Directional analysis of sub-MeV 7Be solar neutrinos in Borexino

Borexino is a liquid scintillator detector located at the Laboratori Nazionali del Gran Sasso (LNGS), Italy with the main goal to measure solar neutrinos. Liquid scintillator detectors are excellent neutrino detectors due to their high light yield and thus a low energy threshold and high energy resolution. But, they suffer from the loss of directional information that is typically present in water Cherenkov neutrino detectors. However, in a liquid scintillator, there is still a subdominant amount of Cherenkov light emitted much faster with respect to the slower, yet dominant scintillation light. Borexino has successfully exploited this Cherenkov light signal from the first few PMT hits of an event to provide the first ever directional measurement of sub-MeV solar neutrinos, through the novel technique called Correlated and Integrated Directionality (CID). This is also the first signature of directionality in neutrinos elastically scattering off electrons in a liquid scintillator detector. Through this measurement, the 7Be interaction rate in the detector has also been extracted. Future liquid scintillator detectors can benefit from this method for background suppression and disentanglement of signals from different directions