Ortho-positronium observation in the Double Chooz Experiment

TL;DR

This paper reports the first observation of ortho-positronium formation in a large liquid scintillator detector, using pulse shape analysis to measure its formation fraction and lifetime, which could improve particle identification in neutrino experiments.

Contribution

It demonstrates the feasibility of detecting ortho-positronium in a large liquid scintillator detector through pulse shape analysis, providing new insights for particle discrimination.

Findings

Measured o-Ps formation fraction: 44% ± 12% (sys.) ± 5% (stat.)

Measured o-Ps lifetime: 3.68 ns ± 0.17 ns (sys.) ± 0.15 ns (stat.)

Results agree with dedicated positron annihilation lifetime spectroscopy

Abstract

The Double Chooz experiment measures the neutrino mixing angle $\theta_{13}$ by detecting reactor $\bar{\nu}_e$ via inverse beta decay. The positron-neutron space and time coincidence allows for a sizable background rejection, nonetheless liquid scintillator detectors would profit from a positron/electron discrimination, if feasible in large detector, to suppress the remaining background. Standard particle identification, based on particle dependent time profile of photon emission in liquid scintillator, can not be used given the identical mass of the two particles. However, the positron annihilation is sometimes delayed by the ortho-positronium (o-Ps) metastable state formation, which induces a pulse shape distortion that could be used for positron identification. In this paper we report on the first observation of positronium formation in a large liquid scintillator detector based on pulse shape analysis of single events. The o-Ps formation fraction and its lifetime were measured, finding the values of 44$\%$ $\pm$ 12$\%$ (sys.) $\pm$ 5$\%$ (stat.) and $3.68$ns $\pm$ 0.17ns (sys.) $\pm$ 0.15ns (stat.) respectively, in agreement with the results obtained with a dedicated positron annihilation lifetime spectroscopy setup.