SHOWER RECONSTRUCTION AND STERILE NEUTRINO ANALYSIS WITH KM3NeT/ORCA AND ANTARES

Neutrino physics is quite a new field of particle physics, since the idea of a neutrino was firstly introduced by W. Pauli in 1930 to try to explain how beta decays could conserve energy, momentum and spin. Since then, a big effort has been done by theoretical and experimental physicists to better understand the neutrino nature and its implications, not only in the more fundamental physics of the Standard Model (SM), but also in astrophysics and cosmology. This led, in the recent past, to two Nobel Prizes which were given to Neutrino Physics. In 2002 Ray Davis and Matoshi Koshiba got the Nobel Prize in Physics “for pioneering contributions to astrophysics, in par- ticular for the detection of cosmic neutrinos” while in 2015 Arthur McDonald and Takaaki Kajita got the Nobel Prize “for the discovery of neutrino oscil- lations, which shows that neutrinos have mass”. While the standard three-flavor framework of neutrino oscillations is by now well established, there are a number of oscillation experiments whose results cannot be explained in this framework and need to invoke more exotic ex- planations. One attempt to solve this tension is to assume the existence of a light sterile neutrino with mass around 1 eV2. The data collected to date present an incomplete, perhaps even contradic- tory picture, where 2-3 σ agreement in favor of and in contradiction to the existence of sterile neutrinos is present. The need thus arises to provide a more precise and complete test of the sterile neutrino hypothesis, which will unambiguously confirm or refute the interpretation of past experimental re- sults. The work of my thesis is focused on the search of sterile neutrinos with deep water neutrino telescopes, using both the real data from ANTARES and the expected performances of the larger KM3NeT/ORCA detector. The aim of the thesis was not only to make such analysis but also to better understand the operation of a neutrino telescope. In particular, for KM3NeT/ORCA, I developed a shower reconstruction algorithm to fully exploit the capability of this detector. In addition, I also developed an online analysis tool to mon- itor the data coming from KM3NeT (ORCA and ARCA) which is already running with the first detection units. The thesis is thought to describe my work in a causal way, in the sense that it starts by introducing the problem I have been focusing on (i.e. the ob- served neutrino oscillation anomalies), then, starting from the flux used in the analysis (i.e. the atmospheric neutrino flux) we will follow the neutrinos journey up to the telecopes, where we will review how neutrinos are detected (i.e. Cherenkov emission of photons) and the related problems of this kind of detection. Once the Cherenkov photons produced by the original neutrinos have hit PMTs, I will describe in more detail the KM3NeT data acquisi- tion system, here I will have the opportunity to describe my work on online data. Once the triggered data are saved to disk, reconstruction algorithms are applied to them, hence here I will describe the shower reconstruction algorithm I developed for KM3NeT/ORCA. Once reconstructions are applied to the data, we can know the detector response and use it in out analyses. In particular we can perform the sterile neutrino analysis with both ORCA and ANTARES, and this is what I will finally describe. To be more precise: Chapter 1 is a general introduction on ster- ile neutrinos anomalies which aims to give a global view on this argument. Chapter 2 explains in more detail neutrinos oscillations from a more theoreti- cal point of view and puts the basis on neutrino properties useful for the final sterile neutrino analysis. Chapter 3 introduces the detection method of deep water neutrino telescopes in a more general way. Chapter 4 enters in the details of the KM3NeT data acquisition system with a particular importance on the online monitoring tool I wrote. Chapter 5 describes the shower recon- struction algorithm that I developed during the PhD. And finally, Chapter 6 shows the results of the sterile neutrino analysis with KM3NeT/ORCA and ANTARES.