The sophisticated design of the ARCA detector of the KM3NeT telescope made it possible to push the limits of accuracy in reconstructing the trajectory and energy of the muon created in the KM3-230213A event. The multi-PMT design of the optical modules (DOMs), enhanced by the sub-nanosecond precision of the time calibration system, allowed for carefully selecting the photomultipliers’ signals to be considered for track reconstruction – important for optimising the reconstruction accuracy – by discriminating the signals which reached the photomultipliers directly from those which were scattered in the seawater. Three sudden releases of energy, ascribed to stochastic energy losses of the muon, were identified along the track, contributing significantly to the determination of the particle’s energy.
The almost horizontal path through the detector together with the record level of energy provides compelling motivation for excluding the possibility that the muon had reached the detector from the atmosphere. It has to be concluded therefore that the particle was created by a neutrino in the vicinity of the detector.
