Search for an exotic three-body decay of orthopositronium

TL;DR

This paper reports a search for an exotic three-body decay of orthopositronium into a photon and two particles, setting upper limits that exclude this decay as the cause of the decay rate discrepancy.

Contribution

It provides the first experimental upper limit on the branching ratio of the proposed exotic decay mode of orthopositronium.

Findings

No evidence found for the decay mode

Set an upper limit on the branching ratio < 4.4 x 10^{-5}

Excluded the decay mode as the cause of the decay rate discrepancy

Abstract

We report on a direct search for a three-body decay of the orthopositronium into a photon and two penetrating particles, o-Ps -> gamma + X1 + X2. The existence of this decay could explain the discrepancy between the measured and the predicted values of the orthopositronium decay rate. From the analysis of the collected data a single candidate event is found, consistent with the expected background. This allows to set an upper limit on the branching ratio < 4.4 \times 10^{-5} (at the 90% confidence level), for the photon energy in the range from 40 keV < E_gamma< 400 keV and for mass values in the kinematical range 0 < mX1+mX2 <900 keV. This result unambiguously excludes the o-Ps -> gamma + X1 + X2 decay mode as the origin of the discrepancy.