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B. K. Jennings

Publications and source records attributed to B. K. Jennings.

35 records · Page 2Linked to original sources
arXiv2003

In-Medium rho^0 Spectral Function Study via the 2H, 3He, 12C(gamma,pi+pi-) Reaction

We report a helicity analysis of sub-threshold rho^0 production on 2H, 3He and 12C at low photo-production energies. The results are indicative of a large longitudinal rho^0 polarization (l=1, m=0) and are consistent with a strong helicity-flip production mechanism. This signature is used to extract in-medium rho^0_L invariant mass distributions for all three nuclei in a manner which is less model-dependent than previous measurements. The results are compared to kinematic and phenomenological models of the rho^0 spectral function. The 2H and 3He data distributions support the role of N*(1520) excitation in shaping the in-medium rho^0_L invariant mass distribution, while the 12C distributions are consistent with quasi-free rho^0_L production. The data support an in-medium modification of the rho^0_L invariant mass distribution.

nucl-ex↗
arXiv2000

Quark-Meson Coupling Model for a Nucleon

The quark-meson coupling model for a nucleon is considered. The model describes a nucleon as an MIT bag, in which quarks are coupled to scalar and vector mesons. A set of coupled equations for the quark and the meson fields are obtained and are solved in a self-consistent way. It is shown that the mass of a nucleon as a dressed MIT bag interacting with sigma- and omega-meson fields significantly differs from the mass of a free MIT bag. A few sets of model parameters are obtained so that the mass of a dressed MIT bag becomes the nucleon mass. The results of our calculations imply that the self-energy of the bag in the quark-meson coupling model is significant and needs to be considered in doing the nuclear matter calculations.

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arXiv2000

Low Energy Behavior of the \protect(^{7}\protect) Be \protect( (p,γ)^{8}\protect) B reaction

Three related features of the astrophysical S-factor, (S_{17}), for low energies are an upturn as the energy of the proton goes to zero, a pole at the proton separation energy and a long ranged radial integral peaking at approximately 40 fm. These features, particularly the last, mean (S_{17}) at threshold is largely determined by the asymptotic normalization of the proton bound-state wave function. In this paper we show that by taking the pole explicitly into account, (S_{17}) can be calculated in terms of purely short range integrals involving the two-body potential. Much of the discusion is relevant to other radiative capture processes especially those wth weakly bound final states.

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arXiv1998

On the equivalence of the Impulse Approximation and the Gersch-Rodriguez-Smith theory for structure functions

We derive for a non-relativistic system an approximation for Final State Interactions in a form, resembling a DWIA which corrects the structure function computed in the PWIA. We then compare the Gersch-Rodriguez-Smith and the IA series for structure functions and prove that to order ${\cal O}(1/q^2)$ the above DWIA representation is contained in the GRS series to the same order. There is an additional term in the GRS series that is missing in the DWIA due to the eikonal approximations in the latter. This strongly suggests that the two approaches, when treated exactly, produce identical structure function to arbitrary order in $1/q$.

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arXiv1998

Extrapolation of the astrophysical S factor for ^7Be(p,gamma)^8B to solar energies

We investigate the energy dependence of the astrophysical $S$ factor for the reaction $^7$Be$(p,γ)^8$B, the primary source of high-energy solar neutrinos in the solar $pp$ chain. Using simple models we explore the model dependence in the extrapolation of the experimental data to the region of astrophysical interest near 20 keV. We find that below approximately 400 keV the energy dependence is very well understood and constrained by the data for the elastic scattering of low energy neutrons from $^7$Li. Above 400 keV nuclear distortion of the wave function of the incident proton introduces a significant model dependence. This is particularly important for the s-wave contribution to the $S$ factor. The extracted value of S(0) is $19.0 \pm 1.0 \pm 0.2$~eVb. The first error is experimental while the second is an estimate of the theoretical error in the extrapolation.

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arXiv1998

S_eff(E) and the 7Be(p,gamma)8B reaction

We explore approximations to the effective $S$ factor for the $^7Be(p,γ)^8B$ reaction using different approximation for the integral over the Gamov peak. In the temperature range of interest for solar neutrino production, $S_{eff}(E)$ may be determined to within 0.5% from S(20) with no direct information on the derivatives of S(E).

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arXiv1998

Direct capture astrophysical S factors at low energy

We investigate the energy dependence of the astrophysical S factors for the reactions 7Be(p,gamma)8B, the primary source of high-energy solar neutrinos in the solar pp chain, and 16O(p,gamma)17F, an important reaction in the CNO cycle. Both of these reactions have predicted S factors which rise at low energies; we find the source of this behavior to be a pole in the S factor at a center-of-mass energy E = -E_B, the point where the energy of the emitted photon vanishes. The pole arises from a divergence of the radial integrals.

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arXiv1996

Change of MIT Bag Constant in Nuclear Medium and Implication for the EMC Effect

The modified quark-meson coupling model, which features a density dependent bag constant and bag radius in nuclear matter, is checked against the EMC effect within the framework of dynamical rescaling. Our emphasis is on the change in the average bag radius in nuclei, as evaluated in a local density approximation, and its implication for the rescaling parameter. We find that when the bag constant in nuclear matter is significantly reduced from its free-space value, the resulting rescaling parameter is in good agreement with that required to explain the observed depletion of the structure functions in the medium Bjorken $x$ region. Such a large reduction of the bag constant also implies large and canceling Lorentz scalar and vector potentials for the nucleon in nuclear matter which are comparable to those suggested by the relativistic nuclear phenomenology and finite-density QCD sum rules.

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arXiv1996

Recovering Relativistic Nuclear Phenomenology from the Quark-Meson Coupling Model

The quark-meson coupling (QMC) model for nuclear matter, which describes nuclear matter as non-overlapping MIT bags bound by the self-consistent exchange of scalar and vector mesons is modified by the introduction of a density dependent bag constant. It is found that when the bag constant is significantly reduced in nuclear medium with respect to its free space value, large canceling isoscalar Lorentz scalar and vector potentials for the nucleon in nuclear matter emerge naturally. Such potentials are comparable to those suggested by relativistic nuclear phenomenology. This suggests that the modification of the bag constant in the nuclear medium may play an important role in low- and medium-energy nuclear physics.

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arXiv1996

Modified Quark-Meson Coupling Model for Nuclear Matter

The quark-meson coupling model for nuclear matter, which describes nuclear matter as non-overlapping MIT bags bound by the self-consistent exchange of scalar and vector mesons, is modified by introducing medium modification of the bag constant. We model the density dependence of the bag constant in two different ways: one invokes a direct coupling of the bag constant to the scalar meson field, and the other relates the bag constant to the in-medium nucleon mass. Both models feature a decreasing bag constant with increasing density. We find that when the bag constant is significantly reduced in nuclear medium with respect to its free-space value, large canceling isoscalar Lorentz scalar and vector potentials for the nucleon in nuclear matter emerge naturally. Such potentials are comparable to those suggested by relativistic nuclear phenomenology and finite-density QCD sum rules. This suggests that the reduction of bag constant in nuclear medium may play an important role in low- and medium-energy nuclear physics.

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arXiv1996

Recovering Relativistic Nuclear Phenomenology from the Quark-Meson Coupling Model

The quark-meson coupling (QMC) model for nuclear matter, which describes nuclear matter as non-overlapping MIT bags bound by the self-consistent exchange of scalar and vector mesons is modified by the introduction of a density dependent bag constant. The density dependence of the bag constant is related to that of the in-medium effective nucleon mass through a scaling ansatz suggested by partial chiral symmetry restoration in nuclear matter. This modification overcomes drawbacks of the QMC model and leads to the recovery of the essential features of relativistic nuclear phenomenology. This suggests that the modification of the bag constant in the nuclear medium may play an important role in low- and medium-energy nuclear physics.

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arXiv1995

The Role of Color Neutrality in Nuclear Physics--Modifications of Nucleonic Wave Functions

The influence of the nuclear medium upon the internal structure of a composite nucleon is examined. The interaction with the medium is assumed to depend on the relative distances between the quarks in the nucleon consistent with the notion of color neutrality, and to be proportional to the nucleon density. In the resulting description the nucleon in matter is a superposition of the ground state (free nucleon) and radial excitations. The effects of the nuclear medium on the electromagnetic and weak nucleon form factors, and the nucleon structure function are computed using a light-front constituent quark model. Further experimental consequences are examined by considering the electromagnetic nuclear response functions. The effects of color neutrality supply small but significant corrections to predictions of observables.

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arXiv1995

Constraints on $Σ$ Nucleus Dynamics from Dirac Phenomenology of $Σ^-$ Atoms

Strong interaction level shifts and widths in $Σ^-$ atoms are analyzed by using a $Σ$ nucleus optical potential constructed within the relativistic mean field approach. The analysis leads to potentials with a repulsive real part in the nuclear interior. The data are sufficient to establish the size of the isovector meson--hyperon coupling. Implications to $Σ$ hypernuclei are discussed.

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arXiv1993

S-wave $π$-Nucleus Repulsion and Dirac Phenomenology

A relativistic $π$-nucleon potential is extended to $m^* \neq m$ to investigate the possibility of generating s-wave $π$-nucleus repulsion. We find that relativity does indeed generate significant repulsion, the exact amount depending on the details of the calculation. In contradistinction the $tρ$ approximation gives very little repulsion.

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arXiv1993

Generalized transparency in semi-inclusive processes

It is argued that the transparency of a medium for passage of a nucleon, knocked-out in a semi-inclusive $(e,e'p)$ reaction and subsequently scattered elastically, is not the same as the one measured in purely elastic scattering. Expressions are given for the properly generalized transparency and those are compared with recently proposed, alternative suggestions. Numerical results are presented for selected nuclear targets and kinematic conditions, applying to the Garino et al and the SLAC NE18 experiment.

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arXiv1992

Realistic Hadronic Matrix Element Approach to Color Transparency

Color transparency occurs if a small-sized wave packet, formed in a high momentum transfer process, escapes the nucleus before expanding. The time required for the expansion depends on the masses of the baryonic components of the wave packet. Measured proton diffractive dissociation and electron deep inelastic scattering cross sections are used to examine and severely constrain the relevant masses. These constraints allow significant color transparency effects to occur at experimentally accessible momentum transfers.

hep-ph↗