Ortho-positronium observation in the Double Chooz experiment

Abstract

The Double Chooz experiment measures the neutrino mixing angle θ 13 by detecting reactorν 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 orthopositronium (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 % ± 12 % (sys.) ± 5 % (stat.) and 3.68 ns ± 0.17 ns (sys.) ± 0.15 ns (stat.) respectively, in agreement with the results obtained with a dedicated positron annihilation lifetime spectroscopy setup.

Experimental aspects of geoneutrino detection: Status and perspectives

“…As an illustration of possible bias due to the spectral shape, two values could be compared: the value Q H =0. 122 assumption of the allowed shape and used in earlier review [3] for 87 Rb evaluation is twice as large as the value in Tab. 1 calculated using the corrected shape.…”

“…14 Wipperfurth et al recently reported the results of their high precision study of the mass ratio in [68], which they found to be M(Th)/M(U)=3.776 +0. 122 −0.075 . We use a commonly accepted value of 3.9 throughout the text, as it has been used in all experimental publications until now, and it is still compatible with the value of Wipperfurth et al within the 1σ uncertainty.…”

“…In the last several years, a significant deviation from the prediction in the 4-6 MeV energy range is observed by a number of reactor antineutrino experiments, see [120,121,122,123,124] and review [125]. More specifically, an excess of events observed in the measured spectrum at these energies has 4.4σ local significance [121].…”

Extraction of neutrino mixing parameters from experiments with multiple identical detectors

“…The Daya Bay Collaboration first reported a successful measurement of mixing angle θ 13 (with a statistical significance of 5.2σ) in 2012 [1] and the RENO Collaboration reported their results (with a statistical significance of 4.9σ) in the same year [2]. More precise results from reactor neutrino experiments were reported in [7][8][9]. To compensate the limitation on the source of electron antineutrinos from nuclear power plants and to reduce systematic uncertainty in estimating absolute neutrino flux from nuclear power plants, Daya Bay and RENO utilized the design of identical detectors placed in group of near and far away from the nuclear reactors.…”

“…The Daya Bay result for θ 13 reported in [1] was obtained by minimizing a χ 2 function, as was done by the other short-baseline neutrino oscillation experiments. The function used by the Daya Bay Collaboration [9] was,…”