The event

On 13 February 2023, KM3NeT/ARCA, operating with 21 detection units comprising 378 optical modules (DOMs), recorded a neutrino event unlike any other: named KM3-230213A, it represents the most energetic neutrino ever detected in the world!

See this movie for an introduction to KM3-230213A.

The signal

In the middle of the night of 13 February 2023, the apparatus recorded an unprecedented event: during a time window of almost 2 microseconds, more than 28,000 photons were detected in the almost 12,000 photomultipliers of the detector; the level of illumination was so high that more than 25% of the photomultipliers which detected a signal got saturated. From the analysis of the data it was possible to identify a single particle, a muon, which had crossed the entire detector. Detailed studies led to an estimate of the muon’s energy at a record value of 120 PeV, 100 trillion times higher than that of visible light photons. This implies that the primary neutrino that generated the muon had an even higher energy, estimated at the level of 220 PeV, significantly higher than any neutrino ever detected before. This result has recently been illustrated in an article published in Nature,

Event display of KM3-230213A. The muon’s reconstructed trajectory is depicted as a red line, accompanied by an artistic representation of the Cherenkov light cone. The hits recorded by individual PMTs are shown as spheres aligned with the PMT orientations. The spheres are color-coded based on their detection time relative to the first triggered hit. Additionally, the size of each sphere corresponds to the number of photons detected by the respective PMT.

The data analysis

The sophisticated design of the KM3NeT/ARCA detector made it possible to push the limits of accuracy in reconstructing the muon’s trajectory and energy. The multi-PMT design of the DOMs, enhanced by the sub-nanosecond precision of the time calibration system, allowed to carefully select the photomultipliers’ signals to consider for track reconstruction – this is very important for optimising the reconstruction accuracy, by discriminating the signals which have reached directly the photomultipliers from those which have undergone scattering in water. Three sudden releases of energy, ascribed to stochastic energy losses of the muon, were identified along the track, contributing significantly to the determination of the particle’s energy.

The almost horizontal path through the detector together with the record level of energy provides compelling motivation for excluding the possibility that the muon had reached the detector from the atmosphere. It has to be concluded therefore that the particle was created by a neutrino in the vicinity of the detector.

The origin

The origin of KM3-230213A is debatable. Due to its extreme energy, the hypothesis that it was generated in an interaction of a cosmic-ray particle with even higher energy in the atmosphere can not reasonably hold. No plausible Galactic sources which can eject neutrinos with such energy have been so far proposed. It is therefore highly likely that this neutrino originated at a very powerful cosmic accelerator well beyond the Milky Way, such as an active galactic nucleus or a gamma-ray burst. Alternatively, it could be a ‘cosmogenic neutrino,’ generated in the interaction of a ultra-energetic cosmic-ray particle with a photon of the background radiation that permeates the Universe.

The KM3NeT Collaboration has performed various studies in order to investigate the possible origin of KM3-230213A and the implications that one can draw from it. No significant correlation has been found so far with potential Galactic and extra-Galactic sources in the direction from which the neutrino was coming (RA = 94.3°, dec. = −7.8°).

These studies have been illustrated in the article published in Nature as well as in a set of articles that were recently made available on arXiv:

Open data and software

Coherently with the open science policy of KM3NeT, public data and notebooks to reproduce some of our findings for KM3-230213A have been made available on zenodo; in alternative, one can directly launch our git repository on mybinder.

What’s next

Several actions have been undertaken in order to improve the accuracy of this event and prepare the detection of the next ones of this kind. An upgrade of the positioning system will take place in the next marine operation: this upgrade, designed for improving the precision of the absolute position and orientation of the detector, will allow to push the accuracy in the determination of the particles’ directions from the level of 1.5° which was estimated for KM3-230213A down to the target value of ∼0.1°. This upgrade will hold for all next detections, but it will also allow for correcting the measurement of KM3-230213A retroactively. Secondly, as the construction of the apparatus continues, both the capacity of collecting events and the reconstruction capabilities will improve. Finally, an online alert system is being set up, which will allow for distributing alerts with the time and direction of interesting neutrino events shortly after the events are detected – a key improvement for multi-messenger searches.

 

The event is also shown in different styles and in different views below.

 

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