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Astroparticle Physics
Abstract:
As the experimental boundaries of the energy and intensity frontiers are pushed forwards, astroparticle physics increasingly becomes a key tool to understand the microscopic and macroscopic mechanisms governing our universe. In this thesis particle physics beyond the Standard Model is explored, especially dark matter and neutrino properties, through the use of astrophysical neutrinos and other messengers. The combined use of neutrinos and photons, as well as cosmic rays and gravitational waves, is at the core of multi-messenger astronomy, a young and rapidly developing field which promises to reshape our understanding of the universe at hugely different energy scales. Neutrinos are of particular interest as they play the double role of possible signal and background. In the first part of the thesis, I present a new analysis of what we will call the “grand unified neutrino spectrum” (GUNS) at Earth, the flux of neutrinos coming from many different sources, both at low and high energies. After a short review of the contributions to the grand unified neutrino spectrum, we will turn to a previously overlooked flux, the low-energy component of neutrinos produced in the Sun by thermal processes, which fills the gap between the cosmic neutrino background and the solar neutrino flux from nuclear reactions. The second part of the thesis is dedicated to the search for physics beyond the Standard Model. First, I will show how solar neutrino observations can be used to constrain neutrino decay to light pseudoscalars, particularly taking advantage of antineutrino searches from the Sun tackled by KamLAND, SNO and Borexino. Finally, I will scrutinize hints for a dark matter signal in the context of multi-messenger, multi-wavelength astronomy, as the decay of axionlike particles with eV mass enhances the infrared cosmic background radiation (as detected by the sounding rocket CIBER), explaining at the same time an existing tension between the observations of Fermi and IceCube, namely that we observe less gamma rays than expected from the measured high-energy neutrino flux.
