J. Dubach

Galactic cosmic rays and N2 dissociation on Titan

The electromagnetic and particle cascade resulting from the absorption of galactic cosmic rays in the atmosphere of Titan is shown to be an important mechanism for driving the photochemistry at pressures of 1 to 50 mbar in the atmosphere. In particular, the cosmic ray cascade dissociates N2, a process necessary for the synthesis of nitrogen organics such as HCN. The important interactions of the cosmic ray cascade with the atmosphere are discussed. The N2 excitation and dissociation rates and the ionization rates of the principal atmospheric constituents are computed for a Titan model atmosphere that is consistent with Voyager 1 observations. It is suggested that HCN may be formed efficiently in the lower atmosphere through the photodissociation of methylamine. It is also argued that models of nitrogen and hydrocarbon photochemistry in the lower atmosphere of Titan should include the absorption of galactic cosmic rays as an important energy source.

Isospin dependences in parity-violating electron scattering

Abstract The role of the isospin structure of nuclear states in parity-violating single-arm electron scattering from nuclei is examined. Specific examples of elastic scattering from even-even N = Z (0 + , M T = 0, nominally T = 0) nuclei, which are supposed to be ideally suited for studying the electroweak interaction, are considered in detail. The aim is to find when nuclear structure effects due to isospin impurities significantly enter into the parity-violating asymmetry, and hence to determine the level of precision at which one can hope to extract the underlying weak neutral current couplings in a relatively model-independent way. The formalism is then extended to the general case of J > 0 states and even-even N ≠ Z nuclei. Finally, it is shown that measurement of the parity-violating asymmetry may provide a determination of the neutron distribution in nuclei.

Theory of Weak Hypernuclear Decay

The weak nomesonic decay of {Lambda}-hypernuclei is studied in the context of a one-meson-exchange model. Predictions are made for the decay rate, the {ital p}/{ital n} stimulation ratio and the asymmetry in polarized hypernuclear decay. Copyright {copyright} 1996 Academic Press, Inc.

Non-mesonic decay modes of hypernuclei

Abstract The present status of theoretical descriptions of non-mesonic ( Λ N → NN) decay modes of hypernuclei is reviewed. One model, which evaluates a strangeness-changing, parity-violating weak interaction “potential” in nuclear matter, is examined in detail to exemplify the physics which can be learned and the problems which must be confronted. The importance of measuring the different isospin decay channels and separating the parity-conserving and parity-violating modes is stressed. Finally, this model is briefly compared and contrasted with other existing calculations.

Cosmic ray ionization of the Jovian atmosphere

Abstract An approximate form of the Boltzmann equation has been used to obtain local ionization rates due to the absorption of galactic cosmic rays in the Jovian atmosphere. It is shown that the muon flux component of the cosmic ray-induced cascade may be especially importannt in ionizing the atmosphere at levels where the total number density exceeds 10 19 cm −3 (well below the ionospheric layers produced by solar euv). A model containing both positive and negative ion reactions has been employed to compute equilibrium electron and ion number densities. Peak electron number densities on the order of 10 3 cm −3 may be expected even at relatively low magnetic latitudes. The dominant positive ions are NH 4 + and C n H m + cluster ions, with n ⩾ 2; it is suggested that the absorption of galactic cosmic ray energy at such relatively high pressures in the Jovian atmosphere (M ∾ 10 18 to 10 20 cm −3 ) and the subsequent chemical reactions may be instrumental in the local formation of complex hydrocarbons.

NN → NNπ at intermediate energies: Predictions of a unitary model

Abstract We explore the NN → NN π reaction in detail within the context of a unified model which satisfies two- and three-body unitary. Predictions for differential cross sections and spin observables are examined and compared with experimental data, where available, for a variety of exclusive [pp → p(n) π + , pp → pp( π 0 ), and np → p(p) π − ] and inclusive [pp → p(X) and pp → n(X)] reactions. Based on predictions of the model, further experiments are suggested to shed additional light on the NN → NN π process. Due to dynamical symmetries the np → p(p) π − observables are strikingly different from the more familiar pp → p(n) π + observables, and we urge that this reaction be investigated experimentally. Experimental tests for the existence of dibaryons are also briefly discussed.

Prospects for using the (e, e′γ) reaction to explore nuclear structure☆

Abstract The (e, e′γ) coincidence reaction is considered in detail. The formalism is developed in a way which emphasizes the connection between this reaction and studies of the scattering of (possibly polarized) electrons from polarized nuclei. A wide variety of examples is given to demonstrate the usefulness of the (e, e′γ) reaction for nuclear structure studies and to demonstrate how this reaction serves as both a complement and a supplement to electron scattering from polarized nuclei. Finally, a few cases of special interest are discussed.

Pion‐exchange contribution to the parity‐violating asymmetry in pp scattering

Charged pion exchange, producing nΔ++ intermediate states, contributes negatively to the weak scattering asymmetry in longitudinally‐polarized pp scattering. Including this contribution moves the theoretical prediction away from the 800 MeV experimental datum. The pion‐exchange contribution has both inelastic and elastic scattering components and is sizeable even below the pion production threshold. Strong distortions enhance the magnitude of the effect.