Invited Seminars

Solar neutrino spectroscopy results and cosmogenic backgrounds analysis with the Borexino detector

by Davide D'Angelo (TUM, INFN-Milano, Italy)




The Borexino experiment located in the Gran Sasso National Laboratory in central Italy, is an organic liquid scintillator detector devoted to the real time spectroscopy of low energy solar neutrinos via the elastic scattering on electrons in the target mass. The data taking campaign started in 2007 and rapidly lead to the first independent measurement of the monocromatic line of 7Be of the solar neutrino spectrum at 862keV. The latest measurement after 41.3 tonxyear of exposure is 49 +- 3 (stat) +- 4 (syst) counts/(dayx100 ton) leaves the hypothesis of no oscillation inconsistent with data at the 4sigma level and it represents the first direct measurement of the survival probability for solar neutrino(e) (0.56 +- 0.10) in the vacuum-driven oscillation region. Recently Borexino was also able to measure of the 8B solar neutrinos interaction rate down to the threshold energy of 3 MeV, the lowest achieved so far. The inferred electron neutrino flux is (2.4 +- 0.4 (stat) +- 0.1 (syst))x10^6 cm-2 s-1, in good agreement with existing measurements and predictions. The corresponding 8B mean electron neutrino survival probability, is 0.29 +- 0.10 at the effective energy of 8.9 MeV. For the first time we confirm, using data from a single detector, the presence of a transition between the low energy vacuum-driven and the high-energy matter-enhanced solar neutrino oscillations, in agreement with the prediction of the MSW-LMA solution for solar neutrinos. The next step for Borexino are the measurements of neutrino fluxes in the transition region such as pep and CNO. Prospects and implication for particle and astrophysics are reviewed together with the background (11C) suppression techniques. Upcoming dark matter experiments as well as other neutrino physics detectors are critically dependent on cosmogenic backgrounds in the underground sites. Borexino is about to publish a detailed review on the cosmic muon signal in Gran Sasso and its modulations as well as on the cosmogenic neutrons, including the production rate and multiplicity, the space-time correlation with the parent muon and the correlated creation of cosmogenic isotopes. Preliminary results of this analysis are discussed here.