Muon detection in early JUNO data

JUNO is a medium-baseline reactor neutrino detector designed to study neutrino oscillation, with the specific goal of determining the neutrino mass ordering. It exploits a 20 000 ton volume of liquid scintillator to detect inverse beta decays produced by reactor antineutrinos, which are mainly coming from nuclear power plant at ∼ 50 km from the experimental site. Before being filled with the liquid scintillator, it was filled with water. The first JUNO data, starting from February 2025, feature cosmic muons detected through Cherenkov light. Then the LS filling started, introducing a mixed LS-water phase in the detector, eventually ending in a fully LS detector. This study focuses on the first two phases, with the main goal of studying the muon flux in JUNO. Muons are one of the main sources of background in JUNO, since they produce, through spallation, cosmogenic isotopes, such as 9Li and 8He, and fast neutrons, which produce a prompt-delayed signal identical to the one of inverse beta decays. The topics covered in this thesis include muon selection, measurement of the muon rate, an algorithm to select the entry and exit points of the muons in the detector, muon track reconstruction, angular distribution, and influence on the background, specifically the production of the fast neutrons. Additionally, muon bundles in JUNO were studied, muon bundles stem from high-energy interactions of primary cosmic rays in the upper atmosphere and can be used to constrain interaction models. Muon bundles are seldom detected by underground experiments because of their limited size. JUNO, instead, featuring a 35m-diameter, is expected to be one of the most sensitive experiments to this class of events.