FINAL-STATE INTERACTION IN THE PD-]PNP REACTION AT 1-GEV

the pd --> pnp reaction at 1 GeV in both the direct and charge-exchang e channel has been investigated. The experimental data come from a lin e reversed beam-target experiment with 3.3 GeV/c deuterons incident on a proton target. In the direct channel data exhibit narrow structures in the np effective-mass spectra: at threshold, at 2.02 and at 2.12 G eV, which have been seen before and we report on a new narrow enhancem ent at 1.95 GeV. In the charge-exchange channel the data show a somewh at broader peak at 2.18 GeV. The purpose of this work is to explain th e data by using a conventional approach, i.e. without sub-nucleonic de grees of freedom, but including the DELTAN channel in NN scattering. T he calculation is based on a two-channel distorted-wave impulse approx imation. The NN --> NN spin-dependent scattering amplitudes have been reconstructed by a summation of partial waves with the parameters supp lied by phase-shift analyses, whereas the NN --> DELTAN amplitudes hav e been calculated from a one-pion-exchange mechanism. The final-state interaction between the slow nucleons is derived from the Paris or Bon n NN potential except for the 1D2 and 3F3 states, which are assumed to be coupled with the DELTAN channel and have been described here by a two-channel phenomenological potential. The finite width of the delta is included by introducing a continuum of DELTAN channels with the del ta-mass distribution given by a Breit-Wigner shape. The calculated spe ctra of the effective mass of the two slow nucleons, imposing various kinematical cuts, are compared with experiment. The effects of the DEL TAN intermediate state are clearly seen as a broad maximum in the effe ctive-mass distribution in the charge-exchange channel. This effect is less pronounced in the direct channel dominated by the isospin non-fl ip amplitude. The overall agreement is very good, but in the direct ch annel narrow peaks are present which are unaccounted for by a conventi onal approach.