This forces scientists to rethink how natural particle factories work in celestial bodies. The research by the international KM3NeT team - including researchers from Poland - was published in Nature. 

NEUTRINO MUST DIE

Neutrinos are extraordinary particles - they are like travellers from distant space, unbothered by nearly anything they pass by. They fly unhindered through the empty spaces between the particles that make up the Universe and do not care about anything.

We, humans, experience the world through electromagnetic interactions. They stop the hand hitting the table from going through the wood and instead stopping on the tabletop with a bang, even if none of the atoms in our hand hit any atom of the table directly. Meanwhile, neutrinos are indifferent to these electromagnetic interactions. That is why these cosmic loners sail across galaxies, stars, planets, mountains, seas, our bodies... They do not see obstacles, and obstacles - do not see them.

With some minor exceptions. Sometimes such a carefree neutrino in its 'empty' path ends up so close to another particle that other interactions come into play - weak ones - and neutrinos do respect those. And then pop, the neutrino is gone. This dying recluse releases particles that are much more capable of cooperating - usually muons (more massive cousins ​​of electrons).

Tough luck for the dying neutrino, but for scientists - it is a treat to record such an event (a bit like for an unscrupulous cameraman - car crash amateur in the movie Nightcrawler). Such a collision is the only opportunity to obtain information about the nature of neutrinos, and maybe also the place from which these space travellers originate.

EXOTIC PLACES WHERE NOTHING HAPPENS

That is why scientists keep coming with ever stranger ideas on how to detect such dying neutrinos. They look for the 'most boring' places on Earth, where ordinary, 'talkative' particles from the atmosphere or from space rarely venture. It would be best if there was nothing there that would distract from the death of stray neutrinos.

That is why neutrino observatories have been placed: in a tunnel inside a mountain (Gran Sasso), in huge artificial underground water reservoirs (Kamioka), in the ice of Antarctica (IceCube). And finally - in the depths of the sea (KM3NeT).

RAIDERS OF THE SUNKEN ARK

The underwater neutrino observatory ARCA, part of the KM3NeT experiment, does not resemble traditional telescopes. 'The detectors are submerged 3.5 km below the surface of the sea, near Sicily. These are vertical strings anchored to the bottom, on which light sensors with photomultipliers hang; they convert light into voltage', explains research participant, Piotr Kalaczyński, PhD from CAMK and AGH UST. There are 230 such strings spaced every 100 meters, each 700 meters long and equipped with 18 sets of sensors.

The task of these sensors is to look for flashes of light in the water. This is called Cherenkov radiation. In simple terms, it is the light released by the atoms that make up water if they are stimulated by an energetic particle.

LIGHT AT THE BOTTOM OF THE SEA

Piotr Kalaczyński explains that in the ARCA observatory at the bottom of the sea, where sunlight no longer reaches, a common source of flashes is bioluminescence - light generated by organisms living in the sea. There are also flashes associated with natural radioactivity, dominated mainly by the potassium-40 isotope. A lot of light is also released by muons coming from the atmosphere. Signals from neutrinos are thousands of times rarer and happen every now and then, more interesting cases - once every few days', he describes.

Neutrinos of cosmic origin are not stopped by Earth or water, so they can fly into the observatory from any possible direction. Strong flashes from strange directions are therefore immediately taken into account.

And that was exactly what happened with the neutrino observed on February 13, 2023. The ARCA detector sensors recorded a flash of light that flew between them more or less horizontally. It was named KM3-230213A, not very affectionately, but it was the most energetic neutrino ever observed. 'It was an order of magnitude or two higher than the neutrinos we had observed before', the physicist says. The energy of this record-breaking neutrino was 220 petaelectronvolts, thousands of times more than the energy of collisions at CERN, where 14 teraelectronvolts were achieved (tera- is a thousand billion, and peta- is a thousand tera-).

'This is such an unusual observation that we will need to correct current models of neutrino production', Kalaczyński believes.

Where did this cosmic wanderer come from? 'This neutrino definitely did not come from our Galaxy', the researcher comments. He explains that this neutrino could have flown to us from one of the active nuclei of other galaxies, called blazars. There are three potential candidates.

The second possibility is that the flash is a so-called cosmogenic neutrino, which is formed from the interaction of photons with ultra-energetic particles of cosmic radiation.

The KM3NeT experiment involves 360 people from 21 countries. Poland is represented in the project by specialists from AGH UST, CAMK PAN and NCBJ.

The researcher reveals that the Polish KM3NeT team is currently working, among other things, on recognising the sounds accompanying neutrinos that enter water. ARCA is equipped with hydrophones that collect sounds from the sea. Sounds carry over long distances in water. In addition to the sounds of dolphins and sea creatures, these devices may also collect information about cosmic outcasts. It would be worth making use of this.

The project 'AstroCeNT - Particle Astrophysics Science And Technology Centre' is implemented within the framework of the International Research Agendas programme of the Foundation for Polish Science.

PAP - Science in Poland, Ludwika Tomala

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