Collective Modes in a Slab of Interacting Nuclear Matter: The effects of finite range interactions

We consider a slab of nuclear matter and investigate the collective excitations, which develop in the response function of the system. We introduce a finite-range realistic interaction among the nucleons, which reproduces the full G-matrix by a linear combination of gaussian potentials in the various spin-isospin channels. We then analyze the collective modes of the slab in the S=T=1 channel: for moderate momenta hard and soft zero-sound modes are found, which exhaust most of the excitation strength. At variance with the results obtained with a zero range force, new "massive" excitations are found for the vector-isovector channel .Comment: 14 pages, TeX, 5 figures (separate uuencoded and tar-compressed postscript files), Torino preprint DFTT 6/9

RPA quasi-elastic responses in infinite and finite nuclear systems

Quasi-elastic responses in nuclear matter and in $^{12}$C and $^{40}$Ca nuclei are calculated in ring approximation to investigate the finite size effects on the electromagnetic quasi-elastic responses. A method to simulate these effects in infinite systems calculations is proposed. The sensitivity of the results to the various terms of the residual interaction is studied. The results of nuclear matter RPA calculations are compared with those obtained in ring approximation to evidence the importance of the exchange terms.Comment: 14 pages, 8 figure

Quantum interference terms in nonmesonic weak decay of Λ\Lambda-hypernuclei within a RPA formalism

Single and double coincidence nucleon spectra in the $\Lambda$-hypernuclei weak decay are evaluated and discussed using a microscopic formalism. Nuclear matter is employed together with the local density approximation which allows us to analyze the $^{12}_{\Lambda}C$ hypernucleus non-mesonic weak decay. Final state interactions (FSI) are included via the first order (in the nuclear residual interaction) terms to the RPA, where the strong residual interaction is modelled by a Bonn potential. At this level of approximation, these FSI are pure quantum interference terms between the primary decay $(\Lambda N \to NN)$ and $(\Lambda N \to NN \to NN)$, where the strong interaction is responsible for the last piece in the second reaction. Also the Pauli exchange contributions are explicitly evaluated. We show that the inclusion of Pauli exchange terms is important. A comparison with data is made. We conclude that the limitations in phase space in the RPA makes this approximation inadequate to reproduce the nucleon spectra. This fact, does not allow us to draw a definite conclusion about the importance of the interference terms.Comment: 39 pages, 10 figure

Towards a methanol economy based on homogeneous catalysis: methanol to H2 and CO2 to methanol

The possibility to implement both the exhaustive dehydrogenation of aqueous methanol to hydrogen and CO2 and the reverse reaction, the hydrogenation of CO2 to methanol and water, may pave the way to a methanol based economy as part of a promising renewable energy system. Recently, homogeneous catalytic systems have been reported which are able to promote either one or the other of the two reactions under mild conditions. Here, we review and discuss these developments

Nonmesonic Weak Decay of Λ\Lambda Hypernuclei within a Nuclear Matter Formalism

The nonmesonic weak decay of $\Lambda$ hypernuclei using nonrelativistic nuclear matter is studied. As the basic building block we use the Polarization Propagator Method developed by Oset and Salcedo. It is shown that the exact calculation of exchange terms is required. Using the Local Density Approximation we evaluate the nonmesonic decay width for $^{12}_{\Lambda}C$ and compare the result with a finite nucleus calculation, obtaining a qualitative agreement.Comment: 28 pages, 3 figure

Strange form factors of the proton: a new analysis of the neutrino (antineutrino) data of the BNL-734 experiment

We consider ratios of elastic neutrino(antineutrino)-proton cross sections measured by the Brookhaven BNL-734 experiment and use them to obtain the neutral current (NC) over charged current (CC) neutrino-antineutrino asymmetry. We discuss the sensitivity of these ratios and of the asymmetry to the electric, magnetic and axial strange form factors of the nucleon and to the axial cutoff mass M_A. We show that the effects of the nuclear structure and interactions on the asymmetry and, in general, on ratios of cross sections are negligible. We find some restrictions on the possible values of the parameters characterizing the strange form factors. We show that a precise measurement of the neutrino-antineutrino asymmetry would allow the extraction of the axial and vector magnetic strange form factors in a model independent way. The neutrino-antineutrino asymmetry turns out to be almost independent on the electric strange form factor and on the axial cutoff mass.Comment: 12 page

Exchange terms in the two--nucleon induced non--mesonic weak decay of Λ\Lambda--hypernuclei

The contribution of Pauli exchange terms to the two--nucleon induced non--mesonic weak decay of $^{12}_\Lambda$C hypernuclei, $\Lambda NN\to nNN$ ($N=n$ or $p$), is studied within a nuclear matter formalism implemented in a local density approximation. We have adopted a weak transition potential including the exchange of the complete octets of pseudoscalar and vector mesons as well as a residual strong interaction modeled on the Bonn potential. The introduction of exchange terms turns out to reduce the two--nucleon induced non--mesonic rate by 18% and, jointly with an increase in the one--nucleon induced rate by the same magnitude, reveals to be significant for an accurate determination of the full set of hypernuclear non--mesonic decay widths in theoretical and experimental analyses.Comment: 14 pages, 3 figure

Quasielastic Electron Scattering from Nuclei: Random-Phase vs. Ring Approximations

We investigate the extent to which the nuclear transverse response to electron scattering in the quasielastic region, evaluated in the random-phase approximation can be described by ring approximation calculations. Different effective interactions based on a standard model of the type g'+V_pi+V_rho are employed. For each momentum transfer, we have obtained the value of g'_0 permitting the ring response to match the position of the peak and/or the non-energy weighted sum rule provided by the random-phase approach has been obtained. It is found that, in general, it is not possible to reproduce both magnitudes simultaneously for a given g'_0 value.Comment: 7 pages, 4 Postscript figures, to appear in Physical Review

Microscopic Approach to Nucleon Spectra in Hypernuclear Non-Mesonic Weak Decay

A consistent microscopic diagrammatic approach is applied for the first time to the calculation of the nucleon emission spectra in the non-mesonic weak decay of Lambda-hypernuclei. We adopt a nuclear matter formalism extended to finite nuclei via the local density approximation, a one--meson exchange weak transition potential and a Bonn nucleon-nucleon strong potential. Ground state correlations and final state interactions, at second order in the nucleon--nucleon interaction, are introduced on the same footing for all the isospin channels of one- and two-nucleon induced decays. Single and double--coincidence nucleon spectra are predicted for 12_Lambda^C and compared with recent KEK and FINUDA data. The key role played by quantum interference terms allows us to improve the predictions obtained with intranuclear cascade codes. Discrepancies with data remain for proton emission.Comment: 11 pages, 6 figures, 2 tables. To be published in Physics Letters

Additional time-dependent phase in the flavor-conversion formulas

In the framework of intermediate wave-packets for treating flavor oscillations, we quantify the modifications which appear when we assume a strictly peaked momentum distribution and consider the second-order corrections in a power series expansion of the energy. By following a sequence of analytic approximations, we point out that an extra time-dependent phase is merely the residue of second-order corrections. Such phase effects are usually ignored in the relativistic wave-packet treatment, but they do not vanish non-relativistically and can introduce some small modifications to the oscillation pattern even in the ultra-relativistic limit.Comment: 9 pages, 3 figure