Neutron matter at low density and the unitary limit

Neutron matter at low density is studied within the hole-line expansion. Calculations are performed in the range of Fermi momentum $k_F$ between 0.4 and 0.8 fm$^{-1}$. It is found that the Equation of State is determined by the $^1S_0$ channel only, the three-body forces contribution is quite small, the effect of the single particle potential is negligible and the three hole-line contribution is below 5% of the total energy and indeed vanishing small at the lowest densities. Despite the unitary limit is actually never reached, the total energy stays very close to one half of the free gas value throughout the considered density range. A rank one separable representation of the bare NN interaction, which reproduces the physical scattering length and effective range, gives results almost indistinguishable from the full Brueckner G-matrix calculations with a realistic force. The extension of the calculations below $k_F = 0.4$ fm$^{-1}$ does not indicate any pathological behavior of the neutron Equation of State.Comment: 17 pages, 7 figures. To be published in Phys. Rev.

Looking for strangeness with neutrino-nucleon scattering

The possibility to determine the axial strange form factor of the nucleon from elastic neutrino-nucleon scattering experiments is studied. The existing experimental information is shortly mentioned and several observables which could be measured in the near future at new neutrino facilities are discussed.Comment: 6 pages, 1 figure, contribution to : "NuFact 03", 5th International Workshop on Neutrino Factories & Superbeams, Columbia University, New York, 5-11 June 200

Spin-orbit correlation energy in neutron matter

We study the relevance of the energy correlation produced by the two-body spin-orbit coupling present in realistic nucleon-nucleon potentials. To this purpose, the neutron matter Equation of State (EoS) is calculated with the realistic two-body Argonne $v_8'$ potential. The shift occuring in the EoS when spin-orbit terms are removed is taken as an estimate of the spin-orbit correlation energy. Results obtained within the Bethe-Brueckner-Goldstone expansion, extended up to three hole-line diagrams, are compared with other many-body calculations recently presented in the literature. In particular, excellent agreement is found with the Green's function Monte-Carlo method. This agreement indicates the present theoretical accuracy in the calculation of the neutron matter EoS.Comment: 5 pages, 2 figures, 2 tables; to appear in Phys. Rev.

Lepton mass effects in the Bethe-Heitler process

We develop the full finite lepton mass formalism for the production of real photons via the Bethe-Heitler reaction of unpolarized leptons off unpolarized nucleons. Genuine lepton mass effects are described, in particular their dependence upon the lepton mass and the initial beam energy, as well as their sensitivity to the nucleon isospin. In the minimum momentum transfer region, these effects dominate the muon induced proton cross section and become significant for electron scattering at small $x_B$.Comment: Final version including errat

Local Fermi gas in inclusive muon capture from nuclei

We compare local Fermi gas and shell model in muon capture in nuclei in order to estimate the effect of finite nuclear size in low energy weak reactions.Comment: 6 pages, 8 figures. To be published in the Proceedings of 20th Max Born Symposium, Wroclaw (Poland), December 7-10, 200

Evolution of the pygmy dipole resonance in nuclei with neutron excess

The electric dipole excitation of various nuclei is calculated with a Random Phase Approximation phenomenological approach. The evolution of the strength distribution in various groups of isotopes, oxygen, calcium, zirconium and tin, is studied. The neutron excess produces $E1$ strength in the low energy region. Indexes to measure the collectivity of the excitation are defined. We studied the behavior of proton and neutron transition densities to determine the isoscalar or isovector nature of the excitation. We observed that in medium-heavy nuclei the low-energy $E1$ excitation has characteristics rather different that those exhibited by the giant dipole resonance. This new type of excitation can be identified as pygmy dipole resonance.Comment: 14 pages, 12 figures, 7 table

Evolution of the pygmy dipole resonance in nuclei with neutron excess

The electric dipole excitation of various nuclei is calculated with a Random Phase Approximation phenomenological approach. The evolution of the strength distribution in various groups of isotopes, oxygen, calcium, zirconium and tin, is studied. The neutron excess produces $E1$ strength in the low energy region. Indexes to measure the collectivity of the excitation are defined. We studied the behavior of proton and neutron transition densities to determine the isoscalar or isovector nature of the excitation. We observed that in medium-heavy nuclei the low-energy $E1$ excitation has characteristics rather different that those exhibited by the giant dipole resonance. This new type of excitation can be identified as pygmy dipole resonance.Comment: 14 pages, 12 figures, 7 table

Non-locality in the nucleon-nucleon interaction and nuclear matter saturation

We study the possible relationship between the saturation properties of nuclear matter and the inclusion of non-locality in the nucleon-nucleon interaction. To this purpose we compute the saturation curve of nuclear matter within the Bethe-Brueckner-Goldstone theory using a recently proposed realistic non-local potential, and compare it with the corresponding curves obtained with a purely local realistic interaction (Argonne v$_{18}$) and the most recent version of the one-boson exchange potential (CD Bonn). We find that the inclusion of non-locality in the two-nucleon bare interaction strongly affects saturation, but it is unable to provide a consistent description of few-body nuclear systems and nuclear matter.Comment: 9 pages, 8 figures; v2: introduction extended, references added, discussion of fig.8 reformulated; to be published in Phys. Rev.