75 research outputs found
Glueballs and topology with O(a)-improved lattice QCD
We present evidence for unquenching effects in N_f=2, 16^3 32 ensembles by
comparing with `equivalent' quenched data at r_0~5.0. A (small) VEV for
torelons signals (weak) string breaking. A 15-20 % reduction in the scalar
glueball mass relative to quenched is argued to be (in part at least) a
discretisation effect. We find a chiral suppression of the topological
susceptibility consistent with expectations, and agreement between fermionic
and gluonic methods for measuring the topological charge.Comment: 4pp LaTeX, 4 EPS figures. Contribution to Lattice2001(confinement
Mixing of scalar glueballs and flavour-singlet scalar mesons
We discuss in detail the extraction of hadronic mixing strengths from lattice
studies. We apply this to the mixing of a scalar glueball and a scalar meson in
the quenched approximation. We also measure correlations appropriate for
flavour-singlet scalar mesons using dynamical quark configurations from UKQCD.
This enables us to compare the results from the quenched study of the mixing
with the direct determination of the mixed spectrum. Improved methods of
evaluating the disconnected quark diagrams are also presented.Comment: 23 pages, 5 postscript figure
NNLO hard-thermal-loop thermodynamics for QCD
We calculate the thermodynamic functions of a quark-gluon plasma for general
N_c and N_f to three-loop order using hard-thermal-loop perturbation theory. At
this order, all the ultraviolet divergences can be absorbed into
renormalizations of the vacuum, the HTL mass parameters, and the strong
coupling constant.We show that at three loops, the results for the pressure and
trace anomaly are in very good agreement with recent lattice data down to
temperatures T~2T_c.Comment: 8 pages, 2 fig
Charmonium Spectrum from Quenched QCD with Overlap Fermions
We present preliminary results using overlap fermions for the charmonium
spectrum, in particular for hyperfine splitting. Simulations are performed on
lattices, with Wilson gauge action at .
Depending on how the scale is set, we obtain 104(5) MeV (using
) or 88(4) MeV (using =0.5 fm) for the hyperfine
splitting.Comment: 3 pages, 5 fiugres. Talk presented at Lattice 2004 (heavy
On the glueball spectrum in O(a)-improved lattice QCD
We calculate the light `glueball' mass spectrum in N_f=2 lattice QCD using a
fermion action that is non-perturbatively O(a) improved. We work at lattice
spacings a ~0.1 fm and with quark masses that range down to about half the
strange quark mass. We find the statistical errors to be moderate and under
control on relatively small ensembles. We compare our mass spectrum to that of
quenched QCD at the same value of a. Whilst the tensor mass is the same (within
errors), the scalar mass is significantly smaller in the dynamical lattice
theory, by a factor of ~(0.84 +/- 0.03). We discuss what the observed m_q
dependence of this suppression tells us about the dynamics of glueballs in QCD.
We also calculate the masses of flux tubes that wind around the spatial torus,
and extract the string tension from these. As we decrease the quark mass we see
a small but growing vacuum expectation value for the corresponding flux tube
operators. This provides clear evidence for `string breaking' and for the
(expected) breaking of the associated gauge centre symmetry by sea quarks.Comment: 33pp LaTeX. Version to appear in Phys. Rev.
Isolating the Roper Resonance in Lattice QCD
We present results for the first positive parity excited state of the
nucleon, namely, the Roper resonance (=1440 MeV) from a
variational analysis technique. The analysis is performed for pion masses as
low as 224 MeV in quenched QCD with the FLIC fermion action. A wide variety of
smeared-smeared correlation functions are used to construct correlation
matrices. This is done in order to find a suitable basis of operators for the
variational analysis such that eigenstates of the QCD Hamiltonian may be
isolated. A lower lying Roper state is observed that approaches the physical
Roper state.
To the best of our knowledge, the first time this state has been identified
at light quark masses using a variational approach.Comment: 7pp, 4 figures; minor typos corrected and one Ref. adde
Cornering New Physics in b --> s Transitions
We derive constraints on Wilson coefficients of dimension-six effective
operators probing the b --> s transition, using recent improved measurements of
the rare decays Bs --> mu+mu-, B --> K mu+mu- and B --> K* mu+mu- and including
all relevant observables in inclusive and exclusive decays. We consider
operators present in the SM as well as their chirality-flipped counterparts and
scalar operators. We find good agreement with the SM expectations. Compared to
the situation before winter 2012, we find significantly more stringent
constraints on the chirality-flipped coefficients due to complementary
constraints from B --> K mu+mu- and B --> K* mu+mu- and due to the LHCb
measurement of the angular observable S_3 in the latter decay. We also list the
full set of observables sensitive to new physics in the low recoil region of B
--> K* mu+mu-.Comment: 18 pages, 6 figures, 4 tables. v3: typos correcte
Heavy Quark Masses from Sum Rules in Four-Loop Approximation
New data for the total cross section in the
charm and bottom threshold region are combined with an improved theoretical
analysis, which includes recent four-loop calculations, to determine the short
distance charm and bottom quark masses. A detailed discussion of
the theoretical and experimental uncertainties is presented. The final result
for the -masses, GeV and GeV, can be translated into GeV and
GeV. This analysis is consistent with but significantly
more precise than a similar previous study.Comment: 29 page
Quantum chromodynamics with advanced computing
We survey results in lattice quantum chromodynamics from groups in the USQCD
Collaboration. The main focus is on physics, but many aspects of the discussion
are aimed at an audience of computational physicists.Comment: 17 pp. Featured presentation at Scientific Discovery with Advanced
Computing, July 13-17, Seattl
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