Correlated and integrated directionality in Borexino : using Cherenkov directionality for the measurement of solar neutrinos in high light-yield liquid scintillator detectors
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Abstract
Observations of low energy neutrinos from astrophysical sources are performed currently by either liquid scintillator detectors or water Cherenkov detectors. To date, liquid scintillator detection in particular is the only technique that allows precision spectroscopy of sub-MeV solar neutrinos, as demonstrated by the Borexino detector at the Gran Sasso National Laboratory in Italy. This is made possible by the high light-yield of the scintillation, which is also responsible for a low energy threshold of ∼ 0.2 MeV, together with the unprecedented radio-purity of the detector materials. In contrast, water Cherenkov detectors are only able to measure solar 8B neutrinos above a few MeV with high precision. They reconstruct the event direction via the corresponding Cherenkov photon hits, to differentiate between the solar neutrino signal and the background events. A combination of the directional Cherenkov information and the high light-yield and low energy threshold of the scintillation spectroscopy can enable an unprecedented precision in the measurement of solar neutrinos and other physics goals. Exploring this novel hybrid detection approach, this thesis describes the first measurement of sub-MeV solar neutrinos using their associated directional Cherenkov photons, as well as a directional measurement of the CNO-neutrino rate, using the Borexino detector. These analyses are based on the specially developed Correlated and Integrated Directionality method, where the individual photon hits of the events are correlated to the known position of the Sun. The integration of these angular hit values over a large number of
events allows the statistical inference of the number of solar neutrino events. The directional measurement of the 0.862 MeV line of the 7 Be-neutrinos is performed in an energy window between 0.56 MeV to 0.76 MeV, which is selected through the dominant scintillation light. The measured rate is R( 7 Be) CID = 51.6 +13.9 −12.5 (stat. + syst.) cpd/100 t, with an exclusion of the zero neutrino hypothesis of > 6σ . The directional measurement of the CNO-neutrino rate is performed in an energy region between 0.85 MeV to 1.3 MeV. The measured rate is R CNO = 7.2 +2.8 −2.7 (stat. + syst.) cpd / 100 t, with an exclusion of the zero CNO-neutrino hypothesis of > 5σ. These results correspond to the first detection of low energy solar neutrinos using only their directional Cherenkov information in a large-scale, high light-yield scintillator detector. Additionally, this is also a very direct proof of the solar origin of the 7 Be- and CNO-neutrino signal events. This demonstration of a directional sensitivity in a monolithic liquid scintillator target provides an experimental proof-of-principle for the concept of hybrid event detection, which could be implemented in future, purpose-built neutrino detectors.