The contribution of Δproduction to the anomalous spin-dependence of low-energy pp scattering

Abstract

The large spin-dependence of the proton-proton total cross section data for laboratory momenta less than 3 GeV/c is shown to be consistent with the interference between the K and p exchange contributions to A production. The recent measurements of the total cross section for proton-proton scattering from specific initial spin states in the momentum range 2 d p,,lb d 6GeV/c (de Boer et a1 1975, Krisch 1975) have presented certain problems for the conventional descriptions of the pp interaction in terms of t-channel exchanges. The quantity measured is the difference between the total cross section of incoming protons with spins perpendicular to the beam direction, in one case parallel (crTt) and in the other case antiparallel (G:~) This cross-section difference 6,,, changes magnitude rapidly with energy, and both Regge pole and cut models fail completely to reproduce this phenomenon (Leader and Wray 1975). It has been suggested (Wilkin 1975) that this dramatic change in spin-dependence at around pi,lb = 2 GeV/c should be associated with the opening up of the A production threshold which is the dominant contribution to the pp total cross section in this energy region. An examination of the pp elastic phase-shifts does indeed indicate that resonance-like behaviour can be seen in Im $2(0) at these energies (Kane and Thomas 1975), and it is then natural to associate such behaviour with the A production threshold. The question remains, however, whether it is possible for A production alone to produce such a large contribution to a,,,, . In this Letter we show that the spin effect 6,,, can be correctly described by a simple dynamical model for pp -+ NA and that this model is also consistent with our knowledge of the spin-averaged differential cross section for this process. The input to our calculation is taken to be n and p exchange in the t-channel. The n and p are assumed to couple as elementary particles to NN and NA. The type of coupling and coupling constants of the nNN, nNA and pNN vertices are well known and reasonably well determined and we use standard forms for these (Pilkuhn et a/ 1973). The pNA vertex is not so well known and we take our vertex function to be