588 research outputs found
Excitations of the nucleon with dynamical fermions
We measure the spectrum of low-lying nucleon resonances using Bayesian
fitting methods. We compare the masses obtained in the quenched approximation
to those obtained with two flavours of dynamical fermions at a matched lattice
spacing. At the pion masses employed in our simulations, we find that the mass
of the first positive-parity nucleon excitation is always greater than that of
the parity partner of the nucleon.Comment: Lattice2002(spectrum) 3 pages, 4 figure
N* Spectrum using an O(a)-Improved Fermion Action
The construction of operators and calculational methods for the determination
of the N* spectrum are discussed. The masses of the parity partners of the
nucleon and delta are computed from the O(a)-improved data of the UKQCD
Collaboration, and a clear splitting observed between the mass of the nucleon
and its parity partner.Comment: Lattice 2000 (subject), 4 pages, 5 postscript figure
Improved Landau Gauge Fixing and Discretisation Errors
Lattice discretisation errors in the Landau gauge condition are examined. An
improved gauge fixing algorithm in which order a^2 errors are removed is
presented. Order a^2 improvement of the gauge fixing condition displays the
secondary benefit of reducing the size of higher-order errors. These results
emphasise the importance of implementing an improved gauge fixing condition.Comment: LATTICE99 (Improvement and Renormalization), 3 pages, 1 figur
The Landshoff-Nachtmann Pomeron on the Lattice
We investigate the Landshoff-Nachtmann two-gluon-exchange model of the
Pomeron using gluon propagators computed in the Landau gauge within quenched
lattice QCD calculations. We first determine an effective gluon-quark coupling
by constraining the Pomeron-quark coupling to its phenomenological value
\beta_0 = 2\, \gev^{-1}. We then provide predictions for a variety of
diffractive processes. As the propagators have been evaluated entirely from QCD
first principles (although in the quenched approximation), our results provide
a consistency check of the Landshoff-Nachtmann model. We address the issue of
the possible gauge-dependence of our results, which will be the object of a
future study.Comment: uuencoded, compressed tar file, 13 pages latex, 4 Postscript figures,
requires epsf.st
Assessing the future threat from vivax malaria in the United Kingdom using two markedly different modelling approaches
Background: The world is facing an increased threat from new and emerging diseases, and there is concern that climate change will expand areas suitable for transmission of vector borne diseases. The likelihood of vivax malaria returning to the UK was explored using two markedly different modelling approaches. First, a simple temperature-dependent, process-based model of malaria growth transmitted by Anopheles atroparvus, the historical vector of malaria in the UK. Second, a statistical model using logistic-regression was used to predict historical malaria incidence between 1917 and 1918 in the UK, based on environmental and demographic data. Using findings from these models and saltmarsh distributions, future risk maps for malaria in the UK were produced based on UKCIP02 climate change scenarios. Results: The process-based model of climate suitability showed good correspondence with historical records of malaria cases. An analysis of the statistical models showed that mean temperature of the warmest month of the year was the major factor explaining the distribution of malaria, further supporting the use of the temperature-driven processed-based model. The risk maps indicate that large areas of central and southern England could support malaria transmission today and could increase in extent in the future. Confidence in these predictions is increased by the concordance between the processed-based and statistical models. Conclusion: Although the future climate in the UK is favourable for the transmission of vivax malaria, the future risk of locally transmitted malaria is considered low because of low vector biting rates and the low probability of vectors feeding on a malaria-infected person
Excited nucleon spectrum using a non-perturbatively improved clover fermion action
We discuss the extraction of negative-parity baryon masses from lattice QCD
calculations. The mass of the lowest-lying negative-parity state
is computed in quenched lattice QCD using an -improved clover
fermion action, and a splitting found with the nucleon mass. The calculation is
performed on two lattice volumes, and three lattice spacings enabling a study
of both finite-volume and finite-lattice-spacing uncertainties. A measurement
of the first excited radial excitation of the nucleon finds a mass considerably
larger than that of the negative-parity ground state, in accord with other
lattice determinations but in disagreement with experiment. Results are also
presented for the lightest negative-parity state.Comment: 7 pages, 5 figures, uses espcrc2. Talk presented at Workshop on
Lattice Hadron Physics, Colonial Club Resort, Cairns, Australia, July 9-18,
2001. Corrected error in determination of mass of excited, positive-parity
nucleon resonanc
An ``Improved" Lattice Study of Semi-leptonic Decays of D-Mesons
We present results of a lattice computation of the matrix elements of the
vector and axial-vector currents which are relevant for the semi-leptonic
decays and . The computations are
performed in the quenched approximation to lattice QCD on a
lattice at , using an -improved fermionic action. In the limit
of zero lepton masses the semi-leptonic decays and are described by four form factors: and ,
which are functions of , where is the four-momentum transferred
in the process. Our results for these form factors at are:
f^+_K(0)=0.67 \er{7}{8} , V(0)=1.01 \err{30}{13} , A_1(0)=0.70
\err{7}{10} , A_2(0)=0.66 \err{10}{15} , which are consistent with the most
recent experimental world average values. We have also determined the
dependence of the form factors, which we find to be reasonably well described
by a simple pole-dominance model. Results for other form factors, including
those relevant to the decays \dpi and \drho, are also given.Comment: 41 pages, uuencoded compressed postscript file containing 14 figures,
LaTeX, Edinburgh Preprint 94/546 and Southampton Preprint SHEP 93/94-3
An Investigation of the Soft Pion Relation in Quenched Lattice QCD
A lattice determination of the form factor and decay constants for the
semileptonic decay of heavy pseudoscalar (PS) mesons at zero recoil is
presented from which the soft pion relation is satisfied. Chiral extrapolation
of the form factor is performed at constant . Pole dominance is used to
extrapolate the form factor in heavy quark mass. At the B mass, the form factor
at zero recoil lies somewhat below the ratio of decay constants; the relation
remains satisfied within error.Comment: 3 pages, 2 figure
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Towards an understanding of the nuclear potential
The formalism for investigating the /bar Q/q/bar Q/q system on the lattice is constructed. We describe how the model may be used to study the nuclear potential, and present some preliminary results on the range of the nuclear force. 6 refs., 4 figs., 1 tab
Negative-parity Baryon Masses using an O(a)-improved Fermion Action
We present a calculation of the mass of the lowest-lying negative-parity
J=1/2- state in quenched QCD. Results are obtained using a non-perturbatively
O(a)-improved clover fermion action, and a splitting is found between the mass
of the nucleon and its parity partner. The calculation is performed on two
lattice volumes and at three lattice spacings, enabling a study of both
finite-volume and finite lattice-spacing uncertainties. A comparison is made
with results obtained using the unimproved Wilson fermion action.Comment: 13 pages, 8 figures, revtex Version accepted for publication in
Physics Letters B. Some changes to discussion of chiral extrapolations;
primary results unchange
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