arXiv subjects

Ryan Bignell

Publications and source records attributed to Ryan Bignell.

At least 19 recordsLinked to original sources

arXiv2026

Heavy quark thermodynamics with anisotropic lattices

We present recent results from the FASTSUM collaboration, using anisotropic lattice QCD to study spectral properties of heavy quarkonia and open heavy flavour systems at high temperature. For heavy quarkonium, our results using a number of different methods suggest a small but significant and robust negative mass shift as well as an increasing thermal width. We present the first lattice results for masses and spectral functions of B mesons at high temperature, and preliminary results for a high-precision calculation of the static quark potential.

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arXiv2026

On the effective restoration of $U(1)_A$ symmetry at finite temperature

The $U(1)_A$ symmetry of the massless QCD Lagrangian is explicitly broken by the axial anomaly, but it may be effectively restored at finite temperature. Determining the temperature at which this occurs is important for understanding the chiral transition and the structure of the QCD phase diagram. A commonly used probe of effective $U(1)_A$ restoration is the degeneracy of flavour non-singlet pseudoscalar and scalar susceptibilities. Using anisotropic lattice QCD ensembles with Wilson-clover fermions generated by the \textsc{Fastsum} collaboration, we study this degeneracy through hadronic correlation functions over a wide range of temperatures. The fine temporal resolution of our Generation 3 ensembles allows us to determine the temperature at which the pseudoscalar and scalar channels become degenerate. We find evidence for the effective restoration of $U(1)_A$ symmetry at $T_{U(1)_A}=319(22)$ MeV, well above the chiral crossover temperature.

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arXiv2026

$U(1)_A$ symmetry restoration at finite temperature with mesonic correlators

The $U(1)_A$ symmetry of the massless QCD Lagrangian is explicitly broken in the quantised theory by the anomaly. It may be effectively restored at some finite temperature, which would have important consequences for the order of the chiral transition and the QCD phase diagram. It has been argued in the literature that one way to probe the effective restoration of $U(1)_A$ is to check for the degeneracy of pseudoscalar and flavour non-singlet scalar correlators. In this work, we consider a new method of examining this degeneracy based upon hadron correlation functions on the anisotropic FASTSUM ensembles. The anisotropic nature and our newest Generation 3 ensembles aid in a determination of the effective restoration of the $U(1)_A$ symmetry which we find to be $T_{U(1)_A} \sim 320$ MeV, well above the chiral transition temperature, which is $T_{\rm pc} \sim 180$ MeV for our choice of Wilson-Clover fermions.

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arXiv2025

Hybrid Charmonium at Finite Temperature

Drawing upon well established zero-temperature techniques, we present, for the first time in a lattice calculation, insight into the fate of the $1^{-+}$ exotic charmonium state at finite temperature. Specifically, using anisotropic FASTSUM ensembles we employ distillation with a wide operator basis which has been extensively used at zero-temperature by the Hadron Spectrum Collaboration to study the charmonium spectrum. The constant contribution to some finite-temperature temporal correlation functions requires particular care with the extended operator basis common to distillation setups and we discuss this effect. As an alternative to derivative based extended operators, we also consider the use of optimal distillation profiles at finite temperature for the first time. Finally, we remark on the temperature dependence of the $1^{-+}$ spectral function by consideration of the reconstructed correlator method.

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arXiv2025

Approaching the continuum with anisotropic lattice thermodynamics

The FASTSUM collaboration has a long-standing programme of using anisotropic lattice QCD to investigate strong interaction thermodynamics, and in particular spectral quantities. Here we present first results from our new ensemble which has a temporal lattice spacing a_t=15am and anisotropy xi=a_s/a_t=7, giving unprecedented resolution in the temporal direction. We show results for the chiral transition, vector-axial-vector degeneracy, and heavy quarkonium, and compare them with earlier results with coarser time resolution.

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arXiv2025

Novel insight into centre-vortex geometry in four dimensions

Centre-vortex surfaces are mapped out in four dimensions within the framework of SU(3) lattice gauge theory to understand the role of secondary loops that develop in three-dimensional visualisations of centre-vortex structure, appearing separate from the percolating cluster. Loops that initially appear disconnected in three-dimensional slices can originate from the same connected surface in four dimensions depending on the surface's curvature. For the first time, these secondary loops are identified as "connected" or "disconnected" with respect to the vortex sheet, allowing new insight into the evolution of centre-vortex geometry through the finite-temperature phase transition. At low temperatures, we find that secondary loops of any length primarily lie in the same sheet percolating the four-dimensional volume. Only a handful of small secondary sheets disconnected from the percolating sheet are identified. Above the phase transition, the vortex structure is still found to be dominated by a single large sheet but one that has aligned with the temporal dimension. With the near absence of any curvature orthogonal to the temporal dimension, connected secondary loops become vanishingly rare. Other novel quantities, such as the four-dimensional density of secondary sheets and the sheet sizes themselves, are analysed to build a complete picture of centre-vortex geometry in four dimensions.

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arXiv2025

Finite temperature hadronic spectral properties

The FASTSUM collaboration has a long-standing project examining hadronic properties using anisotropic lattice QCD. We determine the spectral properties of bottomonia at finite temperature using lattice NRQCD and describe how our newer simulations improve our control over systematic errors. Motivated by these efforts, the temperature dependence of charm hadron masses is determined where it is found that temperature effects can extend into the confining phase and that some species remain stable deep past the pseudo-critical temperature.

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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.

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arXiv2025

Spectral properties of bottomonium at high temperature: a systematic investigation

We investigate spectral features of bottomonium at high temperature, in particular the thermal mass shift and width of ground state S-wave and P-wave state. We employ and compare a range of methods for determining these features from lattice NRQCD correlators, including direct correlator analyses (multi-exponential fits and moments of spectral functions), linear methods (Backus-Gilbert, Tikhonov and HLT methods), and Bayesian methods for spectral function reconstruction (MEM and BR). We comment on the reliability and limitations of the various methods.

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arXiv2025

Lattice gauge ensembles and data management

We summarize the status of lattice QCD ensemble generation efforts and their data management characteristics. Namely, these proceedings combine the contributions to a dedicated parallel session during the 41st International Symposium on Lattice Field Theory (Lattice 2024), during which representatives of 16 lattice QCD collaborations provided details on their simulation program, with focus on plans for publication, data management, and storage requirements. The parallel session was organized by the International Lattice Data Grid (ILDG), following an open call to the lattice QCD community for participation in the session.

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arXiv2025

NRQCD Bottomonium at non-zero temperature using time-derivative moments

A well-known challenge for the lattice community is calculating the spectral function from the Euclidean correlator. We have approximated the spectral function and derived the mass and thermal width of particles through the time derivatives of the lattice correlator moments. We have focused on extracting the properties of bottomonium states, specifically $\Upsilon$ and $\chi_{b1}$. We will give an overview of the time-derivative moments approach and present results for the temperature dependence of the mass and width of both bottomonium states. The zero temperature results are consistent with experimental values, while results at higher temperatures are similar to those obtained using other methods.

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arXiv2025

The NRQCD $\Upsilon$ spectrum at non-zero temperature using Backus-Gilbert regularisations

Understanding how the properties of heavy mesons change as temperature increases is crucial for gaining valuable insights into the quark-gluon plasma. Information about meson masses and decay widths is encoded in the meson spectral function, which, in principle, can be extracted from Euclidean correlation functions via generalised Laplace transformations. However, this inverse problem is ill-posed for lattice correlation functions and requires regularisation. In this work, we present the latest results for bottomonium spectral functions obtained within the lattice NRQCD framework using the Backus-Gilbert regularisation, along with two other variants, one of which is commonly referred to as the HLT method. Our analysis employs Generation 2L anisotropic lattice configurations produced by the \textsc{Fastsum} collaboration.

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arXiv2025

Anisotropic excited bottomonia from a basis of smeared operators

Bottomonia play a crucial role in our understanding of the quark gluon plasma. We present lattice non-relativistic QCD calculations of bottomonia at temperatures in the range $T \in [47, 380]$ MeV using the Fastsum Generation 2L anisotropic $N_f = 2 + 1$ ensembles. The use of a basis of smeared operators allows the extraction of excited-state masses at zero temperature and an investigation of their thermal properties at non-zero temperature. We find that the ground state signal is substantially improved by this variational approach at finite temperature. We also apply the time-derivative moments approach to the projected or optimal correlation functions at finite temperature.

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arXiv2024

The curvature of the pseudo-critical line in the QCD phase diagram from mesonic lattice correlation functions

In the QCD phase diagram, the dependence of the pseudo-critical temperature, $T_{\rm{pc}}$, on the baryon chemical potential, $\mu_B$, is of fundamental interest. The variation of $T_{\rm{pc}}$ with $\mu_B$ is normally captured by $\kappa$, the coefficient of the leading (quadratic) term of the polynomial expansion of $T_{\rm{pc}}$ with $\mu_B$. In this work, we present the first calculation of $\kappa$ using hadronic quantities. Simulating $N_f=2+1$ flavours of Wilson fermions on {\sc Fastsum} ensembles, we calculate the ${\cal O}(\mu_B^2)$ correction to mesonic correlation functions. By demanding degeneracy in the vector and axial-vector channels we obtain $T_{\rm{pc}}(\mu_B)$ and hence $\kappa$. While lacking a continuum extrapolation and being away from the physical point, our results are consistent with previous works using thermodynamic observables (renormalised chiral condensate, strange quark number susceptibility) from lattice QCD simulations with staggered fermions.

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arXiv2024

Centre vortex evidence for a second finite-temperature QCD transition

Evidence for the existence of a second finite-temperature transition in quantum chromodynamics (QCD) is obtained through the study of centre vortex geometry and its evolution with temperature. The dynamical anisotropic ensembles of the FASTSUM Collaboration are utilised to conduct a comprehensive analysis at eight temperatures beyond the established chiral transition. Visualisations of the centre vortex structure in temporal and spatial slices of the lattice reveal that vortex percolation persists through the chiral transition and ceases at a temperature that is approximately twice the chiral transition temperature $T_c$. This implies that confinement is retained through temperatures up to $T \approx 2\,T_c$, pointing toward a second transition corresponding to deconfinement. The loss of percolation is quantified by the vortex cluster extent, providing a clear signal for the deconfinement transition. Additional vortex statistics, including temporal correlations, vortex and branching point densities, the number of secondary clusters and vortex chain lengths between branching points, are scrutinised as a function of temperature. All ten measures investigated herein show the characteristics of two transitions in QCD, encompassing the chiral transition at $T_c$ and the deconfinement transition at $T \approx 2\,T_c$. Performing an inflection point analysis on the vortex and branching point densities produces an estimate of $T_c$ that agrees with the known FASTSUM value. By the same procedure, a precise estimate of the deconfinement point is extracted as $T_d = 321(6)\,$MeV.

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arXiv2024

Thermal lattice QCD results from the FASTSUM collaboration

The FASTSUM Collaboration has developed a comprehensive research programme in thermal lattice QCD using 2+1 flavour ensembles. We review our recent hadron spectrum analyses of open charm mesons and charm baryons at non-zero temperature. We also detail our determination of the interquark potential in the bottomonium system using NRQCD quarks. All of our work uses anisotropic lattices where the temporal lattice spacing is considerably finer than the spatial one allowing better resolution of temporal correlation functions.

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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.

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arXiv2023

Reconstructed (charm) baryon methods at finite temperature on anisotropic lattices

Reconstructed-correlator methods have been used to investigate thermal effects in mesonic correlation functions in a fit-independent manner. This technique has recently been extended to the baryonic sector. In this work different ways of implementing this approach for baryon correlators are examined. Using both real and synthetic data it is found that for heavy baryons, such as the $\Xi_{cc}(ccu)$ baryon, different choices are equivalent and that for the lighter nucleon the effect of different implementations is minimal. Further comparison to the so-called "double ratio" using the FASTSUM Generation 2L thermal ensembles shows that reconstructed-correlator ratios and double ratios contain nearly identical quantitative information.

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