J. Delorme

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.

MESON DEGREES OF FREEDOM IN NUCLEI

Abstract A review is presented of the successes and shortcomings of the theory of meson degrees of freedom in nuclei with special emphasis on recent progress and on the necessity to bridge the gap with the degrees of freedom of QCD theory.

The second-class current problem in nuclear beta decay

Abstract Taking into account in a simple but non-trivial way both off-mass-shell and meson-exchange effects, we calculate the second-class current (scc) contribution to asymmetries in nuclear mirror Gamow-Teller decays. No assumptions on the divergence of the scc are made but the previous result obtained for a divergenceless current is recovered straightforwardly from our formula. Of the plethora of parameters that can be brought in for a current with non-vanishing divergence, we confine ourselves to the minimum number (two) which still renders our calculation sufficiently realistic while retaining its predictive power. The two parameters - one associated with the off-shell phenomenon and the other with meson-exchange current - are determined from two pieces of experimental data on the A = 8 system (the averaged asymmetry δ obtainable from the total decay rates and the slope in the plot of δ versus the energy release) and predictions are made for the scc effects in other mirror transitions. On the average the expected scc contribution to mirror asymmetry δ is 2 to 5%, for both even- and odd- A nuclei. We reach the same conclusion as Wilkinson that the large residual asymmetry (≳10%) left unaccounted in odd- A nuclei must be nuclear in origin. The fundamental nucleon pseudotensor coupling constant implied by this result is still of the same order (∼1/ M , where M is nucleon mass) as that of the weak magnetism or pseudoscalar terms. In the absence of kinematic suppression, this could induce large effects. Thus the ω-decay into π ± e −+ ν is expected to be as fast as any allowed first-class decay of a meson with comparable mass. The asymmetries in various correlation experiments (i.e. eη correlation and up-down asymmetry with respect to nuclear spin polarization) are predicted to be large enough to be measurable. We make a detailed analysis on whether, as conjectured by Lipkin, the meson-exchange current can build up coherently for large nuclei. The conclusion drawn therefrom is that such an enhancement does not materialize.

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.

Relativistic shell models and meson-exchange currents

Abstract We discuss the connection between nonrelativistic pair exchange currents and relativistic mean field theories. Heavy mesons, especially the σ-meson, are shown to yield strong enhancements to the matrix element of “odd” Dirac operators, such as γ and γ 4 γ 5 , in complete analogy to the strong enhancements obtained in relativistic mean field theories. For isoscalar operators, there is a cancelling contribution from ω-pair diagrams, called “backflow” in relativistic theories, but for isovector operators the equivalent ρ-pair diagram is very weak. Agreement between traditional nonrelativistic calculations and experiment is worsened by the introduction of heavy-meson pair diagrams. The situation can be improved on adding vertex form factors and short-range correlation functions that reduce the heavy-meson pair contribution by about a factor of two. The differences between the two approaches are emphasized and experimental tests are discussed.

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.

Critical opalescence of the pion field and the M1 from factor of 12C: An investigation of the role of the rho meson

Abstract It is shown that the inelastic M1 form factor of 12 C can be interpreted by nuclear polarization phenomena with inclusion of the rho meson in the polarizing interaction playing an essential role at large momenta. Whereas much of the observed anomaly can be attributed to standard core polarization effects, a large opalescence of the pion field (a factor 4.5 enhancement) would be needed to get full agreement with experiment.

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.