Deuteron photodisintegration. I. Theory and results at low energies

We extend the “elementary-particle treatment” of deuteron electrodisintegration at small excitation energies, as recently developed by Hwang, Henley, and Miller, by incorporating more (np)-scattering states so that deuteron disintegration by low-energy photons (e.g., ≤150 MeV) can also be investigated. As compared to the well-known formalism obtained by Partovi, the present theory has the following distinct features: The nucleon-only impulse approximation (NOIA), as modified to incorporate gauge invariance (GI), is assumed to be valid only in the Breit frame, in which the initial and final nuclear systems are treated symmetrically. The matrix element of the electromagnetic current is thus determined in the Breit frame and then transformed into the CM frame (i.e., the rest frame for the final n + p, or initial γ + D, system). In addition, the difference between the resultant $\text{NOIA}\text{GI}$ and the NOIA is determined explicitly in every channel.Numerical results are obtained for the Reid soft-core potential with extensions required for J > 2 scattering states. Our major result is that there is no substantial discrepancy between the θ_p = 0° data of Hughes et al. and the prediction of this formalism. General features of this formalism, including angular distributions at low energies, are discussed in quantitative terms. Other important results at these energies include: (1) the ³F_J(np)-scattering states are found to be important for E_γ ≥ 20 MeV, especially at forward and backward angles (i.e., θ_p ～ 0° or 180°); (2) the meson-exchange currents arising from intermediate isobar |Δ(1232)| propagation, with or without energy transfer effects, are not yet of any numerical significance; and (3) the results exhibit sensitivity to details of final-state interactions.     

On the validity of classical estimates of the dynamical Coulomb barrier in heavy-ion collisions

Several recent classical estimates of the effective Coulomb barrier height in heavy-ion collisions have approximated the dynamic deformations of the nuclei as entirely classical (non-quantized) surface oscillations, weakly coupled to the ion trajectories. An examination of the validity of this approximation shows it to be quite reliable when the surface oscillations are small and harmonic.     

Tuning the electronic communication and rates of intramolecular electron transfer of dimers of trinuclear ruthenium clusters: bridging and ancillary ligand effects

Ten new bridged dimers of oxo-centered triruthenium clusters with CO and 4-(dimethylamino)pyridine (dmap), pyridine (py), or 4-cyanopyridine (cpy) as terminal ligands and pyrazine-d(4) (d(4)-pz), 2,5-dimethylpyrazine (dmpz), 2-methylpyrazine (mpz), and 2-chloropyrazine (clpz) as bridging ligands were prepared. The carbonyl stretching frequency, nu(CO), was used as a probe for infrared spectroelectrochemical measurements. In the neutral and doubly reduced states, a single band was observed for each of the dimers, with a shift in frequency due to the oxidation state of the triruthenium clusters. In the singly reduced state, a range of nu(CO) line shapes was observed, depending on the nature of the ligands, from two bands centered at the frequencies of the bands of the neutral and doubly reduced species to one broad band at the average of these two frequencies. By synthesizing new combinations of bridging and ancillary ligands, electronic communication between two bridged triruthenium clusters was effectively tuned, and electron-transfer rates were estimated by IR spectral line-shape analysis. In dimers bridged by the asymmetric ligand mpz, it was possible through selective isotope labeling of one CO ligand to observe "mixed-valence isomers," the two alternate charge distributions of a mixed-valence complex.     

Masses ofq 2 \bar q 2 states in a Bethe-Salpeter model

Ground-state masses ofq ^{2 –2} states (true and mock baryonium) are investigated in the framework of a Bethe-Salpeter formalism motivated from QCD. The four-particle system is described by pairwise interactions betweenqq orq pairs with a spectator approximation for the non-interacting pair. The quark-quark interactions are Coulomb plus harmonic interactions; the harmonic terms have been modified to produce linear confinement for heavier quarks, in agreement with experimental spectra. The confining interaction is proportional to the strong coupling constant _s. Apart from the quark masses, the confining interaction is characterized by three basic parameters: (i) a universal spring constant ₀; (ii) a constantC ₀/ ₀ ² , which defines the vacuum structure; (iii) a constantA ₀, which provides a smooth transition from quadratic to linear confinement as one goes from light to heavy quark systems. These three constants [ ₀ = 0.158 GeV;C ₀=0.296;A ₀=0.0283] have been shown to produce excellent fits to all quarkonia states [q ,q ,Q ] as well as baryon spectra (qqq); thus our predictions forq ^{2 2} states contain no free parameters. In this model, theL=0 ground states occur in the range 1.8–2 GeV, 2.15–2.3 GeV and 6.72–6.75 GeV foru ^{2 2},s ^{2 2} andc ^{2 2} states, respectively. We discuss the prospects for these states to be seen experimentally. In the case of thes ^{2 2} state, this is likely to have a rather narrow width, and may correspond to theX(2.22 GeV) meson observed in radiative decays of theJ/ meson. Thec ^{2 2} state might also be visible as a resonance with an appreciable width.Research supported in part by the National Science Foundation under grant NSF-PHY 86-06364Research supported in part by the U.S. Department of Energy     

Interpretation of the NuTeV experiment

The NuTeV neutrino measurement of the Weinberg angle differs by 3 standard deviations from measurements at the Z pole. We review the status of various possibilities to explain this result, including contributions from physics outside the standard model; effects of radiative corrections; nuclear corrections to parton distributions; isospin-violating effects; and effects due to strange quarks.     

Coupled channel approach to the off-shell transition matrix

We propose a coupled-channel separable potential model with the feature that the off-shell elastic transition matrix can be generated directly from the scattering phase shifts. This should be useful in constructing pion-nucleus optical potentials.     

Inelastic shadowing effects in multiple scattering

The projectile-nucleon scattering amplitudes used as input into multiple scattering theories of projectile-nucleus scattering naturally include the effects of coupling to inelastic (i.e., production) channels. We employ a multichannel separable potential to describe the projectile-nucleon interaction and show that within the fixed nucleon framework we can obtain the nuclear elastic scattering amplitude. This includes terms outside the conventional formalisms, corresponding to intermediate propagation in the inelastic channels both above and below inelastic threshold. We refer to this as inelastic shadowing. In a two-channel approximation, we show that knowledge of the projectile-nucleon elastic scattering phase shifts plus specification of the inelastic threshold energy are sufficient to determine the off-shell coupled-channel transition matrix, implying that the nuclear amplitude can be calculated within this model without any detailed information about the inelastic channels. We study this solution quantitatively for some model problems and for pion scattering, with the general result that inelastic shadowing can be significant whenever the elementary interaction has important channel coupling. For pion scattering in the energy regime characterized by strongly absorptive resonances, we find, for example, that the effect of inelastic shadowing is much more important than that due to two-nucleon correlations.