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

TL;DR

This paper details the development and evaluation of a highly efficient method for identifying and reducing the $^{11}$C background in the Borexino liquid scintillator neutrino experiment, improving the sensitivity to solar neutrinos.

Contribution

It introduces a novel three-fold coincidence technique and a burst-based method for $^{11}$C identification, enhancing background suppression in neutrino measurements.

Findings

Achieved about 90% $^{11}$C tagging efficiency.

Maintained 63-68% of exposure after background rejection.

Demonstrated stable performance across multiple data phases.

Abstract

Cosmogenic radio-nuclei are an important source of background for low-energy neutrino experiments. In Borexino, cosmogenic $^{11}$C decays outnumber solar $pep$ and CNO neutrino events by about ten to one. Highly efficient identification of this background is mandatory for these neutrino analyses. 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 $^{11}$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 $^{11}$C tagging efficiency of $\sim$90 % and $\sim$63-66 % of the exposure surviving the tagging. We present also a novel technique that targets specifically $^{11}$C produced in high-multiplicity during major spallation events. Such $^{11}$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 $\sim$90 % but with a higher fraction of the exposure surviving, in the range of $\sim$66-68 %.