arXiv subjects

Anthony W. Thomas

Publications and source records attributed to Anthony W. Thomas.

At least 37 records · Page 2Linked to original sources

arXiv2024

Electromagnetic form factors for nucleons in short-range correlations

Recent experimental studies have led to the suggestion that short-range correlations may be a major contributor to the nuclear EMC effect. This hypothesis requires that the structure function for nucleons involved in short-range correlations should be heavily suppressed compared to that of a free nucleon. Based on calculations performed within an AdS/QCD motivated, light-front quark-diquark model, we find that this large suppression of the nucleon structure function leads to a strong suppression of the nucleon elastic form factors.

hep-ph↗

arXiv2024

Study of the pion-mass dependence of $ρ$-meson properties in lattice QCD

We collect spectra extracted in the $I=\ell=1$ $ππ$ sector provided by various lattice QCD collaborations and study the $m_π$ dependence of $ρ$-meson properties using Hamiltonian Effective Field Theory (HEFT). In this unified analysis, the coupling constant and cutoff mass, characterizing the $ρ- ππ$ vertex, are both found to be weakly dependent on $m_π$, while the mass of the bare $ρ$, associated with a simple quark-model state, shows a linear dependence on $m_π^2$. Both the lattice results and experimental data can be described well. Drawing on HEFT's ability to describe the pion mass dependence of resonances in a single formalism, we map the dependence of the phase shift as a function of $m_π$, and expose interesting discrepancies in contemporary lattice QCD results.

hep-lat↗

arXiv2024

Structure of the $\mathbf{Λ(1670)}$ resonance

We examine the internal structure of the $Λ(1670)$ through an analysis of lattice QCD simulations and experimental data within Hamiltonian effective field theory. Two scenarios are presented. The first describes the $Λ(1670)$ as a bare three-quark basis state, which mixes with the $πΣ$, $\bar{K}N$, $ηΛ$ and $KΞ$ meson-baryon channels. In the second scenario, the $Λ(1670)$ is dynamically generated from these isospin-0 coupled channels. The $K^-p$ scattering data and the pole structures of the $Λ(1405)$ and the $Λ(1670)$ can be simultaneously described well in both scenarios. However, a comparison of the finite-volume spectra to lattice QCD calculations reveals significant differences between these scenarios, with a clear preference for the first case. Thus the lattice QCD results play a crucial role in allowing us to distinguish between these two scenarios for the internal structure of the $Λ(1670)$.

hep-ph↗

arXiv2024

Sea-quark loop contributions to the $\bar d$-$\bar u$ asymmetry in the proton

QCD interactions for equal-mass fermion flavors are flavor blind. This fact is often used to state that disconnected sea-quark loop contributions are equal for $u$ and $d$ quarks in the mass symmetric case and therefore these disconnected sea-quark loop contributions cannot contribute to the well-known $\bar d - \bar u$ asymmetry in the proton. Instead, it is argued that one must look to the connected sector of lattice QCD correlation functions to find this difference. In this presentation, we note that these statements are true provided unphysical contributions in the sea-quark loop sector are included, contributions from baryons that do not appear in the physical spectrum. To respect the Pauli principle, these unphysical contributions from the disconnected sea-quark loop sector must cancel equally unphysical contributions in the connected sector. The remaining disconnected sea-quark loop contributions no longer have a balance between $\bar d$ and $\bar u$. Upon considering only physically observed baryons in the loop contributions, we illustrate an important contribution from the sea-quark loop sector to $\bar d - \bar u$ that enhances the leading connected contribution by 12\%.

hep-ph↗

arXiv2024

Understanding the nature of baryon resonances

This presentation opens with a brief review of lattice QCD calculations showing the $2s$ radial excitation of the nucleon sits at approximately 2 GeV, well above the Roper resonance position. We then proceed to reconcile this observation with experimental scattering data. While the idea of dressing quark-model states in a coupled-channel analysis to describe scattering data has been around for decades, it's now possible to bring these descriptions to the finite-volume of lattice QCD for confrontation with lattice-QCD calculations. This combination of lattice QCD and experiment demands that we reconsider our preconceived notions about the quark-model and its excitation spectrum. We close with a discussion of an unanticipated resolution to the missing baryon resonances problem.

hep-lat↗

arXiv2023

Excluded Volume Effects on Cold Neutron Star Phenomenology

Observable properties of neutron stars are studied within a hadronic equation of state derived from the quark level. The effect of short-range repulsion is incorporated within the excluded volume framework. It is found that one can sustain neutron stars with masses as large as 2.2$M_\odot$ even including hyperons in $β$ equilibrium, while producing radii and tidal deformabilities consistent with current constraints.

nucl-th↗

arXiv2023

Low-lying odd-parity nucleon resonances as quark-model like states

Recent lattice QCD results for the low-lying odd-parity excitations of the nucleon near the $N^{*}(1535)$ and $N^{*}(1650)$ resonance positions have revealed that the lattice QCD states have magnetic moments consistent with predictions from a constituent-quark-model. Using Hamiltonian Effective Field Theory (HEFT) to describe pion-nucleon scattering in the $I(J^{P}) = \frac{1}{2}(\frac{1}{2}^{-})$ channel, we represent these two quark-model like states as two single-particle bare basis states, dressed and mixed by meson-baryon scattering channels. By constraining the free parameters of the Hamiltonian with $S_{11}$ pion-nucleon scattering data, we perform the first calculation of the finite-volume spectrum using two bare-baryon basis states. By comparing this spectrum to contemporary lattice QCD results at three lattice volumes, we analyse the eigenvectors of the Hamiltonian to gain insight into the structure and composition of these two low-lying resonances. We find that an interpretation of the two low-lying nucleon resonances as quark-model like states dressed by meson-baryon interactions is consistent with both the $S_{11}$ scattering data and lattice QCD. We introduce a novel HEFT formalism for estimating scattering-state contaminations in lattice QCD correlation functions constructed with standard three-quark operators. Not only are historical lattice QCD results described with excellent accuracy, but correlation functions with large scattering-state contaminations are identified.

hep-lat↗

arXiv2023

Effects of multiple single-particle basis states in scattering systems

Low-lying baryon resonances have been explored using Hamiltonian Effective Field Theory (HEFT), in a formalism where resonances with a three-quark component are described by both two-particle meson-baryon states and a bare basis state. Here, we investigate the use of multiple bare states in the Hamiltonian, to extend the formalism to higher energy ranges, and represent a larger portion of the low-lying baryon spectrum. Introducing a second bare state into a toy model extension of the low-energy $Δ(1232)$ system, we explore the influence of the second bare state on the position of poles in the infinite-volume $T$-matrix. Considering the same system in a finite-volume, we analyse the finite-volume energy spectrum in the presence of a second bare state, providing insight into the interplay between two bare basis states, representing quark-model states, and the relationship between infinite-volume poles and finite-volume eigenstates.

nucl-th↗

arXiv2022

Nucleon Structure and Strong Interactions in Dark Matter Capture in Neutron Stars

We outline two important effects that are missing from most evaluations of the dark matter capture rate in neutron stars. As dark matter scattering with nucleons in the star involves large momentum transfer, nucleon structure must be taken into account via a momentum dependence of the hadronic form factors. In addition, due to the high density of neutron star matter, we should account for nucleon interactions rather than modeling the nucleons as an ideal Fermi gas. Properly incorporating these effects is found to suppress the dark matter capture rate by up to four orders of magnitude for the heaviest stars.

hep-ph↗

arXiv2022

Maximum likelihood estimation for left-truncated log-logistic distributions with a given truncation point

The maximum likelihood estimation of the left-truncated log-logistic distribution with a given truncation point is analyzed in detail from both mathematical and numerical perspectives. These maximum likelihood equations often do not possess a solution, even for small truncations. A simple criterion is provided for the existence of a regular maximum likelihood solution. In this case a profile likelihood function can be constructed and the optimisation problem is reduced to one dimension. When the maximum likelihood equations do not admit a solution for certain data samples, it is shown that the Pareto distribution is the $L^1$-limit of the degenerated left-truncated log-logistic distribution. Using this mathematical information, a highly efficient Monte Carlo simulation is performed to obtain critical values for some goodness-of-fit tests. The confidence tables and an interpolation formula are provided and several applications to real world data are presented.

stat.ME↗

arXiv2022

Novel neutron decay mode inside neutron stars

We explore the suggestion that the neutron lifetime puzzle might be resolved by neutrons decaying into dark matter through the process, n \rightarrow χχχ, with χhaving a mass one third of that of the neutron. In particular, we examine the consequences of such a decay mode for the properties of neutron stars. Unlike an earlier suggested decay mode, in order to satisfy the constraints on neutron star mass and tidal deformability, there is no need for a strong repulsive force between the dark matter particles. This study suggests the possibility of having hot dark matter at the core of the neutron star and presents a possible mechanism of dark matter cooling, and examines the possible signal of neutrons decaying in this way inside the neutron star right after its birth.

hep-ph↗

arXiv2022

Dense Nuclear Matter with Baryon Overlap

The possibility of new short-distance physics applicable inside the cores of NS is incorporated into the equation of state generated by the quark-meson coupling model. The contribution of this new physics to the energy density is taken to be proportional to the amount of overlap between the quark cores of the baryons involved. With no change to the properties of symmetric nuclear matter at saturation density, including an incompressibility compatible with data on giant monopole resonances, one can sustain neutron stars with a maximum mass $M_{max}>2.1$ M$_\odot$, even when hyperons are included.

nucl-th↗

arXiv2022

Consequences of neutron decay inside neutron stars

The hypothesis that neutrons might decay into dark matter is explored using neutron stars as a testing ground. It is found that in order to obtain stars with masses at the upper end of those observed, the dark matter must experience a relatively strong self-interaction. Conservation of baryon number and energy then require that the star must undergo some heating, with a decrease in radius, leading to an increase in speed of rotation over a period of days.

hep-ph↗

arXiv2022

Regularisation in Nonperturbative Extensions of Effective Field Theory

The process of renormalisation in nonperturbative Hamiltonian Effective Field Theory (HEFT) is examined in the $Δ$-resonance scattering channel. As an extension of effective field theory incorporating the Lüscher formalism, HEFT provides a bridge between the infinite-volume scattering data of experiment and the finite-volume spectrum of energy eigenstates in lattice QCD. HEFT also provides phenomenological insight into the basis-state composition of the finite-volume eigenstates via the state eigenvectors. The Hamiltonian matrix is made finite through the introduction of finite-range regularisation. The extent to which the established features of this regularisation scheme survive in HEFT is examined. In a single-channel $πN$ analysis, fits to experimental phase shifts withstand large variations in the regularisation parameter, $Λ$, providing an opportunity to explore the sensitivity of the finite-volume spectrum and state composition on the regulator. While the Lüscher formalism ensures the eigenvalues are insensitive to $Λ$ variation in the single-channel case, the eigenstate composition varies with $Λ$; the admission of short distance interactions diminishes single-particle contributions to the states. In the two-channel $πN$, $πΔ$ analysis, $Λ$ is restricted to a small range by the experimental data. Here the inelasticity is particularly sensitive to variations in $Λ$ and its associated parameter set. This sensitivity is also manifest in the finite-volume spectrum for states near the opening of the $πΔ$ scattering channel. Finally, HEFT has the unique ability to describe the quark-mass dependence of the finite-volume eigenstates. The robust nature of this capability is presented and used to confront current state-of-the-art lattice QCD calculations.

hep-lat↗

arXiv2022

Significance of lower energy density region of neutron star and universalities among neutron star properties

We have constructed and compared models of rotating neutron stars and strange stars, within the Hartle framework. The significance of the low energy density region and crust region inside the neutron star has been studied, along with how much the existence of strange matter above the energy density 300 MeV/fm3 can affect the neutron star properties. We have confirmed several universalities among the neutron star properties such as, dimensionless moment of inertia vs dimensionless quadrupole moment, dimensionless tidal deformability vs dimensionless moment of inertia and moment of inertia parameters vs R/2M.

astro-ph.HE↗

arXiv2022

Gluon EMC effects in nuclear matter

We investigate the gluonic structure of nuclei within a mean-field model of nuclear structure based upon the modification of the structure of a bound nucleon, with the nucleon described by the Nambu--Jona-Lasinio model. This approach has been shown to reproduce the European Muon Collaboration (EMC) effect, involving the ratio of the spin-independent structure functions of a heavier nucleus to that of the deuteron. It also predicts a significant nuclear modification for the spin structure functions, known as the polarized EMC effect. Here we report sizeable nuclear modifications of the gluon distributions (a "gluon EMC effect") for the ratios of both the unpolarized and polarized gluon distributions in nuclear matter to those of a free nucleon.

hep-ph↗

arXiv2022

The Role of Baryon Structure in Neutron Stars

Understanding the equation of state of dense nuclear matter is a fundamental challenge for nuclear physics. It is especially timely and interesting challenge as we have reached a period where neutron stars, which contain the most dense nuclear matter in the Universe, are now being studied in completely new ways, from gravitational waves to satellite based telescopes. We review the theoretical approaches to calculating this equation of state which involve a change in the structure of the baryons.

nucl-th↗

arXiv2021

Possible nature of Dark Matter

{We present a study of neutron star models that contain dark matter (DM) in the core. The DM is assumed to have a particle nature and to be self-interacting. Using constraints on the mass and radius of neutron stars, we investigate the allowed properties of either bosonic or fermionic DM particles. We consider cases where it constitutes up to 15\% of the mass of the star, even though conventional mechanisms cannot generate such large fractions. For this purpose three different models of neutron stars are considered, the first involving nucleons only, the second including hyperons, and the last involving strange matter in the core. Different EoSs are constructed for the various cases of fermionic and bosonic DM. These EoSs are solved for selected properties of the DM particles and the results are tested against mass, radius and tidal deformability constraints for neutron stars. The distribution of energy density of DM and ordinary matter inside the neutron stars is also presented. It is found that if the DM is fermionic in nature it does not just sit in the core but it is present everywhere in the star, from the centre to outside the surface and may even envelop it.

hep-ph↗