New spectral analysis of solar 8B neutrino with the Borexino detector

Solar neutrinos were first detected in the 1970’s and have played since then a significant role in the studies related to the construction of the solar model as well as in studies of the intrinsic properties of these particles, such as the features of vacuum and matter-enhanced oscillations. The Borexino detector is a unique experimental apparatus which has spectral sensitivity to all solar neutrino components apart from hep neutrinos. The detector could cover the energy range from 150 keV to as much as 15 MeV and in some cases even higher, while the achieved radiopurity levels minimizes the interference with background components. In this thesis spectral studies of the whole solar neutrino spectrum detected in Phase II with a duration of 1173 live days are reported. In particular, solar neutrinos from 8B are studied from the lowest detection threshold ever used for this component of solar neutrino spectrum. Electrons produced by recoil of these neutrinos are detected above 2.17 MeV of visible energy with a measured total neutrino flux of (5.5 ± 0.4) × 106 cm−2 s −1 under assumption of Large Mixing Angle (LMA) solution of Mikheyev-Smirnov-Wolfenstein(MSW) theory, that is compatible with the Standard Solar Model(SSM) in the high metallicity case. The spectral shape shows statistical compatibility with MSW-LMA. In this thesis it is shown that Borexino detector has statistical sensitivity to CNO and pep neutrino when the dedicated analysis here developed is applied. Central values are (5.2±1.8stat)×108 cm−2s−1 and (1.31±0.35stat)×109 cm−2s−1 respectively, compatible with expectations of high metallicity case of the SSM and MSW-LMA. In spite of the low precision, these results, obtained using the data collected by the Borexino detector could pave the way to future solar neutrino studies aiming to improve our understanding of solar models and neutrino oscillations in matter.