39,226 research outputs found

### Tadpole renormalization and relativistic corrections in lattice NRQCD

We make a comparison of two tadpole renormalization schemes in the context of
the quarkonium hyperfine splittings in lattice NRQCD. Improved gauge-field and
NRQCD actions are analyzed using the mean-link $u_{0,L}$ in Landau gauge, and
using the fourth root of the average plaquette $u_{0,P}$. Simulations are done
for $c\bar c$, $b\bar c$, and $b\bar b$ systems. The hyperfine splittings are
computed both at leading and at next-to-leading order in the relativistic
expansion. Results are obtained at lattice spacings in the range of about
0.14~fm to 0.38~fm. A number of features emerge, all of which favor tadpole
renormalization using $u_{0,L}$. This includes much better scaling behavior of
the hyperfine splittings in the three quarkonium systems when $u_{0,L}$ is
used. We also find that relativistic corrections to the spin splittings are
smaller when $u_{0,L}$ is used, particularly for the $c\bar c$ and $b\bar c$
systems. We also see signs of a breakdown in the NRQCD expansion when the bare
quark mass falls below about one in lattice units. Simulations with $u_{0,L}$
also appear to be better behaved in this context: the bare quark masses turn
out to be larger when $u_{0,L}$ is used, compared to when $u_{0,P}$ is used on
lattices with comparable spacings. These results also demonstrate the need to
go beyond tree-level tadpole improvement for precision simulations.Comment: 14 pages, 7 figures (minor changes to some phraseology and
references

### Quarkonium spin structure in lattice NRQCD

Numerical simulations of the quarkonium spin splittings are done in the
framework of lattice nonrelativistic quantum chromodynamics (NRQCD). At leading
order in the velocity expansion the spin splittings are of $O(M_Q v^4)$, where
$M_Q$ is the renormalized quark mass and $v^2$ is the mean squared quark
velocity. A systematic analysis is done of all next-to-leading order
corrections. This includes the addition of $O(M_Q v^6)$ relativistic
interactions, and the removal of $O(a^2 M_Q v^4)$ discretization errors in the
leading-order interactions. Simulations are done for both S- and P-wave mesons,
with a variety of heavy quark actions and over a wide range of lattice
spacings. Two prescriptions for the tadpole improvement of the action are also
studied in detail: one using the measured value of the average plaquette, the
other using the mean link measured in Landau gauge. Next-to-leading order
interactions result in a very large reduction in the charmonium splittings,
down by about 60% from their values at leading order. There are further
indications that the velocity expansion may be poorly convergent for
charmonium. Prelimary results show a small correction to the hyperfine
splitting in the Upsilon system.Comment: 16 pages, REVTEX v3.1, 5 postscript figures include

### 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 $b$ quarks and Highly Improved Staggered light
quarks in lattice Quantum Chromodynamics (QCD) through $\mathcal{O}(\alpha_s)$
and $\Lambda_{\text{QCD}}/m_b$. We use matrix elements of these operators to
extract $B$ 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 $B$ meson
decay constants and form factors. We provide a new value for the $B$ and $B_s$
meson decay constants from lattice QCD calculations on ensembles that include
$u$, $d$, $s$ and $c$ quarks in the sea and those which have the $u/d$ quark
mass going down to its physical value. Our results are $f_B=0.196(6)$ GeV,
$f_{B_s}=0.236(7)$ GeV and $f_{B_s}/f_B =1.207(7)$, agreeing well with earlier
results using the temporal axial current. By combining with these previous
results, we provide updated values of $f_B=0.190(4)$ GeV, $f_{B_s}=0.229(5)$
GeV and $f_{B_s}/f_B = 1.206(5)$.Comment: 14 pages, 10 figure

### Precision Charmonium Spectroscopy From Lattice QCD

We present results for Charmonium spectroscopy using Non-Relativistic QCD
(NRQCD). For the NRQCD action the leading order spin-dependent and next to
leading order spin-independent interactions have been included with
tadpole-improved coefficients. We use multi-exponential fits to multiple
correlation functions to extract ground and excited $S$ states. Splittings
between the lowest $S$, $P$ and $D$ states are given and we have accurate
values for the $S$ state hyperfine splitting and the $\chi_c$ fine structure.
Agreement with experiment is good - the remaining systematic errors are
discussed.Comment: 23 pages uuencoded latex file. Contains figures in late

### D to K and D to pi semileptonic form factors from Lattice QCD

We present a 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 Quark action (HISQ) for both the charm and the strange and
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 semileptonic
decays, including D to pi and D to K. By using a phased boundary condition we
are able to tune accurately to q^2=0 and explore the whole q^2 range allowed by
kinematics. We can thus compare the shape in q^2 to that from experiment and
extract the CKM matrix element |V_cs|. We show that the form factors are
insensitive to the spectator quark: D to K and D_s to eta_s form factors are
essentially the same, which is also true for D to pi and D_s to K within 5%.
This has important implications when considering the corresponding B/B_s
processes.Comment: To appear in the proceedings of The 5th International Workshop on
Charm Physics (Charm 2012

### NRQCD results on the MILC extra coarse ensemble

We present preliminary results using NRQCD to describe heavy quarks on the
MILC 2+1 flavour dynamical extra coarse ensemble. We calculate the spectra of
low lying states in bottomonium to complement earlier results on the finer MILC
ensembles. We then exploit the coarseness of the lattices to calculate charm
propagators using NRQCD. These are used to examine the charmonium spectrum and
to calclate the mass of the $B_c$ using NRQCD. Finally we look breifly at the
$B_d$ and $B_s$ systems using the imporoved staggered formalism to describe the
light valence quarks.Comment: 6 pages, Talk presented at Lattice 2005 (Heavy Quarks), Dublin, 25-30
July 200

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