224 research outputs found
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
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
Effective Field Theory for Few-Nucleon Systems
We review the effective field theories (EFTs) developed for few-nucleon
systems. These EFTs are controlled expansions in momenta, where certain
(leading-order) interactions are summed to all orders. At low energies, an EFT
with only contact interactions allows a detailed analysis of renormalization in
a non-perturbative context and uncovers novel asymptotic behavior. Manifestly
model-independent calculations can be carried out to high orders, leading to
high precision. At higher energies, an EFT that includes pion fields justifies
and extends the traditional framework of phenomenological potentials. The
correct treatment of QCD symmetries ensures a connection with lattice QCD.
Several tests and prospects of these EFTs are discussed.Comment: 55 pages, 18 figures, to appear in Ann. Rev. Nucl. Part. Sci. 52
(2002
Renormalisation of heavy-light light ray operators
We calculate the renormalisation of different light ray operators with one
light degree of freedom and a static heavy quark. Both - and
-kernels are considered. A comparison with the light-light case suggests
that the mixing with three-particle operators is solely governed by the light
degrees of freedom. Additionally we show that conformal symmetry is already
broken at the level of the one loop counterterms due to the additional
UV-renormalisation of a cusp in the two contributing Wilson-lines. This general
feature can be used to fix the -renormalisation kernels up to a
constant. Some examples for applications of our results are given.Comment: 23 pages, 5 figures; v2: changed some wording, added a few references
and one appendix concerning some subtleties related to gauge fixing and ghost
terms; v3: clarified calculation in section 3.2., added an explicit
calculation in section 5.2, corrected a few typos and one figure, added a few
comments, results unchanged, except for typesetting matches version to appear
in JHE
Determination of the quark condensate from heavy-light current-current correlators in full lattice QCD
14 pages, 4 figuresWe derive the Operator Product Expansion whose vacuum expectation value gives the time-moments of the pseudoscalar heavy-light current-current correlator up to and including terms in multiplying and terms in multiplying , where is the heavy-quark mass. Using lattice QCD results for heavy-strange correlators obtained for a variety of heavy quark masses on gluon field configurations including , and quarks in the sea at three values of the lattice spacing, we are able to show that the contribution of the strange-quark condensate to the time-moments is very substantial. We use our lattice QCD time-moments and the OPE to determine a value for the condensate, fitting the 4th, 6th, 8th and 10th time-moments simultaneously. Our result, , agrees well with HPQCD's earlier, more direct, lattice QCD determination~\cite{McNeile:2012xh}. As well as confirming that the quark condensate is close in value to the light quark condensate, this demonstrates clearly the consistency of the Operator Product Expansion for fully nonperturbative calculations of matrix elements of short-distance operators in lattice QCD
Extracting the rho meson wavefunction from HERA data
We extract the light-cone wavefunctions of the rho meson using the HERA data
on diffractive rho photoproduction. We find good agreement with predictions for
the distribution amplitude based on QCD sum rules and from the lattice. We also
find that the data prefer a transverse wavefunction with enhanced end-point
contributions.Comment: 13 pages, 7 figures, significant improvements over the original
version with a new section on distribution amplitudes adde
Two-Loop Soft Corrections and Resummation of the Thrust Distribution in the Dijet Region
The thrust distribution in electron-positron annihilation is a classical
precision QCD observable. Using renormalization group (RG) evolution in Laplace
space, we perform the resummation of logarithmically enhanced corrections in
the dijet limit, to next-to-next-to-leading logarithmic (NNLL)
accuracy. We independently derive the two-loop soft function for the thrust
distribution and extract an analytical expression for the NNLL resummation
coefficient . To combine the resummed expressions with the fixed-order
results, we derive the -matching and -matching of the NNLL
approximation to the fixed-order NNLO distribution.Comment: 50 pages, 12 figures, 1 table. Few minor changes. Version accepted
for publication in JHE
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