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

TL;DR

This study searched for eta-mesic helium-4 nuclei using the WASA-at-COSY detector, finding no evidence of such states and setting an upper limit on their production cross section, with future experiments planned for improved sensitivity.

Contribution

First experimental search for eta-mesic 4He using WASA-at-COSY, establishing upper limits and planning more sensitive future measurements.

Findings

No structure observed near eta production threshold

Estimated upper limit for eta-mesic production is about 20 nb

Future measurements aim for sensitivity of a few nb

Abstract

In June 2008 we performed a search for the 4He-eta bound state by measuring the excitation function of the dd to 3He p pi- reaction near the eta production threshold using the WASA-at-COSY detector. During the experimental run the momentum of the deuteron beam was varied continuously within each acceleration cycle from 2.185 GeV/c to 2.400 GeV/c, crossing the kinematic threshold for the eta production in the dd to 4He eta reaction at 2.336 GeV/c. The preliminary excitation function indicates no structure in the angular range close to the 180 degree where the signal is expected. The preliminary estimation of the upper limit for the eta-mesic production via the dd to (eta 4He)b.s. to 3He p pi- reaction is equal to about 20 nb on a one sigma level. A two-week measurement with WASA-at-COSY for the dd 3He p pi- channel is scheduled for November 2010. After two weeks of measurement with a luminosity of 4 * 10^(30) cm^(-2) s^(-1), we expect a statistical sensitivity of a few nb sigma.