M. Ericson

Nuclear critical opalescence, a precursor to pion condensation

Abstract It is shown that pion condensation in nuclei, a long-range phenomenon, has a precursor in the disordered phase, the local ordering of spins which becomes of infinite range at the critical point. A new physical effect arising from this short-range order is predicted, namely the enhancement of the static nuclear pion field near the critical momentum. This phenomenon is strongly reminiscent of the critical opalescence observed in the scattering of neutrons by antiferromagnetic substances.

To condense or not to condense? That is the question

Abstract We investigate the dependence of the pion mass in the nuclear medium with density and the condition for s-wave pion condensation. We base our approach on multiple scattering and the consistent use of the chiral off-mass-shell properties of the πN amplitude. We find important modifications of the results based on an effective lagrangian approach. These arise from the energy dependence of the πN amplitude with the small scattering length, which lead to a different behaviour of the density dependence and a suppression of condensation. Our results also suggest that the ratio of the effective to the real pion mass goes to unity in the chiral limit.

Critical opalescence of the nuclear pion field: A possible evidence in the M1 (15.11 MeV) form factor of 12C

Abstract We have computed the nuclear pion field for the transition to the 15.11 MeV (1 + , T = 1) state of 12 C, evaluating the nuclear polarization with a large basis of nucleon- and isobar-hole excitations. The field shows an enhancement (or critical opalescence) in the momentum region beyond 1.5 m π which leads to a substantial increase of the second maximum of the M1 form factor. Agreement with experiment can be obtained if the 12 C nucleus is much closer to the pion condensation threshold than currently expected.

High energy spin-isospin modes in nulcei

Abstract The high energy response of nuclei to a spin-isospin excitation is investigated. We predict the existence of a strong contrast between the spin transverse and spin longitudinal responses. The second one undergoes a shadow effect in the Δ region and displays the occurence of the pionic branch.

Nuclear effects in neutrino-nucleus interactions

Abstract The apparent anomaly in the ratio of muon-to-electron atmospheric neutrinos has been confirmed by several collaborations using different detection techniques. Together with the asymmetry in the zenithal distributions of the ν μ events in Super-Kamiokande, it gives a strong support to the neutrino oscillation hypothesis and to the existence of a non-vanishing mass for the neutrinos. In this work we are interested in the role of nuclear physics in the neutrino-oxygen reactions, which are relevant for the Cherenkov detectors. We point out that multi-nucleon excitations of n p– n h type and nuclear correlations could modify an experimental analysis a la Super-Kamiokande because they lead to a substantial enhancement of the number of 1 Cherenkov ring retained events.

Pauli principle and renormalization of the pion nucleon interaction in nuclei

Abstract We show that the exclusion principle provides an alternative mechanism to the Lorentz-Lorenz effect for the renormalization of the pion-nucleon coupling constant in nuclei. This mechanism proves to be less sensitive to the range of the π - N forces. The implications for pion-nucleus scattering are discussed.

Chiral symmetry restoration and parity mixing

Abstract We derive the expressions of the vector and axial current from a chiral Lagrangian restricted to nucleons and pions. They display mixing terms between the axial and vector currents. We study the modifications in the nuclear medium of the coupling constants of the axial current, namely the pion decay constant and the nucleonic axial one due to the requirements of chiral symmetry. We express the renormalizations in terms of the local scalar pion density. The latter also governs the quark condensate evolution and we discuss the link between this evolution and the renormalizations. In the case of the nucleon axial coupling constant this renormalization corresponds to a new type of pion exchange currents, with two exchanged pions. We give an estimate for the resulting quenching. Although moderate it helps explaining the quenching experimentally observed.

Pion and neutron production by cosmic ray muons underground

The production of positive pions and neutrons by cosmic muons at underground sites of various depths is investigated. We first test the equivalent photon method in the particular case of Δ excitation by the muon. We find that this method, when it neglects the momentum dependence of the transverse response, reproduces remarkably well the theoretical muon cross section. This success has lead us to apply the method to higher energies, where it has not been tested. We evaluate in this way the production of positive pions in liquid scintillator from known photo-absorption cross sections. At a shallow depth of 20 meters our estimate reproduces the measurement. As for the neutron emission, we include the obvious sources, such as the giant-resonance excitation, the quasideuteron process, the quasifree pion production as well as neutrons emitted following pion capture. Our evaluation underestimates the number of neutrons produced and finds a too weak dependence on the depth. This suggests that secondary neutron production is important at all depths.

Quark spin-isospin sum rules and the adler-weisberger relation in nuclei

Abstract We use a quark model to extend the classical Gamow-Teller sum rule for the difference of the β − and β + strengths to excitations of the nucleon (mainly the Δ isobar).A schematic model illustrates the realization of the new sum rule when a particle-hole force is introduced. We discuss the connection of our result with the model-independent Adler-Weisberger sum rule.

Exploration of the spin-isospin nuclear response function by neutrinos

Abstract We study the possibility of exploring the collective character of the nuclear spin-isospin longitudinal response by neutrino experiments. We discuss the kinematical conditions for the observation of the pionic (or optical) branch. We find that the available data present some indications in favour of its existence.