The optical module of the KM3NeT neutrino telescope is an innovative multi-faceted large area photodetection module. It contains 31 three-inch photomultiplier tubes in a single 0.44 m diameter pressure-resistant glass sphere. The module is a sensory device also comprising calibration instruments and electronics for power, readout and data acquisition. It is capped with a breakout-box with electronics for connection to an electro-optical cable for power and long-distance communication to the onshore control station. The design of the module was qualified for the first time in the deep sea in 2013. Since then, the technology has been further improved to meet requirements of scalability, cost-effectiveness and high reliability. The module features a sub-nanosecond timing accuracy and a dynamic range allowing the measurement of a single photon up to a cascade of thousands of photons, suited for the measurement of the Cherenkov radiation induced in water by secondary particles from interactions of neutrinos with energies in the range of GeV to PeV. A distributed production model has been implemented for the delivery of more than 6000 modules in the coming few years with an average production rate of more than 100 modules per month. In this paper a review is presented of the design of the multi-PMT KM3NeT optical module with a proven effective background suppression and signal recognition and sensitivity to the incoming direction of photons.

The production of very high energy muons inside an extensive air shower is observable at ν telescopes and sensitive to the composition of the primary cosmic ray. Here we discuss five different sources of these muons: pion and kaon decays; charmed hadron decays; rare decays of unflavored mesons; photon conversion into a muon pair; and photon conversion into a J/ψ vector meson decaying into muons. We solve the cascade equations for a 1010.5 GeV proton primary and find that unflavored mesons and gamma conversions are the two main sources of E≥ 108.5 GeV muons, while charm decays dominate at 105.5 GeV< E< 108.5 GeV. In inclined events one of these muons may deposite a large fraction of its energy near the surface, implying fluctuations in the longitudinal profile of the shower and in the muon to electron count at the ground level. In particular, we show that 1 out of 6 proton showers of 1010.5 GeV include an E>106 GeV deposition within 500 g/cm2, while only in 1 out of 330 showers it is above 107 GeV . We also show that the production of high energy muons is very different in proton, iron or photon showers (e.g., conversions γ→ μ+ μ− are the main source of E≥ 104 GeV muons in photon showers). Finally, we use Monte Carlo simulations to discuss the validity of our results.

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High energy cosmic rays illuminate the Sun and produce an image that could be observed in up to five different channels: a cosmic-ray shadow (whose energy dependence has been studied by HAWC); a gamma-ray flux (observed at E ≤ 200 GeV by Fermi-LAT); a muon shadow (detected by ANTARES and IceCube); a neutron flux (undetected, as there are no hadronic calorimeters in space); a flux of high energy neutrinos. Since these signals are correlated, the ones already observed can be used to reduce the uncertainty in the still undetected ones. Here we define a simple setup that uses the Fermi-LAT and HAWC observations to imply very definite fluxes of neutrons and neutrinos from the solar disk. In particular, we provide a fit of the neutrino flux at 10 GeV–10 TeV that includes its dependence on the zenith angle and on the period of the solar cycle. This flux represents a neutrino floor in indirect dark matter searches. We show that in some benchmark models the current bounds on the dark matter–nucleon cross section push the solar signal below this neutrino floor.

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