arXiv subjects

A. W. Thomas

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

At least 307 records · Page 17Linked to original sources

arXiv2001

The Stability of Nuclear Matter in the Nambu-Jona-Lasinio Model

Using the Nambu-Jona-Lasinio model to describe the nucleon as a quark-diquark state, we discuss the stability of nuclear matter in a hybrid model for the ground state at finite nucleon density. It is shown that a simple extension of the model to simulate the effects of confinement leads to a scalar polarizability of the nucleon. This, in turn, leads to a less attractive effective interaction between the nucleons, helping to achieve saturation of the nuclear matter ground state. It is also pointed out that that the same effect naturally leads to a suppression of ``Z-graph'' contributions with increasing scalar potential.

nucl-th↗

arXiv2001

A New Slant on Hadron Structure

Rather than regarding the restriction of current lattice QCD simulations to quark masses that are 5--10 times larger than those observed, we note that this presents a wonderful opportunity to deepen our understanding of QCD. Just as it has been possible to learn a great deal about QCD by treating $N_c$ as a variable, so the study of hadron properties as a function of quark mass is leading us to a much deeper appreciation of hadron structure. As examples we cite recent progress in using the chiral properties of QCD to connect hadron masses, magnetic moments, charge radii and structure functions calculated at large quark masses within lattice QCD with the values observed physically.

nucl-th↗

arXiv2001

Pions in isospin asymmetric nuclei

Using a pair of the lightest mirror nuclei, $^3$He and $^3$H, we study the effect of the medium modification of pion fields on the flavor non-singlet structure function. The change of the pion fields leads to an enhancement of the flavor asymmetry of the antiquark distributions in a nucleus.

nucl-th↗

arXiv2001

Non-singlet structure function of the 3He-3H system and divergence of the Gottfried integral

We study shadowing and antishadowing corrections to the flavor non-singlet structure function F_2(3He)-F_2(3H) and show that the difference between the one-particle density distributions of 3He and 3H plays an important role at very small x. We find that the flavor non-singlet structure function in these mirror nuclei is enhanced at small x by nuclear shadowing, which increases the nuclear Gottfried integral, integrated from 10^{-4} to 1, by 11-36 %. When integrated from zero, the Gottfried integral is divergent for these mirror nuclei. It seems likely that, as a consequence of charge symmetry breaking, this may also apply to the proton-neutron system.

hep-ph↗

arXiv2001

Chiral Symmetry and the Intrinsic Structure of the Nucleon

Understanding hadron structure within the framework of QCD is an extremely challenging problem. In order to solve it, it is vital that our thinking should be guided by the best available insight. Our purpose here is to explain the model independent consequences of the approximate chiral symmetry of QCD for two famous results concerning the structure of the nucleon. We show that both the apparent success of the constituent quark model in reproducing the ratio of the proton to neutron magnetic moments and the apparent success of the Foldy term in reproducing the observed charge radius of the neutron are coincidental. That is, a relatively small change of the current quark mass would spoil both results.

hep-ph↗

arXiv2001

Charmonium absorption by nucleons

$J/Ψ$ dissociation in collisions with nucleons is studied within a boson exchange model and the energy dependence of the dissociation cross section is calculated from the threshold for $Λ_c{\bar D}$ production to high energies. We illustrate the agreement of our results with calculations based on short distance QCD and Regge theory. The compatibility between our calculations and the data on $J/Ψ$ photoproduction on a nucleon is discussed. We evaluate the elastic $J/Ψ{+}N$ cross section using a forward dispersion relation and demonstrate the overall agreement with the predictions from QCD sum rules. Our results are compatible with the phenomenological dissociation cross section evaluated from the experimental data on $J/Ψ$ production from $γ{+}A$, $p{+}A$ and $A{+}A$ collisions.

nucl-th↗

arXiv2000

Structure functions for the three nucleon system

The spectral functions and light-cone momentum distributions of protons and neutrons in 3He and 3H are given in terms of the three-nucleon wave function for realistic nucleon-nucleon interactions. To reduce computational complexity, separable expansions are employed for the nucleon-nucleon potentials. The results for the light-cone momentum distributions suggest that they are not very sensitive to the details of the two-body interaction, as long as it has reasonable short-range repulsion. The unpolarised and polarised structure functions are examined for both 3He and 3H in order to test the usefulness of 3He as a neutron target. It is found that the measurement of the spin structure function of polarised 3H would provide a very clear test of the predicted change in the polarised parton distributions of a bound proton.

nucl-th↗

arXiv2000

Can the Lambda pi scattering phase shifts be large?

Motivated by the presence of nearby thresholds in other baryon-meson channels with I=1 and S=-1, we investigate whether the Lambda pi scattering phase shifts at a center-of-mass energy equal to the Xi mass could be larger than suggested by lowest-order chiral perturbation theory. Within a coupled-channel K-matrix approach, we find that the S-wave phase shift could be as large as -7 degrees.

hep-ph↗

arXiv2000

Kaon properties and cross sections in nuclear medium

Results for the $π+ N \to Λ, Σ+ K$ reactions in nuclear matter of Ref. nucl-th/0004011 are presented. To evaluate the in-medium modification of the reaction amplitude as a function of the baryonic density we introduce relativistic, mean-field potentials for the initial, final and intermediate mesonic and baryonic states in the resonance model. These vector and scalar potentials were calculated using the quark meson coupling model. Contrary to earlier work which has not allowed for the change of the cross section in medium, we find that the data for kaon production at SIS energies are consistent with a repulsive $K^+$-nucleus potential.

nucl-th↗

arXiv2000

Measurement of the F2n/F2p and d/u Ratios in Deep Inelastic Electron Scattering off 3H and 3He

We discuss a possible measurement of the ratio of the nucleon structure functions, F2n/F2p, and the ratio of the up to down quark distributions, u/d, at large x, by performing deep inelastic electron scattering from the 3H and 3He mirror nuclei with the 11 GeV upgraded beam of Jefferson Lab. The measurement is expected to be almost free of nuclear effects, which introduce a significant uncertainty in the extraction of these two ratios from deep inelastic scattering off the proton and deuteron. The results are expected to test perturbative and non-perturbative mechanisms of spin-flavor symmetry breaking in the nucleon, and constrain the structure function parameterizations needed for the interpretation of high energy e-p, p-p and p-pbar collider data. The precision of the expected data can also allow for testing competing parameterizations of the nuclear EMC effect and provide valuable constraints on models of its dynamical origin.

nucl-ex↗

arXiv2000

In-medium kaon and antikaon properties in the quark-meson coupling model

The properties of the kaon, $K$, and antikaon, $\kbar$, in nuclear medium are studied in the quark-meson coupling (QMC) model. Employing a constituent quark-antiquark (MIT bag model) picture, their excitation energies in a nuclear medium at zero momentum are calculated within mean field approximation. The scalar, and the vector mesons are assumed to couple directly to the nonstrange quarks and antiquarks in the $K$ and $\kbar$ mesons. It is demonstrated that the $ρ$ meson induces different mean field potentials for each member of the isodoublets, $K$ and $\kbar$, when they are embedded in asymmetric nuclear matter. Furthermore, it is also shown that this $ρ$ meson potential is repulsive for the $K^-$ meson in matter with a neutron excess, and renders $K^-$ condensation less likely to occur.

nucl-th↗

arXiv2000

Strangeness production from $πN$ collisions in nuclear matter

Kaon production in pion-nucleon collisions in nuclear matter is studied in the resonance model. To evaluate the in-medium modification of the reaction amplitude as a function of the baryonic density we introduce relativistic, mean-field potentials for the initial, final and intermediate mesonic and baryonic states. These vector and scalar potentials were calculated using the quark-meson coupling (QMC) model. The in-medium kaon production cross sections in pion-nucleon interactions for reaction channels with $Λ$ and $Σ$ hyperons in the final state were calculated at the baryonic densities appropriate to relativistic heavy ion collisions. Contrary to earlier work which has not allowed for the change of the cross section in medium, we find that the data for kaon production are consistent with a repulsive $K^+$-nucleus potential.

nucl-th↗

arXiv2000

Enhanced Direct CP Violation in $B^{\pm} \to ρ^{0} π^{\pm}$

We study direct CP violation in the hadronic decay $B^{\pm} \to ρ^{0}π^{\pm}$, including the effect of $ρ- ω$ mixing. We find that the CP violating asymmetry is strongly dependent on the CKM matrix elements, especially the Wolfenstein parameter $η$. For fixed $N_{c}$ (the effective parameter associated with factorization), the CP violating asymmetry, $a$, has a maximum of order $30%-50%$ when the invariant mass of the $π^{+}π^{-}$ pair is in the vicinity of the $ω$ resonance. The sensitivity of the asymmetry, $a$, to $N_{c}$ is small. Moreover, if $N_{c}$ is constrained using the latest experimental branching ratios from the CLEO collaboration, we find that the sign of $\sin δ$ is always positive. Thus, a measurement of direct CP violation in $B^{\pm} \to ρ^{0}π^{\pm}$ would remove the mod$(π)$ ambiguity in ${\rm arg}[ - \frac{V_{td}V_{tb}^{\star}}{V_{ud}V_{ub}^{\star}}]$.

hep-ph↗

arXiv2000

Neutron Structure Function and A=3 Mirror Nuclei

We investigate deep inelastic scattering from He-3 and H-3 within a conventional convolution treatment of binding and Fermi motion effects. Using realistic Faddeev wave functions together with a nucleon spectral function, we demonstrate that the free neutron structure function can be extracted in deep-inelastic scattering from A=3 mirror nuclei, with nuclear effects canceling to within 2% for x < 0.85.

nucl-th↗

arXiv2000

Incorporating Chiral Symmetry and Heavy Quark Theory in Extrapolations of Octet Baryon Charge Radii

We extrapolate lattice calculations of electric charge radii of the spin-1/2 baryon octet to the physical regime. The extrapolation procedure incorporates chiral perturbation theory and heavy quark effective theory in the appropriate limits. In particular, this procedure includes the non-analytic, logarithmic terms from pion loops. The electric charge radii of the nucleons and $Σ^-$ obtained from the chiral extrapolations agree well with experimental data. We make predictions for the charge radii of the remaining baryons in anticipation of future experimental measurements.

hep-lat↗

arXiv2000

Dynamical Symmetry Breaking in the Sea of the Nucleon

We derive the non-analytic chiral behavior of the flavor asymmetry dbar - ubar. Such behavior is a unique characteristic of Goldstone boson loops in chiral theories, including QCD, and establishes the unambiguous role played by the Goldstone boson cloud in the sea of the proton. Generalizing the results to the SU(3) sector, we show that strange chiral loops require that the s - sbar distribution be non-zero.

hep-ph↗

arXiv2000

Deep inelastic scattering on asymmetric nuclei

We study deep inelastic scattering on isospin asymmetric nuclei. In particular, the difference of the nuclear structure functions and the Gottfried sum rule for the lightest mirror nuclei, 3He and 3H, are investigated. It is found that such systems can provide significant information on charge symmetry breaking and flavor asymmetry in the nuclear medium. Furthermore, we propose a new method to extract the neutron structure function from radioactive isotopes far from the line of stability. We also discuss the flavor asymmetry in the Drell-Yan process with isospin asymmetric nuclei.

nucl-th↗

arXiv2000

Role of the Delta (1232) in DIS on polarized $^3$He and extraction of the neutron spin structure function $g_{1}^{n}(x,Q^2)$

We consider the effect of the transitions $n \to Δ^{0}$ and $p \to Δ^{+}$ in deep inelastic scattering on polarized $^3$He on the extraction of the neutron spin structure function $g_{1}^{n}(x,Q^2)$. Making the natural assumption that these transitions are the dominant non-nucleonic contributions to the renormalization of the axial vector coupling constant in the A=3 system, we find that the effect of $Δ$ increases $g_{1}^{n}(x,Q^2)$ by $10 ÷40$% in the range $0.05 \le x \le 0.6$, where our considerations are applicable and most of the data for $g_{1}^{n}(x,Q^2)$ exist.

hep-ph↗