Sensitivity of the KM3NeT/ARCA Phase-1 detector to a diffuse neutrino emission from the Southern sky

KM3NeT is a large research infrastructure that will host two deep-sea neutrino detectors in the Mediterranean Sea. The ORCA (Oscillation Research with Cosmics in the Abyss) detector, which will be located 40 km offshore Toulon (France), is optimised for the study of atmospheric neutrino oscillations. Whereas the ARCA (Astroparticle Research with Cosmics in the Abyss) detector, which will be located 80 km offshore Capo Passero (Italy), is dedicated to the search for high-energy neutrinos (above 1 TeV) from astrophysical sources. Moreover ARCA will explore the IceCube signal from a different field of view, with an improved angular resolution and different systematic uncertainties. A grid of thousands of sensitive optical sensors will detect the faint Cherenkov light produced by relativistic charged particles emerging from neutrino interactions. The collected photon arrival times and the spatial positions of these sensors are used to reconstruct the direction of the neutrinos and to associate them with potential distant cosmic sources. The ARCA detector will consist of two structures called “building blocks”. Each building block is composed of 115 strings and each string comprises 18 large spherical sensors, called “digital optical modules”. The novel design of these digital optical modules helps to optimize the photocathode area by implementing 31 small photomultiplier tubes within a glass sphere and makes it possible, with only one digital optical module, to identify atmospheric muon events. Once completed, the ARCA installation will add up to an instrumented volume of about 1 km3. The first construction phase of ARCA will consist of 24 strings and will subsequently amount to an instrumented volume of about 0.1 km3. The underwater neutrino detector can start taking data from the deployment of the fist string. At the Capo Passero site, two strings have been deployed and the first results demonstrate the ability to efficiently reconstruct events, which were then compared to Monte Carlo simulations. The excellent agreement between data and Monte Carlo simulations confirms the ability to accurately reproduce the detector’s performance. The purpose of this work is to predict the capabilities of the first 24 strings of the ARCA detector by relying on Monte Carlo simulations. The work reported in this thesis focuses on the evaluation of the discovery potential of ARCA Phase-1 to the muon neutrino diffuse flux by selecting the upgoing muon tracks which cross the detector volume. The reconstruction of the muon tracks with respect to the direction of the incident muon neutrinos is performant enough to achieve an angular resolution better than 0.15° at high energies (above 10 TeV). Furthermore, the detection of the IceCube isotropic diffuse flux at 3σ significance level is expected to be achieved within some 4 years (about 10 years at 5σ). Two techniques were applied to optimize event selection by using track reconstruction parameters and to derive the corresponding discovery flux with suitable significance. This work concludes that, in spite of the detector’s size at this first construction phase, interesting scientific findings can be expected from the ARCA installation with reasonable sensitivity. Improved results can be obtain by performing a multivariate analysis to better identify background events and also by taking into account cascade-like events.