Search for the eta-mesic 4He with WASA-at-COSY detector

TL;DR

This study searched for eta-mesic helium-4 nuclei using the WASA-at-COSY detector by measuring the dd->3Heppi- reaction across the eta production threshold, but found no evidence of the bound state and set upper cross-section limits.

Contribution

First experimental search for eta-mesic 4He with precise excitation function measurement near threshold, establishing upper limits for bound state formation.

Findings

No eta-mesic 4He signal observed.

Upper cross-section limits range from 20 to 27 nb.

Experiment constrains theoretical models of eta-mesic nuclei.

Abstract

An exclusive measurement of the excitation function for the dd->3Heppi- reaction was performed at the Cooler Synchrotron COSY-Juelich with the WASA-at-COSY detection system. The data were taken during a slow acceleration of the beam from 2.185 GeV/c to 2.400 GeV/c crossing the kinematic threshold for the eta meson production in the dd->4He-eta reaction at 2.336 GeV/c. The corresponding excess energy with respect to the 4He-eta system varied from -51.4MeV to 22MeV. The integrated luminosity in the experiment was determined using the dd->3Hen reaction. The shape of the excitation function for the dd->3Heppi- was examined. No signal of the 4He-eta bound state was observed. An upper limit for the cross-section for the bound state formation and decay in the process dd->(4He-eta)bound->3Heppi- was determined on the 90% confidence level and it varies from 20nb to 27nb for the bound state width ranging from 5MeV to 35MeV, respectively.