The most energetic neutrino

 

Although IceCube is still the largest neutrino telescope around, it is not the only game in town.   The KM3NeT ARCA telescope is now large enough to study astrophysical neutrinos.  ARCA is located in the Mediterranean, of the coast of Nice, France.  It uses seawater as its optical medium, instead of ice; this has plusses and minuses.  

At the Neutrino 2024 conference in Milan, Joao Coelho (from APC - Paris) presented some initial KM3NeT results.  They have observed an enormously energetic event - far more energetic than any neutrino that IceCube has seen.  The event - seen above -  lit up a fair portion of their detector.  Although the collaboration is still working on a paper on the event - and so has been rather tight-lipped about the details, Joao did talk about some early work estimating its energy.  The Collaboration estimated the energy by comparing the observed event with simulated events of different energies.  

The plot below compares the number of hit photomultiplier tubes (PMTs) for the event with several simulations.  It is important to remember that the scale here is not linear; there are only a certain number of PMTs in the detector.  The deposited energy appears considerably higher 10 PeV - probably 20-40 PeV if one ignores the non-linearity, and higher with it.  This is for the muon energy; the neutrino energy will be higher to much higher, depending on how far away the neutrino interaction is from the active volume and how much energy the departing muon carried off.

The event appears to come from slightly below the horizon.  The collaboration has not discussed possible backgrounds.  The only obvious background in this energy range would be a very energetic muon bundle from a cosmic-ray air shower.   However, this would require a significant misreconstruction - large enough to seem unlikely. If one looks carefully (see the top figure), one can see evidence of stochastic (non-uniform) energy loss, as expected from high-energy muons.  

 Without knowing the energy better, it is hard to tell how compatible this event is with the IceCube limits on extremely-high energy (EHE) neutrinos, but there is at least some statistical tension.    Still, this observation is great news for all high-energy neutrino enthusiasts.