arXiv subjects

C. J. Benesh

Publications and source records attributed to C. J. Benesh.

13 recordsLinked to original sources

arXiv2003

Valence Quark Distribution in A=3 Nuclei

We calculate the quark distribution function for 3He/3H in a relativistic quark model of nuclear structure which adequately reproduces the nucleon approximation, nuclear binding energies, and nuclear sizes for small nuclei. The results show a clear distortion from the quark distribution function for individual nucleons (EMC effect) arising dominantly from a combination of recoil and quark tunneling effects. Antisymmetrization (Pauli) effects are found to be small due to limited spatial overlaps. We compare our predictions with a published parameterization of the nuclear valence quark distributions and find significant agreement.

nucl-th↗

arXiv1998

Flavor and Charge Symmetry in the Parton Distributions of the Nucleon

Recent calculations of charge symmetry violation(CSV) in the valence quark distributions of the nucleon have revealed that the dominant symmetry breaking contribution comes from the mass associated with the spectator quark system.Assuming that the change in the spectator mass can be treated perturbatively, we derive a model independent expression for the shift in the parton distributions of the nucleon. This result is used to derive a relation between the charge and flavor asymmetric contributions to the valence quark distributions in the proton, and to calculate CSV contributions to the nucleon sea. The CSV contribution to the Gottfried sum rule is also estimated, and found to be small.

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arXiv1997

Neutrino Annihilation in Stellar Magnetic Fields

The potential enhancement of the cross section $ν\barν\to e^+e^-$ in the presence of a magnetic field is of critical interest for the study of supernovae and as a possible mechanism for gamma ray bursts. While parity violation(PV) in this reaction in free space is forbidden by CP, the presence of a CP non-invariant background gas of electrons creates an asymmetry in the cross section which may contribute to the asymmetry of a supernova and the natal velocities of neutron stars. We calculate the cross section in the presence of fields as high as $B = 10^{16}$G and find no significant enhancement of the cross section with magnetic field strength. By studying the systematics of our results as a function of environmental variables(field strength, density,temperature), we extrapolate the relative strength of the parity violating terms to the weak field limit, and find the parity violation is insufficient to provide the ``kick'' required to explain the observed velocities of neutron stars.

astro-ph↗

arXiv1997

Reply to Baur's and Bertulani's Comment

Shortcomings of a momentum-space treatment of strong absorption, as discussed in the previous Comment [nucl-th/9611025], are only of concern at low projectile energies, $γ$< 1.5. At intermediate and high energies, for which the quantum-mechanical equivalent-photon spectrum is intended, the quantum-mechanical cross sections are reduced relative to the semi-classical results whether one treats strong effects via a momentum-space or an impact-parameter (spatial) cut-off. At these energies the origin of the discrepancy between the predictions of full quantuym-mechanical and the semi-classical calulations cannot be traced to differences in the treatment of strong-interaction effects. Rather, they arise from quantum effects neglected in the semi-classical calculations.

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arXiv1996

Charge Symmetry Breaking in the Valence Quark Distributions of the Nucleon

Using a quark model, we study the effect of charge symmetry breaking on the valence quark distributions of the nucleon. The effect due to quark mass differences and the Coulomb interaction of the electrically charged quarks is calculated and, in contrast to recent claims, found to be small. In addition, we investigate the effect of charge symmetry breaking in the confining interaction, and in the perturbative evolution equations used to relate the quark model distributions to experiment. We find that both these effects are small, and that the strong charge symmetry breaking effect included in the scalar confining interactions may be distinguishable from that generated by quark mass differences.

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arXiv1996

A Quantum-Mechanical Equivalent-Photon Spectrum for Heavy-Ion Physics

In a previous paper, we calculated the fully quantum-mechanical cross section for electromagnetic excitation during peripheral heavy-ion collisions. Here, we examine the sensitivity of that cross section to the detailed structure of the projectile and target nuclei. At the transition energies relevant to nuclear physics, we find the cross section to be weakly dependent on the projectile charge radius, and to be sensitive to only the leading momentum-transfer dependence of the target transition form factors. We exploit these facts to derive a quantum-mechanical ``equivalent-photon spectrum'' valid in the long-wavelength limit. This improved spectrum includes the effects of projectile size, the finite longitudinal momentum transfer required by kinematics, and the response of the target nucleus to the off-shell photon.

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arXiv1995

Quark Model Calculations Of Symmetry Breaking in Parton Distributions

Using a quark model, we calculate symmetry breaking effects in the valence quark distributions of the nucleon. In particular, we examine the breaking of the quark model SU(4) symmetry by color magnetic effects, and find that color magnetism provides an explanation for deviation of the ratio $d_V(x)/u_V(x)$ from $1/2$. Additionally, we calculate the effect of charge symmetry breaking in the valence quark distributions of the proton and neutron and find, in contrast to other authors, that the effect is too small to be seen experimentally.

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arXiv1994

Simultaneous Projectile-Target Excitation in Heavy Ion Collisions

We calculate the lowest-order contribution to the cross section for simultaneous excitation of projectile and target nuclei in relativistic heavy ion collisions. This process is, to leading order, non-classical and adds incoherently to the well-studied semi-classical Weizsäcker-Williams cross section. While the leading contribution to the cross section is down by only $1/Z_P$ from the semiclassical process, and consequently of potential importance for understanding data from light projectiles, we find that phase space considerations render the cross section utterly negligible.

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arXiv1994

J-Psi Suppression in Hadron-Nucleus Collisions

We examine the production of J/$Ψ$ mesons in high energy hadron-nucleus collisions using models that account for both the initial state modification of parton distributions in nuclei and the final state interaction of the produced $c\bar c$ pairs. We show that, at the energies of current fixed target experiments, J/$Ψ$ production through quark-antiquark annihilation gives the largest contribution at $x_F > 0.5$, while gluon-gluon fusion dominates the production at smaller $x_F$. The observed J/$Ψ$ suppression at large $x_F$ is directly connected to nuclear shadowing in deeply inelastic scattering. We find that a $6\sim 8$mb $c\bar c$-nucleon cross section is needed to explain the data on J/$Ψ$ suppression if a simple incoherent multiple scattering formalism is used for the final state interactions.

hep-ph↗

arXiv1994

QCD Evolution by Finite Element Methods

A simple, new method for solving for the $Q^2$ evolution of parton distributions in perturbative QCD using cubic splines is described and applied to the evolution of nonsinglet quark distributions.

nucl-th↗

arXiv1993

Color Magnetic Corrections to Quark Model Valence Distributions

We calculate order $α_s$ color magnetic corrections to the valence quark distributions of the proton using the Los Alamos Model Potential wavefunctions. The spin-spin interaction breaks the model SU(4) symmetry, providing a natural mechanism for the difference between the up and down distributions. For a value of $α_s$ sufficient to produce the $N-Δ$ mass splitting, we find up and down quark distributions in reasonable agreement with experiment.

nucl-th↗

arXiv1992

Electromagnetic Dissociation of Nuclei in Heavy-Ion Collisions

Large discrepancies have been observed between measured Electromagnetic Dissociation(ED) cross sections and the predictions of the semiclassical Weizäcker-Williams-Fermi(WWF) method. In this paper, the validity of the semiclassical approximation is examined. The total cross section for electromagnetic excitation of a nuclear target by a spinless projectile is calculated in first Born approximation, neglecting recoil. The final result is expressed in terms of correlation functions and convoluted densities in configuration space. The result agrees with the WWF approximation to leading order(unretarded electric dipole approximation), but the method allows an analytic evaluation of the cutoff, which is determined by the details of the electric dipole transition charge density. Using the Goldhaber-Teller model of that density, and uniform charge densities for both projectile and target, the cutoff is determined for the total cross section in the nonrelativistic limit, and found to be smaller than values currently used for ED calculations. In addition, cross sections are calculated using a phenomenological momentum space cutoff designed to model final state interactions. For moderate projectile energies, the calculated ED cross section is found to be smaller than the semiclassical result, in qualitative agreement with experiment.

nucl-th↗

arXiv1992

Quadrupole Contribution to Two Neutron Removal in Heavy Ion Collisions

In this report, electric quadrupole corrections to the two neutron removal cross section measured in heavy ion collisions are estimated for $^{197}$Au and $^{59}$Co targets. The quadrupole process is assumed to proceed primarily through excitation of the giant isovector quadrupole resonance, which then decays by neutron emission. For $^{59}$Co, the contribution from E2 radiation is found to be small, while for $^{197}$Au we find the quadrupole contribution resolves the discrepancy between experiment and the simple predictions of the Weissacker-Williams virtual photon method.

nucl-th↗