arXiv subjects

A. W. Thomas

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

At least 379 records · Page 21Linked to original sources

arXiv1998

Rho-Omega Mixing and Direct CP Violation in Hadronic B-Decays

The extraction of CKM-matrix-element information from hadronic B-decays generally suffers from discrete ambiguities, hampering the diagnosis of physics beyond the Standard Model. We show that a measurement of the rate asymmetry, which is CP-violating, in $B^{\pm}\toρ^{\pm}ρ^0(ω)\toρ^{\pm}π^+π^-$, where the invariant mass of the $π^+π^-$ pair is in the vicinity of the $ω$ resonance, can remove the mod($π$) uncertainty in $α\equiv arg [-V_{td} V_{tb}^\ast/(V_{ud}V_{ub}^\ast)]$ present in standard analyses.

hep-ph↗

arXiv1998

The neutron charge form factor in helium-3

In order to measure the charge form factor of the neutron, $G^n_E(Q^2)$, one needs to use a neutron bound in a nuclear target. We calculate the change in the form factor for a neutron bound in $^3He$, with respect to the free case, using several versions of the quark meson coupling model. It is found that the form factor may be suppressed by as much as 12% at $Q^2 = 0.5{GeV}^2$ with respect to that of the free neutron.

nucl-th↗

arXiv1998

Medium dependence of the bag constant in the quark-meson coupling model

Possible variations of the quark-meson coupling (QMC) model are examined in which the bag constant decreases in the nuclear medium. The reduction is supposed to depend on either the mean scalar field or the effective mass of the nucleon. It is shown that the electric and magnetic radii of the bound nucleon are almost linearly correlated with the bag constant. Using the fact that the size of the bound nucleon inside a nucleus is strongly constrained by $y$-scaling data in quasielastic, electron-nucleus scattering, we set a limit for the reduction allowed in the bag constant for these two models. The present study implies that the bag constant can decrease up to 10--17 % at average nuclear density, depending on the details of the model.

nucl-th↗

arXiv1998

Quark and gluon condensates in the quark-meson coupling model

Using the quark-meson coupling (QMC) model, we study the density dependence of the quark and gluon condensates in nuclear matter. We show that the change of the quark condensate is mainly driven by the scalar field in the medium and that the reduction of the quark condensate is suppressed at high density, even in the mean-field approximation. The gluon condensate decreases by 4 - 6 % at nuclear saturation density. We also give a simple relationship between the change of the quark condensate and that of a hadron mass in the medium.

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arXiv1997

A clue to the mechanism of $ΛK^+$ production in $pp$-reactions

We analyse the $p p \to p ΛK^+$ total cross section data which were recently measured by the COSY-11 collaboration at an energy 2~MeV above the reaction threshold. Our analysis suggests that the measurement of the invariant mass spectrum for the $ΛK^+$ at energies around 100 MeV above the threshold can provide a new constraint on the theoretical calculations for this reaction. In particular, the measurement can give a clue as to whether the reaction is dominated by resonance excitation or not. Thus, it should contribute to further understanding of the strangeness production mechanism. We show the invariant mass spectra for the $ΛK^+$ system in these optimal kinematic conditions calculated by several approaches, and demonstrate that our suggestion is experimentally feasible.

nucl-th↗

arXiv1997

Non-Perturbative Structure of the Nucleon

While much attention has been focussed on the successes of perturbative QCD in describing the $Q^2$-dependence of deep-inelastic structure functions, the starting distributions themselves contain important, non-perturbative information on the structure of the nucleon, which has been somewhat neglected. We review some of the most important, recent discoveries resulting from studies of deep-inelastic scattering. There are important connections between these discoveries and low energy properties of the nucleon and wherever possible we shall make these clear. In particular, we shall see that well known features of QCD, such as dynamical symmetry breaking, are reflected in the properties of the measured parton distributions.

hep-ph↗

arXiv1997

The quark-meson coupling model for Lambda, Sigma and Xi hypernuclei

The quark-meson coupling (QMC) model, which has been successfully used to describe the properties of both infinite nuclear matter and finite nuclei, is applied to a systematic study of $Λ, Σ$ and $Ξ$ hypernuclei. Assumptions made in the present study are, (i) the (self-consistent) exchanged scalar, and vector, mesons couple only to the u and d quarks, and (ii) an SU(6) valence quark model for the bound nucleons and hyperon. The model automatically leads to a very weak spin-orbit interaction for the $Λ$ in a hypernucleus. Effects of the Pauli blocking at the quark level, particularly in the open, coupled, $ΣN - ΛN$ channel (strong conversion), is also taken into account in a phenomenological way.

nucl-th↗

arXiv1997

$Q^2$--Dependence of the Gerasimov-Drell-Hearn Sum Rule

We test the Gerasimov-Drell-Hearn (GDH) sum rule numerically by calculating the total photon absorption cross sections $σ_{1/2}$ and $σ_{3/2}$ on the nucleon via photon excitation of baryon resonances in the constituent quark model. A total of seventeen, low-lying, non-strange baryon resonances are included in this calculation. The transverse and longitudinal interference cross section, $σ_{1/2}^{TL}$, is found to play an important role in the study of the $Q^2$ variation of the sum rule. The results show that the GDH sum rule is saturated by these resonances at a confidence level of 94%. In particular, the $P_{33}(1232)$ excitation largely saturates the sum rule at $Q^2 = 0$, and dominates at small $Q^2$. The GDH integral has a strong $Q^2$-dependence below $Q^2= 1.0 {GeV}^2$ and changes its sign around $Q^2= 0.3 {GeV}^2$. It becomes weakly $Q^2$-dependent for $Q^2 > 1.0 {GeV}^2$ because of the quick decline of the resonance contributions. We point out that the $Q^2$ variation of the GDH sum rule is very important for understanding the nucleon spin structure in the non-perturbative QCD region.

nucl-th↗

arXiv1997

Electromagnetic Form Factors of the Nucleon in an Improved Quark Model

Nucleon electromagnetic form factors are studied in the cloudy bag model (CBM) with center-of-mass and recoil corrections. This is the first presentation of a full set of nucleon form factors using the CBM. The center of mass motion is eliminated via several different momentum projection techniques and the results are compared. It is found that the shapes of these form factors are significantly improved with respect to the experimental data if the Lorentz contraction of the internal structure of the baryon is also appropriately taken into account.

nucl-th↗

arXiv1997

Deconfinement at finite chemical potential

In a confining, renormalisable, Dyson-Schwinger equation model of two-flavour QCD we explore the chemical-potential dependence of the dressed-quark propagator, which provides a means of determining the behaviour of the chiral and deconfinement order parameters, and low-energy pion observables. We find coincident, first order deconfinement and chiral symmetry restoration transitions at μ_c = 375 MeV. f_πis insensitive to μuntil μ\approx μ_0 = 0.7 mu_c when it begins to increase rapidly. m_πis weakly dependent on μ, decreasing slowly with μand reaching a minimum 6% less than its μ=0 value at μ=μ_0. In a two-flavour free-quark gas at μ=μ_c the baryon number density would be approximately 3 ρ_0, where ρ_0=0.16 fm^{-3}; while in such a gas at μ_0 the density is ρ_0.

nucl-th↗

arXiv1997

HERA Anomaly and Hard Charm in the Nucleon

We explore the possibility that the excess neutral and charged current events seen by the H1 and ZEUS Collaborations at HERA at large x and Q^2 could arise from a hard charm component of the nucleon. While the symmetric intrinsic charm hypothesis is unable to account for the HERA anomaly, a non-symmetric charm distribution generated non-perturbatively, for which c-bar is much harder than c, can produce significant enhancement of cross sections at the HERA kinematics.

hep-ph↗

arXiv1997

Deuteron Structure Functions in the Context of Few-Body Physics

Few-body systems offer a unique challenge to those interested in deep inelastic scattering. Using the deuteron as the simplest and most easily solvable nuclear system we review the main physics issues in the valence and sea quark regimes. For the former the key issue is the understanding of Fermi motion and binding corrections, and our ability to extract the ratio F_2n/F_2p as x -> 1. The most recent analysis of deuteron data for F_2n/F_2p is consistent with perturbative QCD, but inconsistent with the assumptions common to all standard parton distribution fits. This should be taken into account in calculating event rates at HERA --- especially at large x and Q^2. The extraction of g_1n also presents an opportunity to refine the description of nuclear effects in spin-dependent structure functions. In the region of small x one must correct for shadowing and meson exchange current effects in order to explore the flavor structure of sea, which is, in turn, deeply related to the chiral structure of the nucleon.

hep-ph↗

arXiv1997

Testing Parton Charge Symmetry at HERA

There are strong theoretical indications that the minority valence quark distributions in the nucleon may break charge symmetry by as much as 3-5%. We show that a comparison of $e^{\pm} D$ deep inelastic scattering through the charged current, for example at HERA, could provide a direct test of this effect. This measurement is also be sensitive to an intrinsic component of the strange quark sea which leads to $s \neq \bar{s}$.

hep-ph↗

arXiv1997

Rho-omega mixing, vector meson dominance and the pion form-factor

We review the current status of rho-omega mixing and discuss its implication for our understanding of charge-symmetry breaking. In order to place this work in context we also review the photon-hadron coupling within the framework of vector meson dominance. This leads naturally to a discussion of the electromagnetic form-factor of the pion and of nuclear shadowing.

hep-ph↗

arXiv1997

Direct CP Violation in $B^\pm\toρ^\pmπ^+π^-$ in the $ρ^0$-$ω$ Interference Region

We study direct CP violation in $B^\pm \to ρ^\pmρ^0(ω) \to ρ^\pmπ^+π^-$ and focus specifically on the rate asymmetry in the $ρ^0$-$ω$ interference region. Here the strong phase is dominated by isospin violation, so that it can be essentially determined by $e^+e^- \to ρ^0 (ω) \to π^+π^-$ data. We find the CP-violating asymmetry to be of the order of 20% at the $ω$ invariant mass. Moreover, it is robust with respect to the estimable strong-phase uncertainties, permitting the extraction of $\sinα$ from this channel.

hep-ph↗

arXiv1997

Near-threshold Isospin Violation in the Pion Form Factor from Chiral Perturbation Theory

We examine the isospin violation in the timelike pion form factor near threshold at next-to-leading order in the chiral expansion using the techniques of Chiral Perturbation Theory. This next-to-leading order contribution contains the first nonvanishing isospin violation. This isospin violation is found to be very small near threshold. In particular, the isospin violation at threshold is found to be of order 10^-4, which should be compared with the few percent level seen in the vector meson resonance region.

hep-ph↗

arXiv1997

Self-consistent description of $Λ$ hypernuclei in the quark-meson coupling model

The quark-meson coupling (QMC) model, which has been successfully used to describe the properties of both finite nuclei and infinite nuclear matter, is applied to a study of $Λ$ hypernuclei. With the assumption that the (self-consistent) exchanged scalar, and vector, mesons couple only to the u and d quarks, a very weak spin-orbit force in the $Λ$-nucleus interaction is achieved automatically. This is a direct consequence of the SU(6) quark model wave function of the $Λ$ used in the QMC model. Possible implications and extensions of the present investigation are also discussed.

nucl-th↗

arXiv1997

On extracting the rho-omega mixing amplitude from the pion form-factor

In this paper we improve and extend a recent analysis which showed that the rho-omega mixing amplitude cannot be unambiguously extracted from the pion electromagnetic form-factor in a model independent way. In particular, we focus on the argument that the extraction is sensitive to the presence of any intrinsic omega_I -> pi pi coupling. Our extended analysis confirms the original conclusion, with only minor, quantitative differences. The extracted mixing amplitude is shown to be sensitive to both the intrinsic coupling omega_I -> pi pi and to the value assumed for the mass of the rho^0-meson.

hep-ph↗