A density expansion of the pion-nucleus optical potential. II. Second order terms
1973; Elsevier BV; Volume: 79; Issue: 2 Linguagem: Inglês
10.1016/0003-4916(73)90093-6
ISSN1096-035X
Autores Tópico(s)Cold Atom Physics and Bose-Einstein Condensates
ResumoThe second order terms in a density expansion of the pion optical potential Vopt are evaluated quantitatively. The coefficients of these terms are proportional to various combinations of on- and off-shell nucleon-nucleon T-matrices, averaged over the distribution of two nucleon relative momenta in the Fermi sea. The on-shell contributions can be obtained directly from experimental phase shifts, but the calculation of the off-shell-parts requires a model for the nucleon-nucleon potential. We consider a number of realistic local and nonlocal, separable potentials which fit nucleon-nucleon phase shifts, in order to study the variations in Vopt which arise from differences in the off-shell behavior of T. We find absorptive (imaginary) and dispersive (real) contributions to Vopt which are of comparable magnitude. The dispersive part, which leads to a real energy shift associated with the two nucleon absorptive process, has not been previously estimated quantitatively. We compare our results to empirical potentials obtained by fitting energy level shifts and widths in pi-mesic atoms, as well as theoretical estimates based on threshold cross sections for the processes π + N + N ⇄ N + N.
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