KM3NeT: An infrastructure for underwater Cherenkov neutrino telescopes

The KM3NeT Collaboration is incrementally building two underwater Cherenkov neutrino telescopes in the Mediterranean Sea. Both telescopes share the same technology for neutrino detection, by studying Cherenkov radiation from secondary charged particles produced in neutrino interactions. Photomultipliers are a common choice for the detection of Cherenkov radiation, but the hostile underwater environment, affected by sea currents and bioluminescence demands innovative solutions in KM3NeT. The distinctive technological features of KM3NeT are discussed, such as its sub-nanosecond timing accuracy and the few-centimetre accurate acoustic positioning of the detector elements. This results in a neutrino pointing accuracy below 0.5°at energies above 100 TeV for track-like events. The KM3NeT design is modular and allows for data taking with the telescope still in the construction stage. Early technical and scientific results are enticing. In particular, KM3NeT recently discovered a neutrino of unprecedented energy from outer space. The article covers the design and operation of the KM3NeT telescopes, as well as readout techniques, time synchronisation, data flow and event reconstruction, highlighting both technical and scientific aspects.