Effects of non-perturbatively improved dynamical fermions in UKQCD simulations

We present results for QCD with 2 degenerate flavours of quark using a non-perturbatively improved action on a lattice volume of $16^3\times32$ where the bare gauge coupling and bare dynamical quark mass have been chosen to maintain a fixed physical lattice spacing and volume (1.71 fm). By comparing measurements from these matched ensembles, including quenched ones, we find evidence of dynamical quark effects on the short distance static potential, the scalar glueball mass and the topological susceptibility. There is little evidence of effects on the light hadron spectrum over the range of quark masses studied ($m_{\pi}/m_{\rho}\geq 0.60$).Comment: Lattice 2000 (Spectrum and quark masses), 4 pages, 5 figure

Strangeness-exchange interactions at high energies

In Chapter 1 we discuss the importance of a study of high energy strangeness-exchange reactions in elucidating the exchange mechanisms and symmetry properties of meson-baryon scattering. The cross-section and polarisation data for KN→πΣ, Δ and πKN→πΣ, Δ are reviewed in Chapter 2. An exchange-degenerate poles with weak cuts model for the data is proposed, and the resulting successful polarisation and unsuccessful cross-section descriptions are discussed. The model's failure to reproduce the well-known line reversal cross-section inequality at intermediate energies is interpreted in terms of low energy resonance effects. A unified treatment of non-zero exotic exchange processes and line reversal symmetry violation in terms of Regge-Regge cuts is presented in Chapter 3. After a theoretical appraisal of a multiple scattering approximation for these cuts we present explicit calculations of the exotic strangeness-exchange cross- sections for K(^-)p-π(^+)Σ(^-) and π(^-)p→K(^+)Σ(^-). It is shown that the small size of the data for these is not easily explained. Since we also show that the inclusion of non-leading cuts aggravates the disagreement between data and the leading cuts model, the usefulness of Regge-Regge cut descriptions at intermediate energies is called into question

Approximate actions for dynamical fermions

Recent developments and applications of approximate actions for full lattice QCD are described. We present first results based on the stochastic estimation of the fermion determinant on $12^3\times 24$ configurations at $\beta=5.2$.Comment: 3 pages, Latex, no figures, Contribution to Lattice 97, The XV International Symposium on Lattice Field Theory, Edinburgh 22-26 July 199

Unquenching the Schwinger Model (revised)

We study the quenched and unquenched lattice Schwinger model with Wilson fermions. The lowest non-trivial order of the systematic expansion recently proposed by Sexton and Weingarten is shown to allow good estimates of long distance physics from quenched configurations. Results for the static potential and the lowest bound state mass are presented.Comment: 4 pages, 2 figures, self-unpacking uuencoded compressed postscript Contribution to Lattice 95 [Revision: value corrected on p.3

Scalar glueball and meson spectroscopy in unquenched lattice QCD with improved staggered quarks

We present results of an exploratory study of singlet scalar states in unquenched QCD using both glueball and meson operators. Results for non-singlet non-strange scalar mesons are also presented. We use Asqtad improved staggered fermions and gauge configurations generated by the MILC collaboration at lattice spacings of .12 and .09 fm. In this formulation, the glueball mass is not significantly different from the quenched value at finite lattice spacing. Significant taste violations are present in the scalar sector. At light quark masses, decay channels complicate the mass determinations. There is some evidence that the non-strange singlet meson lies below the non-singlet meson.Comment: Lattice 2005 (hadron spectrum and quark masses), 6 pages, 4 figure

Pseudoscalar singlet physics with staggered fermions

We report on progress in measuring disconnected correlators associated with pseudoscalar flavor-singlet mesons. This will eventually allow us to compute the masses of the eta and eta' mesons. Flavor-singlet physics also presents an interesting test of the staggered fermion formulation, as disconnected correlators are sensitive to whether the same action governs both sea quarks and valence quarks. It can also help test the validity of the ``fourth-root trick'' used in unquenched lattice calculations where the number of flavors $N_f<4$.Comment: Talk presented at Lattice 2005 (Hadron spectrum and quark masses), 6 pages, 3 figure

Glueball mass measurements from improved staggered fermion simulations

We present the first 2+1 flavour spectrum measurements of glueball states using high statistics simulations with improved staggered fermions. We find a spectrum consistent with quenched measurements of scalar, pseudoscalar andtensor glueball states. The measurements were made using 5000 configurations at a lattice spacing of 0.123 fm and pion mass of 280 MeV and 3000 configurations at 0.092 fm with a pion mass of 360 MeV. We see some evidence of coupling to 2 pion states. We compare our results with the experimental glueball candidate spectrum as well as quenched glueball estimates.Comment: 22 pages, 19 figures and 8 tables, minor additions on mixing post-refere

Tuning Actions and Observables in Lattice QCD

We propose a strategy for conducting lattice QCD simulations at fixed volume but variable quark mass so as to investigate the physical effects of dynamical fermions. We present details of techniques which enable this to be carried out effectively, namely the tuning in bare parameter space and efficient stochastic estimation of the fermion determinant. Preliminary results and tests of the method are presented. We discuss further possible applications of these techniques.Comment: 17 pages, 4 eps figures; affiliation correction in this header + minor post-referee addition

Parallel Tempering in Lattice QCD with O(a)-Improved Wilson Fermions

We present details of our investigation of the Parallel Tempering algorithm. We consider the application of action matching technology to the selection of parameters. We then present a simple model of the autocorrelations for a particular parallel tempered system. Finally we present numerical results from applying the algorithm to lattice QCD with O(a)-improved dynamical fermions with twin ensemble system