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Aspects of QCD on the lattice: Scaling and hadron masses. / Aurora Trivini

Swansea University Author: Aurora Trivini

Abstract

This thesis explores different aspects of QCD and QCD-like theories through the use of Lattice Gauge Theory (LGT). This work is composed of three different projects: the first one investigating the scaling behaviour of pure SU(N) gauge theories; the other two investigating and calculating hadron mas...

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Published: 2007
Institution: Swansea University
Degree level: Doctoral
Degree name: Ph.D
URI: https://cronfa.swan.ac.uk/Record/cronfa42604
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Abstract: This thesis explores different aspects of QCD and QCD-like theories through the use of Lattice Gauge Theory (LGT). This work is composed of three different projects: the first one investigating the scaling behaviour of pure SU(N) gauge theories; the other two investigating and calculating hadron masses, using Domain Wall gauge configurations. In the first project (chapter 3) the Lattice-Distorted Perturbation Theory approach to the lack of asymptotic scaling for Monte Carlo data is described. Quenched Monte Carlo data from different sources, different observables and different gauge actions are considered. The main purpose of our calculations is to show the importance of lattice corrections in the relationship between the running coupling g2(a) and the lattice spacing a, where g2(a) is either the bare lattice coupling or some renormalized one. We show not only that the O(an) corrections are not negligible, but also that their coefficients turn out to be the order we expect. We consider a parametrization of the lattice beta-function which includes both the O(an) corrections and higher order terms in since the observed lack of asymptotic scaling is presumably due to a mixture of the two effects: the presence of lattice artefacts due to the finiteness of a and the inclusion of only a finite number of terms in the perturbative expansion (renormalized coupling approach). We study these two effects both together and separately. Furthermore, we apply our approach to SU(N) data for the number of colours N ≠ 3, and show the validity of the Lattice-Distorted PT method in particular at large N. All our investigations lead to a remarkable consistency both in the fitted A parameters and in the size of the O(an) corrections. The second part of this work (chapter 4) uses lattice QCD for spectroscopy studies with Domain Wall gauge configurations generated with the Iwasaki gauge action by the RBC-UKQCD collaboration, at fixed lattice spacing. In particular, a study of nucleon masses is presented on two different lattice sizes. The calculation of the nucleon mass is a very good test for LGT since its value can be compared with the one very accurately determined by experiments. Furthermore, LGT can be a powerful predictive tool for other baryonic states whose experimental values are not known as accurately as the nucleon one. Here we present results using correlation functions at unitary, mval = msea and non-unitary points, multiple sources per configuration and several different types of smearing to improve the signal. Firstly, we determine the masses for each channel via a fit to individual correlators; then in order to increase the precision of the result, we fit the correlation functions simultaneously to the same mass M for each channel. The study also includes the nucleon parity partner, N*. In chapter 5 preliminary results are presented for a study of the Dg meson spectrum. The discoveries of new resonances Ds some years ago have provoked much interest in heavy-light systems in general and in the DsJ mesons in particular. The existence and properties of the multiplet {lcub}0+,1+{rcub},partner of the {lcub}0-,1-{rcub}were predicted from the theory before its discovery. In fact the mass splittings between these different states can be understood in terms of a combination of heavy quark and chiral symmetry. In particular, the quantities mainly investigated in Ds calculations are the parity and hyperfine splittings, in order to compare them with their experimental values, and their independence on the spin and on the parity respectively. We carry out this study using dynamical domain wall configurations. The novelty of our study is considering the charm quark propagating as an overlap fermion and the strange quark as a domain wall fermion. Also a quenched calculation is performed with identical valence quarks as the dynamical case in order to investigate sea quark effects.
Keywords: Theoretical physics.
College: College of Science