A COMPARISON OF n - p * P ” n WITH SINGLE PION PHOTOPRODUCTION AT 15 GeV

We present a comparison of new data on n-p -+ ,o”n at 15 GeV/c with polarized and unpolarized single pion photoproduction data. Particular emphasis is placed upon the behavior of the differential cross sections and asymmetries in the forward direction, (-t < m $ 0 (Submitted to Phys. Rev. Letters.) * Work supported by the U. S. Atomic Energy Commission. Studies of single pion photoproduction at SLAC’ and DESY’ have shown a sharp increase in the differential cross section, do/dt, for momentum transfers to the nucleon -t c mf, over a large energy range. In addition, experiments with linearly polarized photons have shown that the asymmetry parameter, which is zero in the forward direction, rises rapidly to unity3 at -t x rnz. Single pion photoproduction is related to the reaction n-p --) pan (1) through the vector dominance model (VDM); 4 Since the small t behavior of the photoproduction processes is so striking, it is important to look for similar structure in the forward direction in reaction (1) o Furthermore, this reaction is interesting from the point of view of exchange models which also make predictions concerning forward structure in n-p + p’n. The strong cut absorption model5 shows a sharp peak in the transverse cross section for momentum transfers less than rnz. In contrast, the one pion exchange model requires that the cross section should vanish in the forward direction, whereas Avni and Harari! have predicted no pronounced structure in the transverse cross section. Although previous comparisons of n-p-pen with single pion photoproduction have been published, 7 the new data presented in the previous letter8 allow this comparison to be made in the small t region (-t < rn$ for the first time. In the preceding letter we presented the differential cross sections and density matrix elements for the reaction r-p -+ n’n’n at an incident beam momentum of 15 GeV/c. .The density matrix elements for the p” in the helicity frame are shown in Fig. 1. They have been obtained from the dipion density matrix elements described in Ref. 8 by imposing the normalization condition 2p F&.&zz+ FAX. 11+ poo = ltd The S-wave s”* cross section was assumed to be of the form g(S-wave) = B It,’ (t-m3 2 e7t