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Springer, The European Physical Journal C, 12(81), 2021

DOI: 10.1140/epjc/s10052-021-09799-x

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Identification of the cosmogenic $^{11}$C background in large volumes of liquid scintillators with Borexino

Journal article published in 2021 by M. Agostini, K. Altenmüller, S. Appel, V. Atroshchenko, Z. Bagdasarian, D. Basilico, G. Bellini, J. Benziger, R. Biondi, D. Bravo, B. Caccianiga, F. Calaprice, A. Caminata, P. Cavalcante, A. Chepurnov and other authors.
This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Data provided by SHERPA/RoMEO

Abstract

AbstractCosmogenic radio-nuclei are an important source of background for low-energy neutrino experiments. In Borexino, cosmogenic $^{11}$ 11 C decays outnumber solar pep and CNO neutrino events by about ten to one. In order to extract the flux of these two neutrino species, a highly efficient identification of this background is mandatory. We present here the details of the most consolidated strategy, used throughout Borexino solar neutrino measurements. It hinges upon finding the space-time correlations between $^{11}$ 11 C decays, the preceding parent muons and the accompanying neutrons. This article describes the working principles and evaluates the performance of this Three-Fold Coincidence (TFC) technique in its two current implementations: a hard-cut and a likelihood-based approach. Both show stable performances throughout Borexino Phases II (2012–2016) and III (2016–2020) data sets, with a $^{11}$ 11 C tagging efficiency of $∼ 90$ ∼ 90 % and $∼ $ ∼ 63–66 % of the exposure surviving the tagging. We present also a novel technique that targets specifically $^{11}$ 11 C produced in high-multiplicity during major spallation events. Such $^{11}$ 11 C appear as a burst of events, whose space-time correlation can be exploited. Burst identification can be combined with the TFC to obtain about the same tagging efficiency of $∼ 90\%$ ∼ 90 % but with a higher fraction of the exposure surviving, in the range of $∼ $ ∼ 66–68 %.