arXiv subjects

Derek Leinweber

Publications and source records attributed to Derek Leinweber.

At least 19 recordsLinked to original sources

arXiv2026

Structure of QC$_2$D ground state fields at nonzero matter densities

A quantitative investigation into the modification of ground-state field structures in two-color QCD (QC$_2$D) is presented at finite chemical potential. Using lattice simulations with Wilson gauge and fermion actions, we explore the chromo-electromagnetic field strengths under varying matter densities. To ensure accurate measurements, we develop and calibrate two highly improved topological charge operators and evaluate four gradient flow actions. Our results reveal a finite-volume crossover in the regime of the anticipated phase boundary at $\mu = m_\pi/2$, with both chromo-electric and chromo-magnetic field strengths suppressed before recovering and exceeding vacuum values at higher chemical potentials. We find the difference between the squared chromo-electric and chromo-magnetic field strengths, $E^2-B^2$, to increase in magnitude monotonically with increasing chemical potential. At $a\mu=0.7$, we find an $11\%$ suppression of $E^2$, a relatively small effect. A systematic analysis using sigmoid fits of lattice simulations in the crossover regime is performed to confirm the critical chemical potential obtained from the field structure is in agreement with the phase boundary at $m_\pi / 2$. These findings provide new insight into non-Abelian ground-state vacuum field structures and offer a foundation for future studies in real QCD.

hep-lat↗

arXiv2025

Centre vortices in thermal lattice QCD with dynamical fermions

Evidence for a second finite-temperature transition in Quantum Chromodynamics (QCD) is revealed through the study of centre vortex geometry and its temperature-dependent evolution. Using the anisotropic dynamical ensembles from the Fastsum collaboration, we conduct a detailed analysis at several temperatures above and below the established chiral transition. Visualisations of the centre vortex structure in temporal and spatial lattice slices demonstrate that vortex percolation persists through the chiral transition and ceases at approximately twice the chiral transition temperature, T_c. This indicates that confinement is maintained up to T \approx 2T_c, suggesting a second transition associated with deconfinement. The loss of percolation, quantified by vortex cluster extent, provides a clear signal for this transition. Statistical analysis of vortex density and cluster extent is presented which reveals consistent evidence of two transitions in QCD: the chiral transition at T_c and the deconfinement transition at T_d \approx 2T_c. By analysing these, and other quantities, we obtain a precise determination of the deconfinement temperature, T_d = 321(6) MeV.

hep-lat↗

arXiv2025

Distorted Sounds: Unlocking the Physics of Modern Music

In the production of modern music, the musical characteristics of the guitar or keyboard amplifier play an integral role in the creative process. This article explores the physics of music with an emphasis on the role of distortion in the amplification. In particular, we derive and illustrate how a distorted amplifier creates new musical notes that are not played by the musician, greatly simplifying the playing technique. In providing a comprehensive understanding, we commence with a discussion of the physics of music, highlighting the harmonic series and its relation to pleasing harmonies. This is placed in the context of the standard music notation of intervals and their relation to note frequency ratios. We then discuss the problems of tuning an instrument and why the equal temperament of standard guitar tuners is incompatible with good sounding music when amplifier distortion is involved. Drawing on the basic trigonometric identities for angle sums and differences, we show how the nonlinear amplification of a distorted amplifier, generates new notes not played by the musician. Here the importance of setting your guitar tuner aside and using your ear to tune is emphasised. We close with a discussion of how humans decipher musical notes and why some highly distorted guitar chords give the impression of low notes that are not actually there. This article will be of assistance to students interested in the physics of music and lecturers seeking fascinating and relevant applications of mathematical trigonometric relations and physics to capture the attention of their students.

physics.pop-ph↗

arXiv2024

Magnetic polarisability of octet baryons near the physical quark-mass point

The magnetic polarisabilities of octet baryons are calculated close to the physical quark-mass point using the background field method in lattice QCD. This first calculation draws on the identification and elimination of exceptional configurations that have hindered previous attempts. The origin of the exceptional configuration problem lies in the use of a Wilson-type fermion action on electro-quenched gauge field configurations, where the dynamical-fermion gauge-field generation algorithm the electric charges of the quarks. Changes in the fermion determinant that would suppress some gauge fields in the background magnetic field are neglected, leaving improbable gauge fields that generate large additive mass renormalisations which manifest as significant outliers in correlation-function distributions. An algorithm for the systematic identification and removal of these exceptional configurations is described. We find the light up and down quarks to be problematic, particularly the up quark with its larger electric charge. The heavier mass of the strange quark protects the hyperon correlation functions to some extent. However, these also benefit from the removal of exceptional configurations. In many cases, the magnetic polarisability is calculated with good precision. We find our results to be in accord with the behaviour anticipated by chiral perturbation theory.

hep-lat↗

arXiv2024

Quark mass effects in octet baryon magnetic polarisabilities via lattice QCD

The quark mass dependence of octet baryon magnetic polarisabilities is examined at the level of individual quark-sector contributions in the uniform background-field approach of lattice QCD. The aim is to understand the direct impact of increasing the mass of a quark flavour on the magnetic polarisability and indirect or environmental effects associated with changing the mass of spectator quarks, insensitive to the background magnetic field. Noting the need to set the electric charge of some quark flavours to zero, a fractionally charged baryon formalism is introduced. We find that increasing the mass of the charged quark flavour directly causes its contribution to the magnetic polarisability to decrease. However, increasing the mass of the spectator quark flavour indirectly acts to increase the magnetic polarisability. To gain a deeper understanding of these effects, we evaluate the predictions of the constituent quark model in this context. While the model provides a compelling explanation for the environmental effect of varying the spectator quark mass, an explanation of the direct mass dependence is more complicated as competing factors combine in the final result. The lattice results indicate the key factor is a reduction in the constituent quark magnetic moment with increasing quark mass, as it governs the strength of the magnetic transition to the nearby decuplet baryon.

hep-lat↗

arXiv2024

Exploring the $ Ω^- $ spectrum in lattice QCD

We present an exploratory lattice QCD analysis of the $ Ω$-baryon spectrum. Using smeared three-quark operators in a correlation matrix analysis, we report masses for the ground, first and second excited states of the $ J^P = 1/2^\pm,\, 3/2^\pm $ spectra across a broad range in the light quark mass. We investigate the parity and spin quantum numbers for the states observed on the lattice, looking to reconcile these with the resonances encountered in experiment. We find that the $ Ω^-(2012) $ as reported by the Particle Data Group corresponds to two overlapping resonances with $ J^P = 1/2^- $ and $ 3/2^- $. We also propose quantum number assignments for the higher energy resonances, and identify successive radial excitations within the spectra.

hep-lat↗

arXiv2024

Magnetic Polarisability of Octet Baryons via Lattice QCD

Drawing on recent advances in lattice-QCD background-field techniques, the magnetic polarisability of octet baryons is calculated from the first principles of QCD. The results are presented in the context of new constituent quark-model calculations providing a framework for understanding the lattice results and a direct comparison with simulation results at unphysical quark masses. Using smeared quark sources, low-lying Laplacian eigenmode projection and final-state Landau mode projection, considerable attention is devoted to ensuring single-state isolation in the lattice correlation functions. We also introduce new weighting methods to reduce the sensitivity to correlation-function fits, averaging over many fits based on merit drawn from the full correlated $\chi^2$ of the fits. The techniques are implemented on the $32^3 \times 64$, 2+1-flavour dynamical-fermion lattices provided by the PACS-CS collaboration following the introduction of uniform magnetic fields quantised to the lowest nontrivial values available. After some fine tuning of the constituent quark model parameters, we find the model captures the patterns observed in the lattice QCD results very well, providing important insights into the physics underpinning the magnetic polarisabilities. Finally, comparison with the most recent results from experiment proceeds through an effective field theory formalism which incorporates estimates of finite-volume corrections and small electro-quenching corrections as the results are brought to the physical point. We find excellent agreement with experiment where available, including the proton and neutron polarisabilities.

hep-lat↗

arXiv2024

Understanding the nature of the $Δ(1600)$ resonance

We present a coupled-channel analysis of the $ J^P = 3/2^+ Δ$-baryon spectrum, based in the framework of Hamiltonian Effective Field Theory (HEFT). We construct a Hamiltonian which mixes quark model-like single-particle states and two-particle meson-baryon channels, and constrain this via experimentally measured $ πN \to πN $ scattering observables. In the same vein as Lüscher's approach, we then connect this infinite-volume inspired Hamiltonian with finite-volume lattice QCD results. Drawing on lattice correlation-matrix eigenvectors identifying the $ 1s $ and $ 2s $ states in the finite-volume $ Δ(3/2^+) $ spectrum, and utilising the HEFT eigenvectors describing the composition of the energy eigenstates, we resolve the structure of these states and their relation to the $ Δ(1600) $ resonance. We find the dominant contributions to this resonance come from strong rescattering in the $ πN $ and $ πΔ$ channels. This contrasts the long-held view of a dominant quark model-like core for the $ Δ(1600) $. Further discussion of other contemporary lattice results for the $ Δ$ spectrum and $ πN $ scattering states is also presented.

hep-ph↗

arXiv2023

Searching for the first radial excitation of the $Δ(1232)$ in lattice QCD

We present a lattice QCD analysis of the $ Δ$-baryon spectrum, with the goal of finding the position of the $ 2s $ radial excitation of the $ Δ(1232) $ ground state. Using smeared three-quark operators in a correlation matrix analysis, we report masses for the ground, first and second excited states of the $ J^P = 3/2^+ $ spectrum across a broad range of $ m_π^2 $. We identify the lowest lying state as being a $ 1s $ state, consistent with the well known $ Δ(1232) $. The first excitation is identified as a $ 2s $ state, but is found to have a mass of approximately 2.15~GeV on our $ \sim3 $ fm lattice, which does not appear to be associated with the $ Δ(1600) $ resonance in a significant manner. We also report on the spin-$ 1/2 $ and odd-parity states accessible via our methods. The large excitation energies of the radial excitations provide a potential resolution to the long-standing missing baryon resonances problem.

hep-lat↗

arXiv2023

$ Δ$ baryon spectroscopy in lattice QCD

A variational analysis is performed within the framework of lattice QCD to extract the masses of the spin-3/2 positive parity $ Δ^+ $ baryons, including radial excitations. $2+1$ flavour dynamical gauge-field configurations provided by the PACS-CS collaboration via the ILDG are considered. To improve our interpolator basis, we perform an iterative process of source and sink smearing and solve a generalised eigenvalue problem using the resulting fermion operators. We obtain a clear signal for the ground and first excited states at a light quark mass corresponding to $ m_π= 413 $ MeV. Furthermore, we show that one can use the eigenvectors obtained in this method to investigate the nature of these states, allowing us to classify our results as $ 1s $ and $ 2s $ states for the ground and first excited states respectively. Finally, we briefly highlight the method of Hamiltonian Effective Field Theory which can be used to make comparison with quark model expectations.

hep-lat↗

arXiv2022

50 Years of Quantum Chromodynamics

This paper presents a comprehensive review of both the theory and experimental successes of Quantum Chromodynamics, starting with its emergence as a well defined theory in 1972-73 and following developments and results up to the present day. Topics include a review of the earliest theoretical and experimental foundations; the fundamental constants of QCD; an introductory discussion of lattice QCD, the only known method for obtaining exact predictions from QCD; methods for approximating QCD, with special focus on effective field theories; QCD under extreme conditions; measurements and predictions of meson and baryon states; a special discussion of the structure of the nucleon; techniques for study of QCD at high energy, including treatment of jets and showers; measurements at colliders; weak decays and quark mixing; and a section on the future, which discusses new experimental facilities or upgrades currently funded. The paper is intended to provide a broad background for Ph.D. students and postdocs starting their career. Some contributions include personal accounts of how the ideas or experiments were developed.

hep-ph↗

arXiv2022

Dynamical fermions, centre vortices, and emergent phenomena

The non-trivial ground-state vacuum fields of QCD form the foundation of matter. Here we examine the centre vortices identified within the ground-state fields of lattice QCD. We aim to understand the manner in which dynamical fermions in the QCD vacuum alter the centre-vortex structure. Using modern visualisation techniques, the centre-vortex structure of pure-gauge and dynamical-fermion fields is quantified and compared. We then explore the impact this modified structure has on measures of confinement and dynamical mass generation. The string tension of the static quark potential, positivity-violation in the gluon propagator, and dynamical mass generation in the overlap quark propagator are of particular interest. The impact of dynamical fermions is significant and provides new insights into the role of centre vortices in underpinning both confinement and dynamical chiral symmetry breaking in QCD.

hep-lat↗

arXiv2022

Overlap quark propagator near the physical pion mass

The Landau-gauge quark propagator is calculated using overlap fermions on 2+1-flavour dynamical fermion gauge fields from the PACS-CS collaboration with pion mass $m_π\sim 156\text{ MeV}$ and spatial volume $\sim (3 \text{ fm})^3.$ The observed features of the mass and renormalisation functions are discussed, including a comparison with recent results using $\mathcal{O}(a)$-improved Wilson fermions on 2-flavour dynamical gauge fields.

hep-lat↗

arXiv2022

Impact of Dynamical Fermions on the Centre Vortex Gluon Propagator

The impact of $SU(3)$ centre vortices on the Landau-gauge gluon propagator is calculated in the presence of dynamical fermions and compared to the pure Yang-Mills case. The presence of dynamical fermions is found to alter the behaviour of the centre vortex propagator when compared to the established pure-gauge result. The gluon spectral representation is also explored from the centre vortex perspective, where centre vortices are shown to exhibit clear signs of positivity violation, which is an indicator of confinement. Vortex removal subsequently restores positivity, demonstrating the crucial role centre vortices play in the confinement of gluons.

hep-lat↗

arXiv2022

Static quark potential from centre vortices in the presence of dynamical fermions

For the first time, centre vortices are identified on SU(3) lattice ensembles that include dynamical fermions. Using a variational method, the static quark potential is calculated on untouched, vortex-removed, and vortex-only fields. Two dynamical ensembles and one pure gauge ensemble are studied, allowing for an exploration of the impact of dynamical fermions on the centre-vortex vacuum. Novel modifications to the standard Coulomb term are introduced to describe the long range behaviour of the vortex-removed potential. These modifications remove a source of systematic error in the fitted string tension on the original ensembles. Our pure Yang-Mills result is consistent with previous studies, where projected centre-vortex fields only reproduce approximately two thirds of the string tension. Remarkably, we find that the vortex-only fields on both dynamical lattices are able to fully reproduce the respective untouched string tensions.

hep-lat↗

arXiv2022

Centre Vortex Structure of QCD-Vacuum Fields and Confinement

The non-trivial ground-state vacuum fields of QCD form the foundation of matter. Using modern visualisation techniques, this presentation examines the microscopic structure of these fields. Of particular interest are the centre vortices identified within the ground-state fields of lattice QCD. Our current focus is on understanding the manner in which dynamical fermions in the QCD vacuum alter the centre-vortex structure. The impact of dynamical fermions is significant and provides new insights into the role of centre vortices in underpinning both confinement and dynamical chiral symmetry breaking in QCD.

hep-lat↗

arXiv2022

Disconnected contributions to the magnetic polarisability of the neutral pion

The magnetic polarisability of the neutral pion has been calculated using the background field method in lattice QCD. These early results do not consider the effect of the disconnected loop contractions arising from the breaking of charge symmetry in a background magnetic field. Recent work in chiral perturbation theory has shown that these quark-self-annihilation contractions provide the leading loop-order contributions to the magnetic polarisability. A first investigation of these contractions in a background magnetic field is presented.

hep-lat↗

arXiv2022

Impact of centre vortex removal on the Landau-gauge quark propagator in dynamical QCD

The important role of centre vortices in dynamical chiral symmetry breaking and corresponding dynamical mass generation has been demonstrated in pure gauge studies of the Landau-gauge quark propagator. We present initial results of our investigation into the impact of centre vortex removal on the Landau-gauge propagator on dynamical gauge fields. To ensure sensitivity to the topology of the gauge fields, the propagator is computed with overlap fermions. Upon removal of centre vortices we find that dynamical mass generation is suppressed. The quark renormalisation function remains remarkably flat except in the deep infrared where it exhibits significant suppression. These effects become more prominent at lighter quark masses.

hep-lat↗