Cosmogenic C11 production and sensitivity of organic scintillator detectors to pep and CNO neutrinos

Several possible background sources determine the detectability of pep and CNO solar neutrinos in organic liquid scintillator detectors. Among such sources, the cosmogenic C11 nuclide plays a central role. C11 is produced underground in reactions induced by the residual cosmic muon flux. Experimental data available for the effective cross section for C11 by muons indicate that C11 will be the dominant source of background for the observation of pep and CNO neutrinos. C11 decays are expected to total a rate 2.5 (20) times higher than the combined rate of pep and CNO neutrinos in Borexino (KamLAND) in the energy window preferred for the pep measurement between 0.8 and 1.3 MeV. This study examines the production mechanism of C11 by muon-induced showers in organic liquid scintillators with a novel approach: for the first time, we perform a detailed ab initio calculation of the production of a cosmogenic nuclide, C11, taking into consideration all relevant production channels. Results of the calculation are compared with the effective cross sections measured by target experiments in muon beams. This article also discusses a technique for reduction of background from C11 in organic liquid scintillator detectors, which allows to identify on a one-by-one basis and remove from the data set a large fraction of C11 decays. The background reduction technique hinges on an idea proposed by Martin Deutsch, who suggested that a neutron must be ejected in every interaction producing a C11 nuclide from C12. C11 events are tagged by a threefold coincidence with the parent muon track and the subsequent neutron capture on protons.