Eta and eta' mesons production at COSY-11

TL;DR

This study investigates eta and eta' meson production at COSY-11, focusing on spin and isospin effects in nucleon-nucleon collisions near threshold energies, revealing pion exchange dominance in eta production.

Contribution

It provides new experimental data on spin observables and isospin dependence, enhancing understanding of meson production mechanisms in nucleon interactions.

Findings

Evidence that eta production is mainly due to pion exchange.

New spin observable measurements for the p p --> p p eta reaction.

Indication that S11(1535) resonance excitation is dominant in eta production.

Abstract

The low emittance and small momentum spread of the proton and deuteron beams of the Cooler Synchrotron COSY combined with the high mass resolution of the COSY-11 detection system permit to study the creation of mesons in the nucleon-nucleon interaction down to the fraction of MeV with respect to the kinematical threshold. At such small excess energies, the ejectiles possess low relative momenta and are predominantly produced with the relative angular momentum equal to zero. Taking advantage of these conditions we have performed investigations aiming to determine the mechanism of the production of eta and eta' mesons in the collision of hadrons as well as the hadronic interaction of these mesons with nucleons and nuclei. In this proceedings we address the ongoing studies of the spin and isospin dependence for the production of the eta and eta' mesons in free and quasi-free nucleon-nucleon collisions. New results on the spin observables for the vec(p)p --> pp eta reaction, combined with the previously determined total cross section isospin dependence, reveal a statistically significant indication that the excitation of the nucleon to the S11(1535) resonance, the process which intermediates the production of the eta meson in the nucleon-nucleon interactions, is predominantly due to the exchange of the pi meson between the colliding nucleons.