Eur. Phys. J. A (2009) 40: 11-22
https://doi.org/10.1140/epja/i2008-10735-x

Regular Article - Experimental Physics

A new analysis improving the evidence of a narrow peak in the invariant-mass distribution of the p system observed in the annihilation at rest on ⁴He

¹ Dipartimento di Fisica Nucleare e Teorica, Università di Pavia, and Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Pavia, Italy
² Dipartimento di Fisica Sperimentale, Università di Torino, and Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Pavia, Italy
³ Dipartimento di Fisica Nucleare e Teorica and Dipartimento di Chimica Generale, Università di Pavia, Pavia, Italy

^* e-mail: Paola.Salvini@pv.infn.it

Received: 29 October 2008
Revised: 4 February 2009
Accepted: 19 February 2009
Published online: 31 March 2009

Abstract

The presence of a narrow peak in the p invariant-mass distribution observed in the annihilation reaction at rest is discussed again through an analysis procedure which improves the ratio signal/background in comparison with the previous analysis. The peak is centred at 2223.2±3.2_stat±1.2_syst MeV and has a statistical significance of 4.7 , values compatible with those published previously. If interpreted as the result of the decay into p of a bound system, the corresponding binding energy should be B = - 151.0±3.2_stat±1.2_syst MeV and the width < 33.9±6.2 MeV. The production rate has a lower limit of 1.2 10^-4. Data on the annihilation reaction at rest ⁴He p p p _s X , analyzed for the first time, lead to a result in qualitative agreement with the previous one.