Title: Observation of an ultra-high-energy cosmic neutrino with KM3NeT

Authors: The KM3NeT Collaboration

Status: Published in Nature [open access]

The most energetic neutrino ever

Neutrinos are an important cosmic messenger, which travel almost completely unimpeded from their sources to Earth – which is why they’re nicknamed the ghost particle! They’re understood to be produced from cosmic rays, which are energetic, charged particles produced in the universe’s most extreme environments, such as supernovae, potentially in active galactic nuclei (AGN), and more. Cosmic rays get easily deflected in magnetic fields on their way to Earth, making it near impossible to track them back to where they came from. We also expect electromagnetic radiation – in the form of gamma rays – to be produced alongside neutrinos and cosmic rays. Gamma rays are neutral and much easier to detect than neutrinos, whose ghost-like ability also makes it easy for them to pass through detectors without actually being detected.

Today’s authors report on a neutrino called KM3-230213A, the most energetic neutrino ever observed – with the new neutrino telescope KM3NeT (short for Cubic Kilometre Neutrino Telescope), while it was still under construction! This new neutrino has an energy of a whopping 120 PeV, which is over a hundred thousand times the particles accelerated in the Large Hadron Collider. 

KM3NeT is made up of two detectors, each consisting of over a hundred detection lines deep in the Mediterranean sea. These detection lines are made up of optical modules, which look for tracks of light left by the muons produced in neutrino interactions in water. These tracks point back to where the original neutrino came from and the size and brightness of the tracks give  us information about the neutrino’s energy. Figure 1 illustrates how the exceptional KM3-230213A looked as it travelled through KM3NeT’s detectors. 

An out-of-this-world event!

Many of the events (individual particle tracks) seen by KM3NeT are “noise” events, which are really just muons produced in our atmosphere by cosmic rays, which masquerade as neutrino signals. It looks like KM3-230213A is the real astrophysical deal though and really does come from outer space. The authors conclude this because the neutrino’s energy is much higher than the energy of atmospheric muons, and we can trace back to KM3-230213A to see that its path crossed too much water and atmosphere for a muon to survive (since they lose more energy per unit distance than a neutrino). 
Where did it come from then? The authors look around the reconstructed direction of KM3-230213A to find that there’s a few known objects there (see Figure 2), but nothing that obviously could’ve made such a high energy neutrino. We’d expect to see some sort of gamma-ray counterpart at around TeV energies to be produced alongside the neutrino, which could be a known object, like a blazar (an AGN with its jet pointed toward us) or a transient source, like a gamma-ray burst. They don’t find any compelling sources around the neutrino location, although they can’t completely rule out that maybe one of these sources in the neutrino’s neighbourhood could’ve somehow spat out the most energetic neutrino ever seen.

Origins: untraceable (probably)

The most reasonable explanation (so far) for the neutrino’s origin seems to be that it’s a cosmogenic neutrino, which comes from the interactions of cosmic rays with the photon fields that exist within our universe, like the cosmic microwave background (CMB) or the extragalactic background light (EBL). This would mean that the neutrino doesn’t come from a source that makes any other observable particles or photons and therefore can’t be traced back to anything from known catalogs of astrophysical objects. This is a nice solution, but the problem is that we’ve never seen a cosmogenic neutrino before, which could either mean that this is an exciting first discovery or a far-fetched explanation for something we don’t have enough information to understand. 

Regardless of its origin, KM3-230213A is the highest energy neutrino ever detected (see Figure 3) and is indicative of a new population of ultra-high energy neutrinos that have never been seen before! This detection is even slightly in tension with IceCube, a huge neutrino experiment at the South Pole, which constrained that neutrinos at this energy and flux shouldn’t exist, as determined by almost a decade of observations. Though there’s a lot of uncertainty about KM3-230213A, it’s created quite a buzz in the astroparticle physics community, with origin stories anywhere from primordial black holes to ultraheavy dark matter. With many new neutrino detectors coming in the next decade, such as the finished KM3NeT, IceCube Gen2, and P-ONE, this will hopefully just be the first of many PeV-scale neutrinos to be discovered!

For further reading, KM3NeT has a webpage dedicated to KM3-230213A!

Astrobite edited by Cole Meldorf

Featured image adapted from: KM3NeT