arXiv subjects

A. W. Thomas

Publications and source records attributed to A. W. Thomas.

At least 55 records · Page 3Linked to original sources

arXiv2019

Refined analysis on the parton distribution functions of the proton

We explore the application of a two-component model of proton structure functions in the analysis of deep-inelastic scattering (DIS) data at low $Q^2$ and small $x$. This model incorporates both vector meson dominance and the correct photo-production limit. The CJ15 parameterization is applied to the QCD component, in order to take into account effects of order $1/Q^2$ effects, such as target mass corrections and higher twist contributions. The parameters of the leading twist parton distribution functions and higher twist coefficient functions are determined by fitting deep inelastic scattering data. The second moments of the parton distribution functions are extracted and compared with other global fits and lattice determinations.

hep-ph↗

arXiv2019

On introducing Charge-Symmetry-Breaking terms to nuclear Energy Density Functionals

The Charge-Symmetry-Breaking (CSB) character of the nucleon-nucleon interaction is well established. This work presents two different ways of introducing such effects into a nuclear Energy Density Functional (EDF). CSB terms are either coming from the effective theory expansion or are derived from electromagnetic mixing of $ρ^0$ and $ω$ mesons. These terms are then introduced to Skyrme and Quark-Meson-Coupling EDFs, respectively.

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arXiv2019

Physics of even-even superheavy nuclei with 96 < Z < 110 in the Quark-Meson-Coupling Model

The Quark-Meson-Coupling (QMC) model has been applied to the study of the properties of even-even super-heavy nuclei with 96 < Z < 110, over a wide range of neutron numbers. The aim is to identify the deformed shell gaps at N = 152 and N = 162 predicted in macroscopic-microscopic (macro-micro) models, in a model based on the mean-field Hartree-Fock+BCS approximation. The predictive power of the model has been tested on proton and neutron spherical shell gaps in light doubly closed (sub)shell nuclei. In the super-heavy region, the ground state binding energies of 98 < Z < 110 and 146 < N < 160 differ, in the majority of cases, from the measured values by less than 2.5 MeV, with the deviation decreasing with increasing Z and N. The axial quadrupole deformation parameter, calculated over the range of neutron numbers 138 < N < 184, revealed a prolate-oblate coexistence and shape transition around N = 168, followed by an oblate-spherical transition towards the expected N = 184 shell closure in Cm, Cf, Fm and No. The closure is not predicted in Rf, Sg, Hs and Ds as another shape transition to a highly deformed shape in Sg, Hs and Ds for N > 178 appears, while 288Rf (N = 184) remains oblate. The bulk properties predicted by QMC are found to have a limited sensitivity to the deformed shell gaps at N = 152 and 162. However, the evolution of the neutron single-particle spectra with 0 < beta2 < 0.55 gives unambiguous evidence for the location and size of the N = 152 and 162 gaps as a function of Z and N. In addition, the neutron number dependence of neutron pairing energies provides supporting evidence for existence of the energy gaps.

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arXiv2019

Predicting the SUSY breaking scale in SUGRA models with degenerate vacua

In N=1 supergravity the scalar potential may have supersymmetric (SUSY) and non-supersymmetric Minkowski vacua (associated with supersymmetric and physical phases) with vanishing energy density. In the supersymmetric Minkowski (second) phase some breakdown of SUSY may be induced by non-perturbative effects in the observable sector that give rise to a tiny positive vacuum energy density. Postulating the exact degeneracy of the physical and second vacua as well as assuming that at high energies the couplings in both phases are almost identical, one can estimate the dark energy density in these vacua. It is mostly determined by the SUSY breaking scale M_S in the physical phase. Exploring the two-loop renormalization group (RG) flow of couplings in these vacua we find that the measured value of the cosmological constant can be reproduced if M_S varies from 20 TeV to 400 TeV. We also argue that this prediction for the SUSY breaking scale is consistent with the upper bound on M_S in the higgsino dark matter scenario.

hep-ph↗

arXiv2019

Parton distributions from nonlocal chiral SU(3) effective theory: Flavor asymmetries

Using recently derived results for one-loop hadronic splitting functions from a nonlocal implementation of chiral effective theory, we study the contributions from pseudoscalar meson loops to flavor asymmetries in the proton. Constraining the parameters of the regulating functions by inclusive production of $n$, $Δ^{++}$, $Λ$ and $Σ^{*+}$ baryons in $pp$ collisions, we compute the shape of the light antiquark asymmetry $\bar{d}-\bar{u}$ in the proton and the strange asymmetry $s-\bar{s}$ in the nucleon sea. With these constraints, the magnitude of the $\bar{d}-\bar{u}$ asymmetry is found to be compatible with that extracted from the Fermilab E866 Drell-Yan measurement, with no indication of a sign change at large values of $x$, and an integrated value in the range $\langle \bar d-\bar u \rangle \approx 0.09-0.17$. The $s-\bar s$ asymmetry is predicted to be positive at $x > 0$, with compensating negative contributions at $x=0$, and an integrated $x$-weighted moment in the range $\langle x (s-\bar s) \rangle \approx (0.9-2.5) \times 10^{-3}$.

hep-ph↗

arXiv2019

Do Delta Baryons Play a Role in Neutron Stars?

The presence of exotic hadrons, such as hyperons and $Δ$ isobars, in the dense nuclear matter in their cores has been shown to produce important changes in the properties of neutron stars. Within the quark-meson coupling model, we show that the many-body forces generated by the change in the internal quark structure of the baryons in the strong scalar mean fields generated in dense nuclear matter prohibit the appearance of $Δ$ isobars.

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arXiv2019

Exposing Novel Quark and Gluon Effects in Nuclei

The fundamental theory of the strong interaction -- quantum chromodynamics (QCD) -- provides the foundational framework with which to describe and understand the key properties of atomic nuclei. A deep understanding of the explicit role of quarks and gluons in nuclei remains elusive however, as these effects have thus far been well-disguised by confinement effects in QCD which are encapsulated by a successful description in terms of effective hadronic degrees of freedom. The observation of the EMC effect has provided an enduring indication for explicit QCD effects in nuclei, and points to the medium modification of the bound protons and neutrons in the nuclear medium. Understanding the EMC effect is a major challenge for modern nuclear physics, and several key questions remain, such as understanding its flavor, spin, and momentum dependence. This manuscript provides a contemporary snapshot of our understanding of the role of QCD in nuclei and outlines possible pathways in experiment and theory that will help deepen our understanding of nuclei in the context of QCD.

nucl-ex↗

arXiv2019

Roles of crust and core in the tidal deformability of neutron stars

With the recent measurement of GW170817 providing constraints on the tidal deformability of a neutron star, it is very important to understand what features of the equation of state have the biggest effect on it. We therefore study the contribution of the crust to the tidal deformability and the moment of inertia of a neutron star for a variety of well-known equations of state. It is found that the contributions to these quantities from the low density crust are typically quite small and as a result the determination of the tidal deformability provides an important constraint on the equation of state of dense matter.

astro-ph.HE↗

arXiv2019

Isovector Effects in Neutron Stars, Radii and the GW170817 Constraint

An isovector-scalar meson is incorporated self-consistently into the quark-meson coupling description of nuclear matter and its most prominent effects on the structure of neutron stars are investigated. The recent measurement of GW170817 is used to constrain the strength of the isovector-scalar channel. With the imminent measurements of the neutron star radii in the NICER mission it is particularly notable that the inclusion of the isovector-scalar force has a significant impact. Indeed, the effect of this interaction on the neutron star radii and masses is larger than the uncertainties introduced by variations in the parameters of symmetric nuclear matter at saturation, namely the density, binding energy per nucleon and the symmetry energy. In addition, since the analysis of GW170817 has provided constraints on the binary tidal deformability of merging neutron stars, the predictions for this parameter within the quark-meson coupling model are explored, as well as the moment of inertia and the quadrupole moment of slowly rotating neutron stars.

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arXiv2019

$Φ$ and $J/Ψ$ Mesons in Cold Nuclear Matter

We present $Φ$- and $J/Ψ$--nuclear bound state energies and absorption widths for some selected nuclei, using potentials in the local density approximation computed from an effective Lagrangian approach combined with the quark-meson coupling model. Our results suggest that these mesons should form bound states with all the nuclei considered provided that these mesons are produced in nearly recoilless kinematics.

hep-ph↗

arXiv2018

Parton distributions from nonlocal chiral SU(3) effective theory. I. Splitting functions

We present a new formulation of pseudoscalar meson loop corrections to nucleon parton distributions within a nonlocal covariant chiral effective field theory, including contributions from SU(3) octet and decuplet baryons. The nonlocal Lagrangian, constrained by requirements of local gauge invariance and Lorentz-invariant ultraviolet regularization, generates additional interactions associated with gauge links. We use these to compute the full set of proton $\to$ meson + baryon splitting functions, which in general contain on-shell and off-shell contributions, in addition to $δ$-function terms at zero momentum, along with nonlocal contributions associated with the finite size of the proton. We illustrate the shapes of the various local and nonlocal functions numerically using a simple example of a dipole regulator.

hep-ph↗

arXiv2018

Flavor symmetry breaking in the $Δ$ sea

The discovery of a sizeable asymmetry in the $\bar u$ and $\bar d$ distributions in the proton was one of the more consequential experimental findings in hadron physics last century. Although widely believed to be related to the fundamental role of chiral symmetry in QCD, a definitive verification of this hypothesis has remained elusive. We propose a novel test of the role of chiral symmetry in generating the sea flavor asymmetry by comparing the $\bar d-\bar u$ content in the proton with that in the $Δ^+$ baryon, where a significant enhancement is expected around the opening of the $N π$ decay channel. Recent developments in lattice QCD suggest a promising way to test this prediction in the near future.

hep-ph↗

arXiv2018

Generation of baryon asymmetry in the E6CHM

The strongly interacting sector in the E6 inspired composite Higgs model (E6CHM) with baryon number violation possesses SU(6)\times U(1)_L global symmetry. In the weakly-coupled sector of this model the U(1)_L symmetry associated with lepton number conservation is broken down to a Z^L_2 discrete symmetry, which stabilizes the proton. Near the scale f > 10 TeV the SU(6) symmetry is broken down to its SU(5) subgroup, giving rise to a set of pseudo-Nambu-Goldstone bosons (pNGBs) that involves the SM-like Higgs doublet, a scalar coloured triplet and a SM singlet boson. Because f is so high, all baryon number violating operators are sufficiently strongly suppressed. Nevertheless, in this variant of the E6CHM the observed matter-antimatter asymmetry can be induced if CP is violated. The pNGB scalar coloured triplet plays a key role in this process and leads to a distinct signature that may be detected at the LHC in the near future.

hep-ph↗

arXiv2018

Neutron to Dark Matter Decay in Neutron Stars

Recent proposals have suggested that a previously unknown decay mode of the neutron into a dark matter particle could solve the long lasting measurement problem of the neutron decay width. We show that, if the dark particle in neutron decay is the major component of the dark matter in the universe, this proposal is in disagreement with modern astro-physical data concerning neutron star masses.

nucl-th↗

arXiv2018

Charge symmetry violation in the doubly charmed cascade masses

We investigate the electromagnetic contribution to the charge symmetry breaking in the $Ξ_{cc}$ baryon masses using a subtracted dispersion relation based on the Cottingham formula, following the formalism developed in an analysis of the octet baryon mass differences. In the absence of experimental information on the structure of charmed baryons, we use parameters for the electromagnetic structure of the $Ξ_{cc}$ and the difference in its charge states obtained from lattice QCD and estimates of SU(4) symmetry breaking. We report a conservative estimate for the mass splitting of the doubly charmed cascades to be 8 $\pm$ 9 MeV. While a smaller isospin splitting is compatible with this result, surprisingly it does not preclude the large splitting reported by the SELEX Collaboration. We identify those quantities which could be determined from lattice QCD and which would reduce the quoted theoretical uncertainty.

nucl-th↗

arXiv2018

Quark-Meson-Coupling (QMC) model for finite nuclei, nuclear matter and beyond

The Quark-Meson-Coupling model, which self-consistently relates the dynamics of the internal quark structure of a hadron to the relativistic mean fields arising in nuclear matter, provides a natural explanation to many open questions in low energy nuclear physics, including the origin of many-body nuclear forces and their saturation, the spin-orbit interaction and properties of hadronic matter at a wide range of densities up to those occurring in the cores of neutron stars. Here we focus on four aspects of the model (i) a full comprehensive survey of the theory, including the latest developments, (ii) extensive application of the model to ground state properties of finite nuclei and hypernuclei, with a discussion of similarities and differences between the QMC and Skyrme energy density functionals, (iii) equilibrium conditions and composition of hadronic matter in cold and warm neutron stars and their comparison with the outcome of relativistic mean-field theories and, (iv) tests of the fundamental idea that hadron structure changes in-medium.

nucl-th↗

arXiv2018

Implications of Neutron Star Properties for the Existence of Light Dark Matter

It was recently suggested that the discrepancy between two methods of measuring the lifetime of the neutron may be a result of an unseen decay mode into a dark matter particle which is almost degenerate with the neutron. We explore the consequences of this for the properties of neutron stars, finding that their known properties are in conflict with the existence of such a particle.

nucl-th↗

arXiv2017

$ϕ$ meson in nuclear matter and nuclei

The mass and decay width of the $ϕ$ meson in cold nuclear matter are computed in an effective Lagrangian approach, with the medium dependence of these properties obtained by evaluating kaon-antikaon loop contributions to the $ϕ$ self-energy. At normal nuclear matter density, we find a downward shift of the $ϕ$ mass by a few percent, while the decay width is enhanced by an order of magnitude. We also present $ϕ$--nucleus bound state energies and absorption widths for some selected nuclei, using complex optical potentials obtained in the local density approximation. Our results suggest that the $ϕ$ should form bound states with all the nuclei considered. However, the identification of the signal for these predicted bound states will need careful investigation because of their sizable absorption widths.

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