Open problems in neutrino astrophysics

In my thesis I addressed a selection of the open problems in neutrino astrophysics. These open problems regard the fundamental properties of neutrinos, as well as their role as an astrophysical messenger, on which we focussed our efforts. In the context of solar neutrinos, the main contribution of my work concerns the luminosity constraint, a strict relation between the solar luminosity in photons and in neutrinos. Such relation is based on few assumptions, among which the stationarity of the Sun, and that 4He is the only accumulating nuclear specie. We reformulated the derivation of the luminosity constraint in a simpler and clearer way, and generalised it by including the contribution of other accumulating elements and of the variation of the solar gravitational potential. The resulting updated luminosity constraint is more general, accurate, and powerful at connecting pp and CNO neutrinos, which are of paramount interest for solar neutrino detectors and for solving the solar metallicity problem. Atmospheric neutrinos have been studied both as a product of cosmic rays and as a background to the cosmic neutrino analyses. Two primary cosmic-ray flux models have been defined by fitting the data by ARGO-YBJ and KASCADE-Grande, which measure the knee of p+He at, respectively, ~700 TeV and ~5 PeV. Two atmospheric neutrino fluxes have been computed using such primary models as input, and have been compared to the available data to discriminate the knee position. Unfortunately, the uncertainties on the data only allow to slightly favour the KASCADE-Grande knee. The potential of atmospheric neutrinos as a proxy for cosmic-ray physics was discussed. The previous work pushed me to assess the possibility to detect prompt neutrinos as well as to investigate their role in cosmic neutrino analyses. A cosmic neutrino flux model was built by combining the expectation on the neutrino spectrum produced in pp collisions in starburst Galaxies with the experimental result of the through-going muons analysis. Comparing the components of the neutrino spectra, it was clear that prompt neutrinos cannot be extracted from samples rich in νμ-induced events. The cascades dataset resulted to be the most promising, and the yearly rates of cascade events in IceCube due to all components (and flavours) of the neutrino spectrum were computed for 1 TeV ≤ Eν ≤ 10 PeV. Prompt neutrinos resulted to contribute to less than 3% of the total rate of shower-like events. The detection of prompt neutrinos could be feasible by studying inclined cascades with a higher energy threshold, so as to reduce the conventional background. Finally, it was concluded that the contribution of prompt neutrinos could cause the spectral difference between the fluxes resulting from the HESE and through-going muons analyses. This spectral tension has been investigated also in my first paper, in which the results of the IceCube analyses have been discussed on the basis of the common set of assumptions used to interpret them. In fact, the spectra resulting from the HESE and through-going muons dataset would not be in tension if the astrophysical signal were anisotropic and distributed differently from an unbroken power law. An alternative two-component astrophysical neutrino spectrum proved to be compatible with all IceCube data, and was used to compute the expected rates of events due to tau neutrinos (double cascades) and Glashow resonances. Double cascade events were the focus of my second paper, in which standard neutrino oscillations and the through-going muons spectrum were used to obtain the astrophysical flux of all neutrino flavours. These were convolved with the (analytically approximated) effective areas of IceCube, IceCube-gen2, and KM3NeT to compute the rates of double cascade events above 100 TeV. IceCube is predicted to be very close to the first detection of double cascade events, and the recent preliminary data by IceCube agree with such prediction. The non-observation of double cascade events would lead to new physics or to disproving the discovery of cosmic neutrinos.