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

The Pion in Electromagnetic and Weak Neutral Current Nuclear Response Functions

The impact of pionic correlations and meson--exchange currents in determining the (vector) response functions for electroweak quasielastic lepton scattering from nuclei is discussed. The approach taken builds on previous work where the Fermi gas model is used to maintain consistency in treating forces and currents (gauge invariance) and to provide a Lorentz covariant framework. Results obtained in first-order perturbation theory are compared with infinite-order summation schemes (HF and RPA) and found to provide quite successful approximations for the quasielastic response functions. The role of pionic correlations in hardening the responses R_L and R_T is investigated in some detail, including studies of the relative importance of central and tensor pieces of the force and of exchange and self-energy diagrams; in addition, their role in significantly modifying the longitudinal parity-violating response R_{AV}^L is explored. The MEC are shown to provide a small, but non-negligible, contribution in determining the vector responses.Comment: TeX, 21 figures (Postscript, available from the authors), MIT preprint CTP\#219

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

Static properties of nuclear matter within the Boson Loop Expansion

The use of the Boson Loop Expansion is proposed for investigating the static properties of nuclear matter. We explicitly consider a schematic dynamical model in which nucleons interact with the scalar-isoscalar sigma meson. The suggested approximation scheme is examined in detail at the mean field level and at the one- and two-loop orders. The relevant formulas are provided to derive the binding energy per nucleon, the pressure and the compressibility of nuclear matter. Numerical results of the binding energy at the one-loop order are presented for Walecka's sigma-omega model in order to discuss the degree of convergence of the Boson Loop Expansion.Comment: 40 pages, 13 figure

Functional approach to the non-mesonic decay of Lambda-hypernuclei

We present an evaluation of the non-mesonic decay widths for Lambda-hypernuclei (Lambda N --> NN, Lambda NN --> NNN) within the framework of the polarization propagator method. The full Lambda self-energy is evaluated microscopically in nuclear matter by using the functional approach, which supplies a theoretically well grounded approximation scheme for the classification of the relevant diagrams, according to the prescriptions of the bosonic loop expansion. We employ average Fermi momenta, suitably adapted to different mass number regions (medium-light, medium and heavy hypernuclei). Moreover, we study the dependence of the decay rates on the NN and Lambda-N short range correlations. With a proper choice of the parameters which control these correlations in the new approximation scheme, it is possible to reproduce the experimental decay widths for A > 10 hypernuclei.Comment: 25 pages, 8 figure

Is it possible to determine the S-factor of the hep process from a laboratory experiment?

We discuss the problem of solar hep neutrinos originating from the reaction p + 3He -> 4He + e+ + nu and obtain a relation between the astrophysical S-factor of the hep process and the cross section of the process e- + 4He -> 3H + n + nu near threshold. The relation is based on the isotopic invariance of strong interactions. The measurement of the latter cross section would allow to obtain experimental information on S(hep), the value of which, at the moment, is known only from theoretical calculations.Comment: 10 pages, no figure

The semi-classical approach to the exclusive electron scattering

The semiclassical approach, successfully applied in the past to the inelastic, inclusive electron scattering off nuclei, is extended to the treatment of exclusive processes. The final states interaction is accounted for in the mean field approximation, respecting the Pauli principle. The impact on the exclusive cross section of the shape of the potential binding the nucleons into the nucleus and of the distortion of the outgoing nucleon wave are explored. The exclusive scattering is found to be quite sensitive to the mean field final states interaction, unlike the inclusive one. Indeed we verify that the latter is not affected, as implied by unitarity, by the distortion of the outgoing nucleon wave except for the effect of relativity, which is modest in the range of momenta up to about 500 MeV/c. Furthermore, depending upon the correlations between the directions of the outgoing and of the initial nucleon, the exclusive cross-section turns out to be remarkably sensitive to the shape of the potential binding the nucleons. These correlations also critically affect the domain in the missing energy-- missing momentum plane where the exclusive process occurs.Comment: 39 pages, latex, including 9 figures (fig.ps

The 2p-2h electromagnetic response in the quasielastic peak and beyond

The contribution to the nuclear transverse response function R_T arising from two particle-two hole (2p-2h) states excited through the action of electromagnetic meson exchange currents (MEC) is computed in a fully relativistic framework. The MEC considered are those carried by the pion and by Delta degrees of freedom, the latter being viewed as a virtual nucleonic resonance. The calculation is performed in the relativistic Fermi gas model in which Lorentz covariance can be maintained. All 2p-2h many-body diagrams containing two pionic lines that contribute to R_T are taken into account and the relative impact of the various components of the MEC on R_T is addressed. The non-relativistic limit of the MEC contributions is also discussed and compared with the relativistic results to explore the role played by relativity in obtaining the 2p-2h nuclear response.Comment: 27 pages, 12 figures, revtex4; minor modifications in the discussion of the results, references adde

Thermal imaginary part of a real-time static potential from classical lattice gauge theory simulations

Recently, a finite-temperature real-time static potential has been introduced via a Schr\"odinger-type equation satisfied by a certain heavy quarkonium Green's function. Furthermore, it has been pointed out that it possesses an imaginary part, which induces a finite width for the tip of the quarkonium peak in the thermal dilepton production rate. The imaginary part originates from Landau-damping of low-frequency gauge fields, which are essentially classical due to their high occupation number. Here we show how the imaginary part can be measured with classical lattice gauge theory simulations, accounting non-perturbatively for the infrared sector of finite-temperature field theory. We demonstrate that a non-vanishing imaginary part indeed exists non-perturbatively; and that its value agrees semi-quantitatively with that predicted by Hard Loop resummed perturbation theory.Comment: 18 pages. v2: clarifications and a reference added; published versio

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