Real time solar neutrino spectroscopy with Borexino phase 2

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For decades the study of missing solar neutrinos was one of the biggest problems in particle astrophysics. Meanwhile this problem is considered to be solved as neutrino oscillations in matter describe all observations well,including the ones from Borexino. Borexino is the worldwide cleanest - in terms of radio purity- scintillation detector and also has a very low threshold, which allowed a first real time measurement of even pp-neutrinos from the fundamental fusion process. This grant application is linked to phase 2 of Borexino, a phase with even lower background due to a performed purification campaign. To improve measurements further two new calibration sources will be built in this grant to extend the calibrated region to higher energies. From the analysis point of few two topics will be dealt with in this grant. Of high priority is the first ever measurement of CNO-neutrinos, which would experimentally prove the existence of this cycle. Within this context the solar abundance problem will be explored. Recent 3-D modeling of photosphere lines lead to a lower abundance of heavier elements like C,N and O by 20-40%. However, this causes some larger discrepancy between solar models and helioseismological observations. The only way to probe the abundancies in the interior of the Sun are CNO-neutrinos. This will also shed some light on fundamental assumptions in stellar astrophysics, namely the homogeneous distribution of elements. In the same analysis an improved measurement of the pep-neutrinos is possible. The major background for these searches is 11C produced by myon interactions in the scintillator. Within this grant a new method based on a likelihood analysis will be developed to remove these events with a higher efficiency than the current one. Another research question tackled in this grant is the low energy behavior of the 8B spectrum. The lowest data point of all real time solar experiments shows the opposite behavior one would expect from oscillations. Thus, for a long time other non-standard interaction (NSI) scenarios are discussed. The improved phase 2 data will be analyzed in this direction and will distinguish between the different solutions or at least strongly constrain the parameters.