} = 1.1947(26)(37), where the errors are statistical and total
systematic, respectively. This is our first physics result from our N_f = 2+1+1
ensembles, and the first calculation of f_{K^+}/f_{\pi^+} from lattice-QCD
simulations at the physical point. Our result is the most precise lattice-QCD
determination of f_{K^+}/f_{\pi^+}, with an error comparable to the current
world average. When combined with experimental measurements of the leptonic
branching fractions, it leads to a precise determination of |V_{us}|/|V_{ud}| =
0.2309(9)(4) where the errors are theoretical and experimental, respectively.Comment: 6 pages, 1 table, 2 figures; v3: result for f_{K^+}/f_{pi^+} updated
to include additional data; typo in some values of L in Table 1 corrected;
typo in sign of 1-|V_{ud}|^2-|V_{us}|^2-|V_{ub}|^2 corrected; version to be
published in Phys. Rev. Let
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