Neutrinos are weird elementary particles. They have no electrical charge. They have very tiny masses, yet to be measured with any certainty. They interact very weakly with “ordinary” matter.
About 100 trillion of them pass through a typical human body each second. Very few interact with any nucleus. A neutrino might have a 50% chance of such interaction as it passes through a light-year of lead!
Some neutrinos carry a lot of energy, and that can be detected when they hit atoms. To catch such events, the ARCA detector array was set up 2,450 m below the surface in the Mediterranean near Sicily. That was the work of the KM3NeT collaboration of about 300 scientists from 18 countries on 5 continents.
On February 13, 2023, one neutrino hit the ARCA array. This single ultraminuscule neutrino carried a whopping amount of energy of about 220 septillion electron volts, the kinetic energy of a good-sized bullet. The collision with an atom in the sea created a shower of many subatomic particles.
The locations of the various particles were reconstructed from the flashes of light they created. The neutrino almost incontestably came from outside even our Milky Way galaxy! Underwater fireworks!
This has been an outreach activity of the Las Cruces Academy, viewable at GreatSchools.org.
Source: Nature, 13 Feb. 2025, pp. 376 ff.
Image: Same