Measurements of the Lifetime of Orthopositronium in the LAB-Based Liquid Scintillator of JUNO

Abstract

Electron antineutrinos are detected in organic liquid scintillator based neutrino experiments by means of the inverse beta decay, producing both a positron and a neutron. The positron may form a bound state together with an electron, called positronium (Ps). The longer-lived spin state of Ps, orthopositronium (o-Ps) has a lifetime of about $3\,\mathrm{ns}$ in organic liquid scintillators (LS). Its formation changes the time distribution of photon emission, which affects positron reconstruction algorithms and allows the application of pulse shape discrimination (PSD) to distinguish electron from positron events. In this work, we measured the lifetime $\tau_2$ of o-Ps in the linear alkylbenzene (LAB) based LS of the JUNO (Jiangmen Underground Neutrino Observatory) experiment including wavelength shifters, obtaining $\tau_2 = 2.97\,\mathrm{ns} \pm 0.04\,\mathrm{ns}$. Due to systematics, which are not yet completely understood, we are not able to give a final result for the o-Ps formation probability $I_2$. We use a novel type of setup, which allows a better background suppression as compared to commonly used PALS (positron annihilation lifetime spectroscopy) measurements.