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Physics, Instrumentation and Detectors, physics.ins-det,High Energy Physics - Experiment, hep-ex
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.
