First simultaneous measurement of low-energy pp-chain solar neutrinos and prospects for CNO neutrino detection in Borexino

This thesis presents the first simultaneous measurement of low energy neutrinos produced in the Sun through the chain of reactions started from the proton–proton fusion (pp chain) and detected by the Borexino experiment. The Borexino experiment uses about 300 tons of liquid scintillator as target material. Solar neutrinos interact with the electrons of the scintillator and the energy released in the interaction is converted into scintillation light and detected by photo-multipliers. The measurement presented in this thesis is based on a multivariate analysis that exploits the energy, position and pulse shape distribution of the events in order to constrain the background and signal rates. To perform the analysis, new software tools and procedures have been developed, benchmarked and optimized in the framework of this thesis work. The sensitivity and systematic uncertainties have been studied using ensembles of pseudo data produced with Monte Carlo techniques considering all physical processes involved in the neutrino interaction, in the detector response, and in the read-out electronics. The results presented in this thesis improves previous Borexino findings. The interaction rate of neutrinos produced in the pp reaction which drives the energy production in the Sun has been measured to be 134 ± 10(stat)+6−10(sys counts per day per 100 ton of scintillator (cpd/100 t). The rate of neutrinos produced by the electron capture on 7Be has been constrained to 48.3 ± 1.1+0.4−0.7 cpd/100 t that is a factor 2 more accurate than the current prediction of the models that describe the structure and evolution of the Sun. The presence of neutrinos from the pep reaction has been established with a 5σ significance, its rate being 2.43 ± 0.36+0.15−0.22 cpd/100 t (2.65 ± 0.36+0.15−0.24 cpd/100 t) assuming an high (low) fraction of metals in the Sun. Under mild assumptions on the nuclear physics, an upper limit on the rate of neutrinos produced in the cycle of nuclear reactions catalyzed by C, N, and O (CNO cycle) has been constrained below 8.1 cpd/100 t at 95% C.L. The sensitivity to a CNO signal is primarily limited by 210Bi and pep events that have an almost indistinguishable energy distribution. To improve the sensitivity an independent analysis has been designed to constrain the rate of 210Bi and pep, breaking the correlation with the CNO signal. The requirements on the constraints have been defined through an extensive sensitivity study based which shows that under plausible assumptions on the actual CNO neutrino flux, an uncertainty of 1.75 cpd/100 t in the determination of the 210Bi background would result in a 3σ median significance. In this connection, a strategy for estimating the 210Bi background level based on the measurement of the 210Po daughter has been developed, tested and applied to real data, showing that the precision requested is achievable, and it is currently under consideration within the Borexino Collaboration. The measurements presented in this thesis along with an independent estimate of 8B neutrinos also based on Borexino data have been used to perform frequentist and Bayesian hypothesis testing between the predictions of the two most popular solar models. The results show an excellent agreement with the temperature profile expected assuming that the Sun has a high fraction of metals, that is favoured with respect to the predictions based on a lower metal content with odds 5 : 1 or, equivalently, Bayes factor 4.9. Assuming the fluxes predicted by the solar models it is possible to compute the electron neutrino survival probability and study the properties of neutrino oscillation. Borexino measurements establish the existence of matter effect at 98.2% C.L. The results presented in this thesis are the most accurate estimates of all the low-solar neutrino fluxes to date and provide the foundation of the future measurement of CNO that can be achieved thanks to innovative methods proposed to constrain the background.