Waseem Kamleh

Isolating the Λ(1405) in lattice QCD.

The odd-parity ground state of the Λ baryon lies surprisingly low in mass. At 1405 MeV, it lies lower than the odd-parity ground-state nucleon, even though it has a valence strange quark. Using the PACS-CS (2+1)-flavor full-QCD ensembles, we employ a variational analysis using source and sink smearing to isolate this elusive state. For the first time we reproduce the correct level ordering with respect to nearby scattering thresholds. With a partially quenched strange quark to produce the appropriate kaon mass, we find a low-lying, odd-parity mass trend consistent with the experimental value.

Unquenching effects in the quark and gluon propagator

In this work we examine the fat-link irrelevant clover (FLIC) overlap quark propagator and the gluon propagator on both dynamical and quenched lattices. The tadpole-improved Luscher-Weisz gauge action is used in both cases. The dynamical gauge fields use the FLIC fermion action for the sea quark contribution. We observe that the presence of sea quarks causes a suppression of the mass function, quark renormalization function, and gluon dressing function in the infrared. The ultraviolet physics is unaffected.

Polynomial filtered HMC–an algorithm for lattice QCD with dynamical quarks

Abstract Polynomial approximations to the inverse of the fermion matrix are used to filter the dynamics of the upper energy scales in Hybrid Monte Carlo (HMC) simulations. The use of a multiple time-scale integration scheme then allows the filtered pseudofermions to be evolved using a coarse step size. The algorithm is applied to N f = 2 flavour simulations at two different quark masses. Polynomial filtering yields a reduction in the computational expense of the molecular dynamics integration of between three- and fivefold when compared to standard HMC. Significantly, the observed speedup is better at the lower quark mass.

Polynomial Filtering for HMC in Lattice QCD

The use of short order polynomial approximations to 1/x as a UV filter for HMC is investigated. Previous work in the Schwinger model (2D QED) showed that the integration step size may be increased within a multiple time scale integration by separating the UV and IR dynamics with a polynomial filter. We test the same method within the framework of Lattice QCD. c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.

Nucleon excitations in 2+1 flavor QCD

A determination of the excited state spectrum of hadrons presents a significant challenge to the first-principles approach of Lattice QCD. Here a brief overview of the correlation-matrix methods developed recently by the CSSM Lattice Collaboration for the isolation of excited states of the nucleon is presented. The utility of the method is shown by exploring the first twelve states of the nucleon in the positive parity channel. Of particular interest is the Roper resonance, the first positive-parity excited-state of the nucleon. A low-lying Roper state is observed in full QCD calculations for the first time, showing significant curvature as the chiral regime is approached. The negative parity results are also explored, where the extracted first negative parity excited state approaches the physical value.

Centre Vortices in SU(3)

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Electromagnetic Form Factors Of Nucleon Eexcitations from Lattice QCD

Variational analysis techniques in lattice QCD are powerful tools that give access to the full spectrum of QCD. At zero momentum, these techniques are well established and can cleanly isolate energy eigenstates of either positive or negative parity. In order to compute the form factors of a single energy eigenstate, we must perform a variational analysis at non-zero momentum. When we do this with baryons, we run into issues with parity mixing in the Dirac spinors, as boosted baryons are not eigenstates of parity. Due to this parity mixing, care must be taken to ensure that the projected correlation functions provided by the variational analysis correspond to the same states at zero momentum. This can be achieved through the parity-expanded variational analysis (PEVA) technique, a novel method developed at the University of Adelaide for ensuring the successful and consistent isolation of boosted baryons. Utilising this technique, we are able to compute the form factors of baryon excitations without contamination from other states. We present world-first calculations of excited state nucleon form factors using this new technique.

Electromagnetic Form Factors through Parity-Expanded Variational Analysis

Variational analysis techniques in lattice QCD are powerful tools that give access to the excited state spectrum of QCD. At zero momentum, these techniques are well established and can cleanly isolate energy eigenstates of either positive or negative parity. In order to compute the form factors of a single energy eigenstate, we must perform a variational analysis at non-zero momentum. When we do this with baryons, we run into issues with parity mixing, as boosted baryons are not eigenstates of parity. The parity-expanded variational analysis (PEVA) technique is a novel method for ensuring the successful and consistent isolation of boosted baryon eigenstates. This is achieved through a parity expansion of the operator basis used to construct the correlation matrix. World-first calculations of excited state nucleon form factors using this new technique are presented, showing the improvement over conventional methods.

Transition of ρ →πγ in lattice QCD

With the ongoing experimental interest in exploring the excited hadron spectrum, evaluations of the matrix elements describing the formation and decay of such states via radiative processes provide us with an important connection between theory and experiment. In particular, determinations obtained via the lattice allow for a direct comparison of QCD-expectation with experimental observation. Here we present the first light quark determination of the

Exploring excited states of the nucleon in 2+1 flavor lattice QCD

rho rightarrow pi gamma