arXiv subjects

A. Hart

Publications and source records attributed to A. Hart.

At least 37 records · Page 2Linked to original sources

arXiv2004

The locality of the square-root method for improved staggered quarks

We study the effects of improvement on the locality of square-rooted staggered Dirac operators in lattice QCD simulations. We find the localisation lengths of the improved operators (FAT7TAD and ASQTAD) to be very similar to that of the one-link operator studied by Bunk et al., being at least the Compton wavelength of the lightest particle in the theory, even in the continuum limit. We conclude that improvement has no effect. We discuss the implications of this result for the locality of the nth-rooted fermion determinant used to reduce the number of sea quark flavours, and for possible staggered valence quark formulations.

hep-lat↗

arXiv2004

Improvement and Taste Symmetry Breaking for Staggered Quarks

We compare several improved actions for staggered quarks. We study the effect of improvement on the taste changing interactions by calculating the splitting in the pion spectrum. We investigate the effect of the improvement on some topological properties.

hep-lat↗

arXiv2004

The Index Theorem and Universality Properties of the Low-lying Eigenvalues of Improved Staggered Quarks

We study various improved staggered quark Dirac operators on quenched gluon backgrounds in lattice QCD generated using a Symanzik-improved gluon action. We find a clear separation of the spectrum into would-be zero modes and others. The number of would-be zero modes depends on the topological charge as expected from the Index Theorem, and their chirality expectation value is large (approximately 0.7). The remaining modes have low chirality and show clear signs of clustering into quartets and approaching the random matrix theory predictions for all topological charge sectors. We conclude that improvement of the fermionic and gauge actions moves the staggered quarks closer to the continuum limit where they respond correctly to QCD topology.

hep-lat↗

arXiv2004

Perturbation theory vs. simulation for tadpole improvement factors in pure gauge theories

We calculate the mean link in Landau gauge for Wilson and improved SU(3) anisotropic gauge actions, using two loop perturbation theory and Monte Carlo simulation employing an accelerated Langevin algorithm. Twisted boundary conditions are employed, with a twist in all four lattice directions considerably improving the (Fourier accelerated) convergence to an improved lattice Landau gauge. Two loop perturbation theory is seen to predict the mean link extremely well even into the region of commonly simulated gauge couplings and so can be used remove the need for numerical tuning of self-consistent tadpole improvement factors. A three loop perturbative coefficient is inferred from the simulations and is found to be small. We show that finite size effects are small and argue likewise for (lattice) Gribov copies and double Dirac sheets.

hep-lat↗

arXiv2004

Discretisation effects in the topological susceptibility in lattice QCD

We study the topological susceptibility, chi, in QCD with two quark flavours using lattice field configurations that have been produced with an O(a)-improved clover quark action. We find clear evidence for the expected suppression at small quark mass, m_q, and examine the variation of $χ$ with this mass and the lattice spacing, a. A joint continuum and chiral extrapolation yields good agreement with theoretical expectations as (a, m_q) -> 0. A moderate increase in autocorrelation is observed on the more chiral ensembles, but within large statistical errors. Finite volume effects are negligible for Leutwyler-Smilga parameter x_LS > 10, and no evidence for a nearby phase transition is observed.

hep-lat↗

arXiv2003

Topological susceptibility with the improved Asqtad action

Chiral perturbation theory predicts that in quantum chromodymamics light dynamical quarks suppress the topological (instanton) susceptibility. We investigate this suppression through direct numerical simulation using the Asqtad improved lattice fermion action. This action holds promise for carrying out nonperturbative simulations over a range of quark masses for which chiral perturbation theory is expected to converge. To test the effectiveness of the action in capturing instanton physics, we measure the topological susceptibility as a function of quark masses with 2+1 dynamical flavors. Our results, when extrapolated to zero lattice spacing, are consistent with predictions of leading order chiral perturbation theory. Included in our study is a comparison of three methods for analyzing the topological susceptibility: (1) the Boulder hypercubic blocking technique with the Boulder topological charge operator, (2) the more traditional Wilson cooling method with the twisted plaquette topological charge operator and (3) the improved cooling method of de Forcrand, Perez, and Stamatescu and their improved topological charge operator.

hep-lat↗

arXiv2003

The contribution of O(alpha) radiative corrections to the renormalised anisotropy and application to general tadpole improvement schemes: addendum to "One loop calculation of the renormalised anisotropy for improved anisotropic gluon actions on a lattice" [hep-lat/0208010]

General O(alpha) radiative corrections to lattice actions may be interpreted as counterterms that give additive contributions to the one-loop renormalisation of the anisotropy. The effect of changing the radiative coefficients is thus easily calculable. In particular, the results obtained in a previous paper for Landau mean link improved actions apply in any tadpole improvement scheme. We explain how this method can be exploited when tuning radiatively improved actions. Efficient methods for self-consistently tuning tadpole improvement factors are also discussed.

hep-lat↗

arXiv2002

The monopole mass in the three-dimensional Georgi-Glashow model

We study the three-dimensional Georgi-Glashow model to demonstrate how magnetic monopoles can be studied fully non-perturbatively in lattice Monte Carlo simulations, without any assumptions about the smoothness of the field configurations. We examine the apparent contradiction between the conjectured analytic connection of the `broken' and `symmetric' phases, and the interpretation of the mass (i.e., the free energy) of the fully quantised 't Hooft-Polyakov monopole as an order parameter to distinguish the phases. We use Monte Carlo simulations to measure the monopole free energy and its first derivative with respect to the scalar mass. On small volumes we compare this to semi-classical predictions for the monopole. On large volumes we show that the free energy is screened to zero, signalling the formation of a confining monopole condensate. This screening does not allow the monopole mass to be interpreted as an order parameter, resolving the paradox.

hep-lat↗

arXiv2002

One loop calculation of the renormalised anisotropy for improved anisotropic gluon actions on a lattice

Using the infrared dispersion relation of the on shell gluon, we calculate the renormalisation of the the anisotropy to one loop in perturbation theory for lattice Yang-Mills theories, including the Wilson action and actions with Symanzik and/or tadpole improvement. Using twisted boundary conditions as a gauge invariant infrared regulator, we show for an SU(3) gauge group in D=3+1 dimensions that the one loop anisotropy is accurate to O(3%) for a range of g^2 and chi covering current simulations. In doing so we also present Feynman rules for SU(N) gauge groups with generic anisotropy structure (including `3+1' and `2+2' cases).

hep-lat↗

arXiv2002

Lattice perturbation theory for gluonic and fermionic actions

We calculate the two loop Landau mean links and the one loop renormalisation of the anisotropy for Wilson and improved SU(3) gauge actions, using twisted boundary conditions as a gauge invariant infrared regulator. We show these accurately describe simulated results, and outline a method for generating Feynman rules for general lattice field theories, in a form suitable for efficient numerical calculation of perturbative loop diagrams.

hep-lat↗

arXiv2001

The topological susceptibility and f_pi from lattice QCD

We study the topological susceptibility, chi, in QCD with two quark flavours using lattice field configurations that have been produced with an O(a) improved quark action. We find clear evidence for the expected suppression at small quark mass, and examine the variation of chi with this mass. The resulting estimate of the pion decay constant, f_pi = 105 +/- 6 ^{+18}_{-10} MeV, is consistent with the experimental value of ~93 MeV. We compare chi to the large-N_c prediction and find consistency over a large range of quark masses. We discuss the benefits of the non-perturbative action improvement scheme and of the stategy of keeping the lattice spacing (nearly) fixed as the quark mass is varied. We compare our results with other studies and suggest why such a quark mass dependence has not always been seen.

hep-lat↗

arXiv2001

On the glueball spectrum in O(a)-improved lattice QCD

We calculate the light `glueball' mass spectrum in N_f=2 lattice QCD using a fermion action that is non-perturbatively O(a) improved. We work at lattice spacings a ~0.1 fm and with quark masses that range down to about half the strange quark mass. We find the statistical errors to be moderate and under control on relatively small ensembles. We compare our mass spectrum to that of quenched QCD at the same value of a. Whilst the tensor mass is the same (within errors), the scalar mass is significantly smaller in the dynamical lattice theory, by a factor of ~(0.84 +/- 0.03). We discuss what the observed m_q dependence of this suppression tells us about the dynamics of glueballs in QCD. We also calculate the masses of flux tubes that wind around the spatial torus, and extract the string tension from these. As we decrease the quark mass we see a small but growing vacuum expectation value for the corresponding flux tube operators. This provides clear evidence for `string breaking' and for the (expected) breaking of the associated gauge centre symmetry by sea quarks.

hep-lat↗

arXiv2001

Effects of non-perturbatively improved dynamical fermions in QCD at fixed lattice spacing

We present results for the static inter-quark potential, lightest glueballs, light hadron spectrum and topological susceptibility using a non-perturbatively improved action on a $16^3\times 32$ lattice at a set of values of the bare gauge coupling and bare dynamical quark mass chosen to keep the lattice size fixed in physical units ($\sim 1.7$ fm). By comparing these measurements with a matched quenched ensemble, we study the effects due to two degenerate flavours of dynamical quarks. With the greater control over residual lattice spacing effects which these methods afford, we find some evidence of charge screening and some minor effects on the light hadron spectrum over the range of quark masses studied ($M_{PS}/M_{V}\ge0.58$). More substantial differences between quenched and unquenched simulations are observed in measurements of topological quantities.

hep-lat↗

arXiv2001

Glueballs and topology with O(a)-improved lattice QCD

We present evidence for unquenching effects in N_f=2, 16^3 32 ensembles by comparing with `equivalent' quenched data at r_0~5.0. A (small) VEV for torelons signals (weak) string breaking. A 15-20 % reduction in the scalar glueball mass relative to quenched is argued to be (in part at least) a discretisation effect. We find a chiral suppression of the topological susceptibility consistent with expectations, and agreement between fermionic and gluonic methods for measuring the topological charge.

hep-lat↗

arXiv2001

The topological susceptibility and f_pi from lattice QCD

We study the topological susceptibility, chi, in QCD with two quark flavours using the lattice field configurations recently produced by the UKQCD collaboration. We find clear evidence for the expected suppression at small quark mass, and examine the variation of chi with this mass. The resulting estimate of the pion decay constant, f_pi = 105 +/- 5 +18/-10 MeV, is consistent with the experimental value of approximately 93 MeV. We compare chi to the large-N_c prediction and find consistency over a large range of quark masses. The benefits of the non-perturbative action improvement scheme and the matched lattice spacings between simulation ensembles are discussed. We compare our results with other studies and suggest a reason why such a quark mass dependence has not been previously seen.

hep-ph↗

arXiv2001

Testing imaginary vs. real chemical potential in finite-temperature QCD

One suggestion for determining the properties of QCD at finite temperatures and densities is to carry out lattice simulations with an imaginary chemical potential whereby no sign problem arises, and to convert the results to real physical observables only afterwards. We test the practical feasibility of such an approach for a particular class of physical observables, spatial correlation lengths in the quark-gluon plasma phase. Simulations with imaginary chemical potential followed by analytic continuation are compared with simulations with real chemical potential, which are possible by using a dimensionally reduced effective action for hot QCD. We find that for imaginary chemical potential the system undergoes a phase transition at |mu/T| \approx pi/3, and thus observables are analytic only in a limited range. However, utilising this range, relevant information can be obtained for the real chemical potential case.

hep-lat↗

arXiv2000

Z2 monopoles in D=2+1 SU(2) lattice gauge theory

We calculate the Euclidean action of a pair of Z2 monopoles (instantons), as a function of their spatial separation, in D=2+1 SU(2) lattice gauge theory. We do so both above and below the deconfining transition at T=Tc. At high T, and at large separation, we find that the monopole `interaction' grows linearly with distance: the flux between the monopoles forms a flux tube (exactly like a finite portion of a Z2 domain wall) so that the monopoles are linearly confined. At short distances the interaction is well described by a Coulomb interaction with, at most, a very small screening mass, possibly equal to the Debye electric screening mass. At low T the interaction can be described by a simple screened Coulomb (i.e. Yukawa) interaction with a screening mass that can be interpreted as the mass of a `constituent gluon'. None of this is unexpected, but it helps to resolve some apparent controversies in the recent literature.

hep-lat↗

arXiv2000

The topological susceptibility and pion decay constant from lattice QCD

We study the topological susceptibility, chi, in two flavour lattice QCD. We find clear evidence for the expected suppression of chi at small quark mass. The estimate of the pion decay constant, f_pi = 105 +/-5 +18/-10 MeV, is consistent with the experimental value of approximately 93 MeV. We compare chi to the large-N_c prediction and find consistency over a large range of quark masses.

hep-lat↗