Readout and trigger system of the KM3NeT detectors
2019 July 22
The KM3NeT collaboration is currently constructing a new generation neutrino telescope in the depths of the Mediterranean Sea. By recording the Cherenkov light originating from charged products of neutrino interactions in the water, the collaboration ultimately aims to investigate the sources of a high-energy neutrino flux with astrophysical origin and to determine the unknown neutrino mass hierarchy. To this aim, two independent detectors are built with an homologous design and data acquisition system. Digital Optical Modules contain the photo-multiplier tubes and data acquistion and control electronics and are organized along vertical assemblies called Detection Units. The photon arrival times are time-stamped to nanosecond accuracy, required to achieve background reduction and the required resolution on neutrino direction and energy. All recorded photon signals are sent to shore via an optical network able to cope with rates up to O(100) Gbps. An on-line software trigger system filters the photons from neutrino interactions from an overwhelming background (O(10$^6$ )) of photons originating from 40 K decay and bioluminescence. The flexible software based trigger system allows for multiple algorithms to operate in parallel. This system is also the intended gateway for generating and receiving external triggers in the multi-messenger context. This contribution discusses the design of the data acquisition system with an emphasis on the trigger and reports on the experience of the first detection units.