Ortho-positronium observation in the Double Chooz Experiment

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.

arXiv comment17 pages, 5 figures

References in corpus (11)

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• Spectral measurement of electron antineutrino oscillation amplitude and frequency at Daya Bay
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Cited by in corpus (8)

• Calibration Strategy of the JUNO Experiment
• Measurement of ${\theta }_{13}$ in Double Chooz using neutron captures on hydrogen with novel background rejection techniques
• Suppression of Cosmic Muon Spallation Backgrounds in Liquid Scintillator Detectors Using Convolutional Neural Networks
• Particle Identification at MeV Energies in JUNO
• Combining Hybrid and Opaque Scintillator Techniques in the Search for Double Beta Plus Decays
• Novel event classification based on spectral analysis of scintillation waveforms in Double Chooz
• A new anti-neutrino detection technique based on positronium tagging with plastic scintillators
• A high precision calibration of the nonlinear energy response at Daya Bay