Kieran Holland

Quenched spectroscopy with fixed-point and chirally improved fermions

Abstract We present results from quenched spectroscopy calculations with the parametrized fixed-point and the chirally improved Dirac operators. Both these operators are approximate solutions of the Ginsparg–Wilson equation and have good chiral properties. This allows us to work at small quark masses and we explore pseudoscalar-mass to vector-mass ratios down to 0.28. We discuss meson and baryon masses, their scaling properties, finite volume effects and compare our results with recent large scale simulations. We find that the size of quenching artifacts of the masses is strongly correlated with their experimentally observed widths and that the gauge and hadronic scales are consistent.

Testing the fixed point QCD action and the construction of chiral currents

Abstract We present the first set of quenched QCD measurements using the recently parametrized fixed-point Dirac operator D FP . We also give a general and practical construction of covariant densities and conserved currents for chiral lattice actions. The measurements include (a) hadron spectroscopy, (b)xa0corrections of small chiral deviations, (c) the renormalized quark condensate from finite-size scaling and, independently, spectroscopy, (d) the topological susceptibility, (e) small eigenvalue distributions and random matrix theory, and (f) local chirality of near-zero modes and instanton-dominance.

The deconfinement phase transition of Sp(2) and Sp(3) Yang–Mills theories in 2+1 and 3+1 dimensions

Abstract Some time ago, Svetitsky and Yaffe have argued that—if the deconfinement phase transition of a ( d +1)-dimensional Yang–Mills theory with gauge group G is second order—it should be in the universality class of a d -dimensional spin model symmetric under the center of G . For d =3 these arguments have been confirmed numerically only in the SU (2) case with center Z (2) , simply because all SU ( N ) Yang–Mills theories with N ⩾3 seem to have non-universal first order phase transitions. The symplectic groups Sp ( N ) also have the center Z (2) and provide another extension of SU (2)= Sp (1) to general N . Using lattice simulations, we find that the deconfinement phase transition of Sp (2) Yang–Mills theory is first order in 3+1 dimensions, while in 2+1 dimensions stronger fluctuations induce a second order transition. In agreement with the Svetitsky–Yaffe conjecture, for (2+1)d Sp (2) Yang–Mills theory we find the universal critical behavior of the 2d Ising model. For Sp (3) Yang–Mills theory the transition is first order both in 2+1 and in 3+1 dimensions. This suggests that the size of the gauge group—and not the center symmetry—determines the order of the deconfinement phase transition.

Probing technicolor theories with staggered fermions

One exciting possibility of new physics beyond the Standard Model is that the fundamental Higgs sector is replaced by a strongly-interacting gauge theory, known as technicolor. A viable theory must break chiral symmetry dynamically, like in QCD, to generate Goldstone bosons which become the longitudinal components of the W ± and Z. By measuring the eigenvalues of the Dirac operator, one can determine if chiral symmetry is in fact spontaneously broken. We simulate SU(3) gauge theory with ns = 2 and 3 staggered flavors in the fundamental representation, corresponding to Nf = 8 and 12 flavors in the continuum limit. Although our first findi ngs show that both theories are consistent with dynamically broken chiral symmetry and QCD-like behavior, flavor breaking effects in the spectrum may require further c larifications before final conclusions can be drawn. We also compare various improved staggered actions, to suppress this potentially large flavor breaking.

The gradient flow running coupling scheme

The Yang-Mills gradient flow in finite volume is used to define a running coupling scheme. As our main result the discrete β -function, or step scaling function, is calculated for scale change s = 3/2 at several lattice spacings for SU(3) gauge theory coupled to N f = 4 fundamental massless fermions. The continuum extrapolation is performed and agreement is found with the continuum perturbative results for small renormalized coupling. The case of SU(2) gauge group is briefly commented on.

Another weak first order deconfinement transition: three-dimensional SU(5) gauge theory

We examine the finite-temperature deconfinement phase transition of (2+1)-dimensional SU(5) Yang-Mills theory via non-perturbative lattice simulations. Unsurprisingly, we find that the transition is of first order, however it appears to be weak. This fits naturally into the general picture of ``large gauge groups having a first order deconfinement transition, even when the center symmetry associated with the transition might suggest otherwise.

Can a light Higgs impostor hide in composite gauge models

The frequently discussed strongly interacting gauge theory with a fermion flavor doublet in the two-index symmetric (sextet) representation of the SU(3) color gauge group is investigated. In previous studies [1] the chiral condensate and the mass spectrum were shown to be consistent with chiral symmetry breaking (χSB) at vanishing fermion mass. The recently reported β -function [2] is not inconsistent with this observation, suggesting that the model is very close to the conformal window and a light “Higgs impostor” could emerge as a composite state. In this work we describe the methodology and preliminary results of studying the emergence of the light composite scalar with 0++ quantum numbers.

Chiral symmetry breaking in fundamental and sextet fermion representations of SU(3) color

We report new results for lattice gauge theories with twelve fermion flavors in the fundamental representation and two fermion flavors in the two-index symmetric (sextet) representation of the SU(3) color gauge group. Both models are important in searching for a viable composite Higgs mechanism in the Beyond the Standard Model (BSM) paradigm. We subject both models to opposite hypotheses inside and outside of the conformal window. In the first hypothesis we test chiral symmetry breaking (

New Higgs physics from the lattice

chi{rm SB}

Triviality and the Higgs mass lower bound

) with its Goldstone spectrum,