Near threshold production of the eta meson via the quasi-free pn --> pn eta reaction

TL;DR

This study measures the total cross sections for the quasi-free pn --> pn eta reaction near threshold energies, revealing a decreasing ratio of pn to pp eta production cross sections as energy approaches threshold, influenced by final state interactions.

Contribution

First measurement of total cross sections for the pn --> pn eta reaction near threshold, highlighting the energy dependence of final state interactions and their effect on cross section ratios.

Findings

Cross sections exceed those of pp eta near threshold by a factor of about three.

The ratio of pn to pp eta cross sections decreases towards threshold.

Final state interactions significantly influence the energy dependence of the reaction.

Abstract

Total cross sections for the quasi-free pn --> pn eta reaction in the range from the kinematical threshold up to 20 MeV excess energy have been determined. At threshold they exceed corresponding cross sections for the pp --> pp eta reaction by a factor of about three in contrast to the factor of six established for higher excess energies. To large extent, the observed decrease of the ratio sigma(pn --> pn eta)/sigma(pp --> pp eta) towards threshold may be assigned to the different energy dependence of the proton-proton and proton-neutron final state interactions. The experiment has been conducted using a proton beam of the cooler synchrotron COSY and a cluster jet deuteron target. The proton-neutron reactions were tagged by the spectator proton whose momentum was measured for each event. Protons and neutron outgoing from the pn --> pn eta reaction have been registered by means of the COSY-11 facility, an apparatus dedicated for threshold meson production.