Involving scientists from Warsaw and Kraków, the facility operates as two neutrino telescopes: ORCA, studying neutrinos from Earth’s atmosphere, and ARCA, capturing very high-energy neutrinos from space.

Unlike conventional telescopes, KM3NeT is not a single detector but a sprawling “city” of sensors anchored to the seabed off Toulon, France, for ORCA, and near Portopalo di Capo Passero, Sicily, for ARCA.

Each telescope unit consists of 115 slender cables anchored to the sea floor, with 18 pressure-resistant glass spheres hanging on each cable. Each sphere contains 31 photomultipliers—highly sensitive “eyes” that detect flashes of light. ARCA alone includes more than 2,000 spheres and over 64,000 photomultipliers in a single block.

In ARCA, cables are spaced approximately every 90 meters, with spheres on the cable 36 meters apart. ORCA, being denser, has cables spaced every 20 meters and spheres every 9 meters. The volume of a single block is approximately 0.56 cubic kilometres for ARCA and 7.6 million cubic metres for ORCA—equivalent to roughly 224,000 and 3,000 Olympic-sized swimming pools, respectively. Construction is ongoing: ARCA is planned to have two blocks, ORCA one, but both are already operational.

Because neutrinos barely interact with matter, they cannot be observed directly. Instead, researchers detect brief flashes of Cherenkov radiation, which occur when a fast charged particle is created after a rare neutrino interaction in water.

“The problem is that the sea has its own sources of flashes: natural radioactivity in the water and bioluminescence of organisms,” the paper notes. Scientists must therefore separate the “neutrino story” from the “sea story” in real time.

The KM3NeT telescopes operate in “all-data-to-shore” mode, converting underwater signals into digital data and sending them via optical fiber to shore-based servers. The raw data stream reaches roughly 25 Gb/s per detector block.

The online filter functions in several stages. All recorded pulses are first collected, then checked for coincident flashes within a single sphere. If two or more photomultipliers flash almost simultaneously, the event is more likely to be real. The filter then narrows the time window between signals, adjusts geometry and the minimum number of hits, and gradually sifts out random coincidences. Preview data, generated about ten times per second, monitor the detector’s performance and environmental conditions, and will be made available for interdisciplinary research.

The system’s final event trigger evaluates whether recorded flashes form a causally consistent image, considering how quickly light can travel between known sphere positions. When conditions are met, a snapshot of the data from a short time window is saved, with a margin accounting for the detector size and uncertainties—2.5 microseconds for ORCA, 10 microseconds for ARCA.

Scientists distinguish two basic physical signatures: a single, long muon trace, and a particle shower. Separate triggering strategies are applied to each, with a “mixed trigger” for ORCA capturing weaker, low-energy events relevant to atmospheric neutrino physics.

The most important aspect, described in the paper, is verifying that the system meets three requirements simultaneously: it does not miss too many real events (efficiency), it does not flood the archive with false alarms (purity), and it fits within realistic computational resources (capacity). The filter has been operational since the instruments’ commissioning and has been used to capture events such as KM3-230213A.

The researchers include physicists from AstroCeNT (Nicolaus Copernicus Astronomical Center PAS) and AGH University of Science and Technology in Kraków. Funding came from the Foundation for Polish Science, the Excellence Initiative Research University at AGH, and the Polish National Science Centre. (PAP)

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