The Cubic Kilometre Neutrino Telescope, or KM3NeT, is a European research infrastructure located on the bed of the Mediterranean Sea at depths of over 2 kilometres. It hosts water Cherenkov neutrino telescopes designed to detect and study neutrinos from distant astrophysical sources as well as from our own atmosphere, contributing significantly to both astrophysics and particle physics knowledge. Arrays of thousands of optical sensor modules detect the faint Cherenkov light in the deep sea from charged particles originating from interactions of neutrinos in water or rock in the vicinity of the detector. The position and direction of the optical modules and the time of arrival of the light on the photomultipliers inside are recorded with high precision. Properties of the particles, like their trajectory and energy, are reconstructed from these measurements.
Background The KM3NeT project foresees the construction of several of these detectors in the depths of the Mediterranean Sea along the southern coasts of Europe: KM3NeT-Fr (offshore Toulon, France) houses the ORCA (Oscillation Research with Cosmics in the Abyss) detector, and KM3NeT-It (offshore Portopalo di Capo Passero, Sicily, Italy) houses the ARCA (Astroparticle Research with Cosmics in the Abyss) detector. Both detectors are collecting data. KM3NeT-Gr (offshore Pylos, Peloponnese, Greece) will expand the KM3NeT Research Infrastructure in a next phase. The KM3NeT project continues the work done by the ANTARES neutrino telescope, which operated off the coast of France between 2008 and 2022. The oversight, governance and management of the implementation and operation of KM3NeT is conducted by an international collaboration with more than 68 institutions from 21 countries all over the world being involved. The KM3NeT community consists of about 360 scientists, along with engineers and technicians.
Scientific goals The main objectives of the KM3NeT Collaboration are as follows:
The discovery and subsequent observation of high-energy neutrino sources in the universe, probing a wide variety of cosmic objects such as supernova remnants, gamma-ray bursts, supernovae and colliding stars. By identifying neutrinos from these sources, KM3NeT aims to provide insight into the origins of cosmic rays and the mechanisms driving some of the most extreme events in the universe. In-depth investigations of fundamental neutrino properties, particularly neutrino oscillations, especially to determine the neutrino mass ordering by measuring the oscillations of atmospheric neutrinos. The ability to distinguish between different neutrino mass states will provide crucial information about the nature of neutrinos and their role in the Standard Model of particle physics. In addition to these primary scientific goals, the telescope is a powerful tool in the search for dark matter in the universe. Furthermore, the research infrastructure houses instrumentation for other sciences like marine biology, oceanography and geophysics for long-term and real-time monitoring of the deep-sea environment and the sea bottom at depths of several kilometres. The ARCA detector is the cubic kilometre-sized telescope searching for neutrino sources in the cosmos. The ORCA detector is optimised for the measurement of neutrino properties, and thus for investigating questions related to particle physics.
Design The infrastructures in France and Italy are designed to consist of almost 200 000 light sensors (photomultiplier tubes, or PMTs) distributed in three so-called building blocks: two for KM3NeT/ARCA and one for KM3NeT/ORCA. A building block comprises 115 flexible vertical strings - or detection units (DUs) - anchored at the seabed. Each string supports 18 pressure-resistant spherical sensor modules and each optical module comprises 31 photomultiplier tubes. Each building block thus constitutes a three-dimensional array of photo sensors that can be used to detect the Cherenkov light produced when relativistic particles emerging from neutrino interactions travel through sea water. The KM3NeT-It site (36°16′N 16°06′E) hosting the ARCA detector is at a depth of 3500 m. It is optimised for the detection of high-energy cosmic neutrinos in the TeV–PeV range by widely spacing the optical modules: the 18 modules are approximately equally spaced on strings that are about 700 m long, and spaced about 90 m apart. The KM3NeT-Fr site (42°48′N 06°02′E) hosting the ORCA detector is at a depth of 2450 m. The more closely spaced optical modules make the ORCA detector optimised for the detection of neutrinos in the GeV range. ORCA will consist of 115 strings in a 20 m triangular grid, with a 9 m spacing between the optical modules in a string. Overall, the array is about 210 m in diameter, and the strings are 200 m long. The position of the modules and the time of arrival of light on the photomultipliers inside are measured with high precision. Each optical module is about 44 centimetres (17 in) in diameter, contains 31 three-inch photomultiplier tubes with supporting electronics, and is connected to shore via a high-bandwidth optical network. Via an electro-optical network of cables and junction boxes on the sea floor the optical modules are connected to control stations on shore for electrical power, for detector control and for data transmission. Since the strings with optical modules move with the currents in the deep sea, the position and orientation of the optical modules and thus of the photomultiplier tubes inside is dynamically monitored using an acoustic system and a compass system, respectively. In each optical module controlled LED pulsers are used for time calibration. At the shore of each KM3NeT installation site, a farm of computers performs the first data filter, prior to streaming the data to a central KM3NeT data centre for storage and further analysis by the KM3NeT scientists. The construction and deployment of many of the detector pieces are illustrated in multiple videos.
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