* **Introduction**: A recent groundbreaking discovery by the KM3NeT Collaboration has detected an exceptionally high-energy cosmic neutrino. This event, named KM3-230213A, is significant because its energy far exceeds any neutrino previously observed.

* **What are Cosmic Neutrinos?**: Cosmic neutrinos are electrically neutral particles that travel vast distances without being deflected by magnetic fields or significantly absorbed by matter. They are produced when cosmic rays interact with matter or photons, making their detection a key to understanding high-energy astrophysical processes.

* **The KM3NeT Experiment**: The KM3NeT is a deep-sea neutrino telescope located in the Mediterranean Sea. It consists of two detector arrays: ARCA, optimized for high-energy cosmic neutrinos, and ORCA, for neutrino oscillations. These detectors utilize optical sensors to detect Cherenkov light produced by charged particles resulting from neutrino interactions.

* **The Ultra-High-Energy Event**: The detected event, KM3-230213A, is a muon with an estimated energy of **120 PeV**. The neutrino that produced this muon is estimated to have had an even higher energy. The muon was detected traversing the ARCA detector on February 13, 2023.

* **How it was Detected**: The muon's trajectory was reconstructed using the arrival times and positions of the first hits recorded on the photomultiplier tubes (PMTs). The energy was estimated by counting the number of PMTs that triggered. The large amount of light detected saturated the PMTs closest to the muon trajectory, and large showers resulting from energy loss processes were observed along the track.

* **Significance**: This event may indicate a different source of cosmic neutrinos or could be the first detection of a cosmogenic neutrino, produced by interactions of ultra-high-energy cosmic rays with background photons. The detected energy significantly exceeds previous detections, suggesting new astrophysical phenomena.

* **Background and Analysis**: The possibility of the event being caused by atmospheric muons or neutrinos was considered. The probability of an atmospheric origin is extremely low, especially given the near-horizontal direction and high energy. The direction of the neutrino matches expectations for an isotropic flux of ultra-high-energy neutrinos, where downgoing neutrinos are obscured by atmospheric muons, and upgoing neutrinos are absorbed by the Earth.

* **Searches for Source**: Extensive searches were conducted for a source counterpart within a 3° radius of the event using multiwavelength data. Various catalogs of gamma-ray, X-ray, infrared, and radio sources were examined, but no conclusive source association has been made.

* **Flux Measurement:** The steady isotropic flux that would produce one event like KM3-230213A is **5.8 x 10^-8 GeV cm^-2 s^-1 sr^-1**. This flux measurement exceeds current limits from IceCube and Auger, possibly indicating an upward fluctuation or a new component in the flux. This event could be from cosmogenic neutrino production or from transient emitters such as gamma-ray bursts or tidal-disruption events.

* **Conclusion**: The detection of KM3-230213A provides significant evidence for the existence of ultra-high-energy neutrinos and enhances our understanding of the universe's most energetic phenomena.

* **Reference**: The KM3NeT Collaboration. "Observation of an ultra-high-energy cosmic neutrino with KM3NeT." *Nature* (2025).

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: KM3NeT