Astroparticle physics: Record-breaking neutrino detected

On February 13, 2023, the ARCA detector, part of the neutrino telescope KM3NeT located off the southeast coast of Sicily, recorded a neutrino with an estimated energy of approximately 220 quadrillion electronvolts (220 PeV for short) – a value that has never before been seen in neutrinos. The researchers from the KM3NeT collaboration have now reported on the unusual discovery from the Mediterranean and its significance in the journal Nature. Researchers from the Erlangen Center for Astroparticle Physics (ECAP) at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) played a leading role in designing and constructing the experiment and in analyzing the relevant data.

The event referred to as “KM3-230213A” describes how a single muon traversed the ARCA detector in February two years ago. This movement was recorded by more than one third of the active sensors. The flight path and enormous energy of this muon led the KM3NeT research team to the conclusion that the muon very probably originated from a cosmic neutrino. Neutrinos come in different “flavors” depending on their corresponding charged leptons or partner particles: electron, tau and muon. If neutrinos interact with matter, for example rock, they may undergo a reaction and transform into their counterpart. That is what happened two years ago near the detector.

Gaining a better understanding of cosmic events thanks to neutrinos

When events such as radiation bursts from supermassive black holes, supernova explosions or bursts of gamma rays happen in the Universe, these act as powerful accelerators for cosmic rays. When the rays interact with the surrounding material or cosmic microwave background radiation, this can produce neutrinos and high-energy photons. Like KM3-230213A, for example: The energy of the recorded neutrino was approximately 10,000 times greater than the energy of the particle beams at the most powerful particle accelerator on Earth, the Large Hadron Collider (LHC) of the European Organization for Nuclear Research (CERN) near Geneva. The neutrino itself has an infinitesimally small mass.

“We have known for approximately one hundred years now that particles are accelerated to extremely high energies in our Universe, but it is still largely unknown where these particles originate from and what causes them to accelerate in this way,” explains Prof. Dr. Claudio Kopper, head of the KM3NeT group at ECAP and holder of the Chair of Experimental Astroparticle Physics at FAU. “Neutrinos allow us to find the sources of these particles. We can also gain an understanding of the processes driving these cosmic particle accelerators, as neutrinos are barely absorbed at all and can therefore deliver very “direct” information from inside these astrophysical accelerators.”

Energy of the event puzzles researchers

KM3-230213A has been the only event of its kind until now, but the KM3NeT team now hopes to detect further similar occurrences. “The neutrino we found using KM3NeT is puzzling,” explains Prof. Kopper. “It has such a high energy, much higher than many of us had expected. As it is the only neutrino with these levels of energy that we have detected to date from this direction, it is currently difficult to come to a conclusion concerning its source. But that is exactly what makes it so interesting, as it is often while doing fundamental research that we make surprising discoveries that lead to new insights.”

Another reason for the researchers to be optimistic is that the discovery from February 13, 2023 was made using only one tenth of the planned detector configuration. The installation of the KM3NeT telescope is a lengthy process that is not yet completed. Further sensors will still be added in the coming years. Once KM3NeT has been installed completely, the deep sea observatory will cover more than one cubic kilometer.

The KM3NeT telescope

Work began on constructing the KM3NeT telescope in December 2015. FAU astroparticle physicists have taken a leading role in the project from the outset. In the event that has now been recorded, they were mainly responsible for visualizing and providing further data to the scientific community.
“It is great that we were able to make a discovery like this in the opening phase, showing us that we are on the right path with KM3NeT,” says Prof. Uli Katz from the Chair of Astroparticle Physics at ECAP, and one of the founding members of the KM3NeT project.

The telescope uses seawater both as a detection medium and to act as a shield against interference from particles from cosmic rays. Its optical high-tech sensor modules detect the Cherenkov light, a bluish glow that appears due to the movement of extremely rapid particles which are themselves created from the interaction of neutrinos in water.
KM3NeT consists of two components. The ARCA detector – ARCA stands for Astroparticle Research with Cosmics in the Abyss – is used to investigate high-energy neutrinos and their sources in the Universe. It is located at a depth of 3450 meters below the sea, roughly 80 kilometers off the coast of Portopalo di Capo Passero, Sicily. Its partner component, the ORCA detector – ORCA stands for Oscillation Research with Cosmics in the Abyss – is used for investigating the basic properties of neutrinos themselves. It is located at a depth of 2450 meters below the sea, roughly 40 kilometers off the coast of Toulon, France.

The KM3NeT collaboration currently involves more than 360 scientists, engineers, technicians and students from almost 70 institutions in more than 20 countries across the globe.