Cross-section and analyzing-power data are presented for the "C(p, p') "C reaction at bombarding energies of 155 and 200 MeV. Comparisons are made with the systematics of data from other energies. Distorted-wave impulseapproximation calculations for the "pion-like" 1+, T = 1 transition to the 15.11-MeV state are successful for q & 280MeV/c, but are substantially smaller than the data at larger momentum transfers. The implications of the data regarding precritical behavior associated with the pion-condensation threshold are discussed. NUCLEAR REACTIONS C(p, p'), E=155 MeV, measured 0(E, 8); E~200 MeV, measured 0(E, 8), A~(8); 8= 6-50, 4 8= 2-3', E"=11-17MeV. DWIA analysis. Precritical phenomena near pion-condensation threshhold.
I. INTRPDUCTIONThe recent suggestion" that precritical phenomena associated with the pion-condensation threshhold might be observable in (p, p') reactions has focused attention on the inelastic excitation of the 1', 2'=1 state at 15.11 MeV in "C. This transition is said to be "pion-like" in that the transfer of a virtual pseudoscalar pion requires both spin transfer and isospin transfer (AS= c T =1). If nuclei are sufficiently close to the critical point for pion condensation, the polarization of the medium by the pion field will produce special collective effects. These should appear as enhancements of (p, p') differential cross sections for isovector unnatural-parity transitions in the range of momentum transfer q near 300-450Existing data for the 15.11-MeV transition at F& --155 MeV indeed show a minimum near q =250 MeV/c followed by a prominent second maximum. 4 Microscopic effective-interaction calculations were unable to describe this shape. However, these features are absent in recent data at E =122 MeV (Ref. 5), where the large-q region of the angular distribution is characterized by a shoulder. Microscopic calculations ' were quite successful in reproducing the absolute cross sections for q & 300 MeV/c; above this point they fell signif icantly below the data.Arguments have also been advanced, based on the large-q behavior of the (e, e') transverse form factor for the same transition, that ' C might actually be close to the pion-condensation threshhold. ' ' Analysis of elastic electron scattering data from ' C seems to support this interpretation. ' It has been noted, however, ' that the (p, p') reaction may be a more sensitive probe of pion phenomena than (e, e') reactions. The reasons are related to the longitudinal (o q) nature of the dominant one-pion-exchange process as opposed to the transverse (oxq) nature of virtual photon transfer, although these distinctions are blurred by surface effects for light nuclei. ' ' In order to obtain new and detailed data on this issue, cross sections for the "C(p, p')"C reaction to the 15.11-MeV and nearby states have been remeasured at 155 MeV with particular attention given to the region q & 200 MeV/c. These will be discussed and compared with additional data, including analyzing-power data, at 120 MeV (Refs. 5, 10) and 200 MeV. '~T he tra...
