Pion Condensation In Symmetric Nuclear Matter

Current calculations of pion condensation phenomena in symmetric nuclear matter are reviewed. The RPA and MFA methods are compared. Latest results (LBL-10572) with a relativistic MFA theory constrained by bulk nuclear properties are presented. The differences between equilibrium (condensation) and nonequilibrium (dynamic) instabilities are discussed. Finally, two-proton correlation experiments aimed at looking for critical scattering phenomena and two-pion correlation experiments aimed at looking for pion field coherence are analyzed. 10 figures, 2 tables.

A review of recent developments which indicate that neutron or nuclear matter at sufficiently high density undergoes a phase transition into a pion condensate is presented. Following an introductory discussion of the simplest possible models, realistic features are added stepwise, like the role of short-range nucleon-nucleon correlations and the importance of the .delta.(1230) isobar. Results of realistic calculations of the critical density for pion condensation in neutron matter are summarized, which show that the phase transition appears around or beyond twice the density of nuclear matter. The description of pion condensation is summarized within the framework of the sigma model, based on chiral symmetry, and generalized to include short-range correlations and .delta.(1230) isobars. This model permits construction of the equation of state for pion condensed neutron matter. The presence of a pion condensate generally leads to a softening of the equation of state. Some implications on the structure of neutron stars are studied.

It is first noted that if the coupling of spin, isospin modes in nuclei to pions were of anywhere near the strength required to produce pion condensation in nuclear matter, then there would be strong effects in nuclear spectra, which are not seen. A model proposed by A.B. Migdal is worked through and it is shown that addition of a strong short-range repnlsion in the nucleon-nuclear interaction modifies the effective interaction in such a way as to remove the main spin-isospin interaction which originally gave rise to the pion condensate. It is believed that short-range repulsions also strongly modify the work of Sawyer and Scalapino, but have not yet shown that they destroy the condensate in neutron stars. The conclusion is that the off-shell p-wave attraction of pions and nucleons lies, at least in the region of low momenta, completely in the domain of the DELTA -isobars. Effects from these would be incapable of producing pion condensates, except at extremely high densities, equal to several times normal nuclear density. (auth).

A relativistic field theory of nuclear matter is solved for the self-consistent field strengths inthe mean-field approximation. The theory is constrained to reproduce the bulk properties of nuclear matter. A weak pion condensate is compatible with this constraint. At least this is encouraging as concerns the possible existence of a new phase of nuclear matter. In contrast, the Lee-Wick density isomer is probably not compatible with the properties of nuclear matter. 3 figures.

Pion condensation is investigated in a self-consistent. relativistic mean field theory that is constrained to reproduce the bulk properties of nuclear matter. This constraint and self-consistency provide stringent constraints on the existence and energy of the condensate.

The pion plays an outstanding role in nuclear physics as a generator of the nuclear force and as an important part of the nuclear many-body problem. At the same time, pion beams are used as probes to explore the nucleus and its interactions. As pion physics has matured over the years, a rich and diverse variety of phenomena has been uncovered. The aim of this book is to examine the underlying physical picture behind these phenomena in a systematic and coherent way. The authors emphasize the interplay between physical concepts and experimental facts more than the formal tools, which are presented in a series of appendices. The book is directed towards advanced students as well as research workers.