Identification of the cosmogenic 11 C background in large volumes of liquid scintillators with Borexino

Abstract

<jats:title>Abstract</jats:title><jats:p>Cosmogenic radio-nuclei are an important source of background for low-energy neutrino experiments. In Borexino, cosmogenic <jats:inline-formula><jats:alternatives><jats:tex-math>$$^{11}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow /> <mml:mn>11</mml:mn> </mml:msup> </mml:math></jats:alternatives></jats:inline-formula>C decays outnumber solar <jats:italic>pep</jats:italic> 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 <jats:inline-formula><jats:alternatives><jats:tex-math>$$^{11}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow /> <mml:mn>11</mml:mn> </mml:msup> </mml:math></jats:alternatives></jats:inline-formula>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 <jats:inline-formula><jats:alternatives><jats:tex-math>$$^{11}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow /> <mml:mn>11</mml:mn> </mml:msup> </mml:math></jats:alternatives></jats:inline-formula>C tagging efficiency of <jats:inline-formula><jats:alternatives><jats:tex-math>$$\sim 90$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>90</mml:mn> </mml:mrow> </mml:math></jats:alternatives></jats:inline-formula> % and <jats:inline-formula><jats:alternatives><jats:tex-math>$$\sim $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> </mml:math></jats:alternatives></jats:inline-formula> 63–66 % of the exposure surviving the tagging. We present also a novel technique that targets specifically <jats:inline-formula><jats:alternatives><jats:tex-math>$$^{11}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow /> <mml:mn>11</mml:mn> </mml:msup> </mml:math></jats:alternatives></jats:inline-formula>C produced in high-multiplicity during major spallation events. Such <jats:inline-formula><jats:alternatives><jats:tex-math>$$^{11}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow /> <mml:mn>11</mml:mn> </mml:msup> </mml:math></jats:alternatives></jats:inline-formula>C appear as a <jats:italic>burst</jats:italic> 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 <jats:inline-formula><jats:alternatives><jats:tex-math>$$\sim 90\%$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>90</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math></jats:alternatives></jats:inline-formula> but with a higher fraction of the exposure surviving, in the range of <jats:inline-formula><jats:alternatives><jats:tex-math>$$\sim $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> </mml:math></jats:alternatives></jats:inline-formula> 66–68 %.</jats:p>