Final-state interaction in the pd → pnp reaction at 1 GeV

Abstract

The pd → pnp reaction at 1 GeV in both the direct and charge-exchange channel has been investigated. The experimental data come from a line 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 GeV, which have been seen before and we report on a new narrow enhancement at 1.95 GeV. In the charge-exchange channel the data show a somewhat broader peak at 2.18 GeV. The purpose of this work is to explain the data by using a conventional approach, i.e. without sub-nucleonic degrees of freedom, but including the Δ N channel in NN scattering. The calculation is based on a two-channel distorted-wave impulse approximation. The NN → NN spin-dependent scattering amplitudes have been reconstructed by a summation of partial waves with the parameters supplied by phase-shift analyses, whereas the NN → ΔN amplitudes have been calculated from a one-pion-exchange mechanism. The final-state interaction between the slow nucleons is derived from the Paris or Bonn N N potential except for the 1D 2 and 3F 3 states, which are assumed to be coupled with the Δ N 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 Δ N channels with the delta-mass distribution given by a Breit-Wigner shape. The calculated spectra of the effective mass of the two slow nucleons, imposing various kinematical cuts, are compared with experiment. The effects of the Δ N intermediate state are clearly seen as a broad maximum in the effective-mass distribution in the charge-exchange channel. This effect is less pronounced in the direct channel dominated by the isospin non-flip amplitude. The overall agreement is very good, but in the direct channel narrow peaks are present which are unaccounted for by a conventional approach.