New ultra-high energy Neutrino

[Andrew Mason]

TL;DR

An ultra-high energy neutrino has been discovered, apparently, in a detector at the bottom of the Mediterranean Sea

The NY Times is reporting the discovery of a new ultra-high energy (UHE) neutrino. This is reporting on a paper published in Nature today:
https://www.nature.com/articles/d41586-025-00444-1

According to the article, the muon produced by the collision of a cosmic neutrino with a molecule in the Earth had an energy of 120 PeV or $1.2 \times 10^{17}$ eV. It appears that 350 physicists were consulted on this who agree that this was, in fact, a neutrino.

I am not up on my neutrino physics so I am not sure what kind of event could have produced such a neutrino, but I would be interested to know. The article speculates "possibilities range from giant black holes to stellar explosions called gamma-ray bursts". I was wondering how a neutrino would escape from a black hole but perhaps from the relativistic jets that are seen coming from these super-massive black holes as matter accretes into it.

Anyhow, it seemed like an event that should generate some interest here.

AM

 

[Orodruin]

TL;DR Summary: An ultra-high energy neutrino has been discovered, apparently, in a detector at the bottom of the Mediterranean Sea

It appears that 350 physicists were consulted on this who agree that this was, in fact, a neutrino.

350 people work in the KM3NeT collaboration. They have all contributed different things, but are not likely to all have independently checked the particular analysis.

There is some relevant discussion here regarding the fact that KM3NeT saw this event essentially right after turning on the detector whereas IceCube has not seen any events of such high energy over a decade of data taking or more. Even in the case of a transient point source, there is some amount of tension.

 

[ohwilleke]

I am not up on my neutrino physics so I am not sure what kind of event could have produced such a neutrino, but I would be interested to know. The article speculates "possibilities range from giant black holes to stellar explosions called gamma-ray bursts". I was wondering how a neutrino would escape from a black hole but perhaps from the relativistic jets that are seen coming from these super-massive black holes as matter accretes into it.

Thanks for noting it. There is a large literature on the source of UHE neutrinos, and of course, it is possible for there to be multiple kinds of processes that produce them, so it doesn't necessary have a single answer. But the source of high energy neutrinos is mostly an unsolved question in physics - some possible sources are known but which are involved in the highest energy ones and what their relative contributions to the total flux of high energy neutrinos is not known and is the subject on ongoing investigation.

O. Deligny, "Various constraints on BSM physics from extensive air showers and from ultra-high energy gamma-ray and neutrino searches" arXiv:2501.19322 (January 31, 2025), recently commented on possible new physics explanations (particularly for high energy neutrinos that go through the Earth before hitting a detector, two of which are seemingly anomalous). But, there are plenty of Standard Model vanilla explanations as well.

The IceCube collaboration (with neutrino telescopes near the South Pole in Antarctica) has led the charge in this area for over ten years and the Mediterranean collaboration that produced the new observation is a new comer with an apparatus that hasn't even been fully built out to its ultimate projected size yet.

As the link explains:

Interactions between cosmic rays—high-energy protons and heavier nuclei, also produced in our galaxy–and galactic gas and dust inevitably produce both gamma rays and neutrinos. Given the observation of gamma rays from the galactic plane, the Milky Way was expected to be a source of high-energy neutrinos.

“A neutrino counterpart has now been measured, thus confirming what we know about our galaxy and cosmic ray sources,” says Steve Sclafani, a physics PhD student at Drexel University, IceCube member, and co-lead analyzer.

The search focused on the southern sky, where the bulk of neutrino emission from the galactic plane is expected near the center of our galaxy. However, until now, the background of muons and neutrinos produced by cosmic-ray interactions with the Earth’s atmosphere posed significant challenges.

To overcome them, IceCube collaborators at Drexel University developed analyses that select for “cascade” events, or neutrino interactions in the ice that result in roughly spherical showers of light. Because the deposited energy from cascade events starts within the instrumented volume, contamination of atmospheric muons and neutrinos is reduced. Ultimately, the higher purity of the cascade events gave a better sensitivity to astrophysical neutrinos from the southern sky.

The link also cites to: “Observation of high-energy neutrinos from the Galactic plane,” The IceCube Collaboration: R. Abbasi et al., Science 380, 6652 (2023), DOI:10.1126/science.adc9818, arXiv:2307.04427

"Binary neutron star (BNS) mergers can be sources of ultrahigh-energy (UHE) cosmic rays and potential emitters of UHE neutrinos." See https://arxiv.org/abs/2406.19440 (published in a peer reviewed journal in 2024).

Efforts to pin down particular point sources have been challenging but are an area of a great deal of research. See, e.g., https://arxiv.org/abs/2205.15985 As of 2022, IceCube had identified 12 likely sources. “Searches for Neutrinos from LHAASO ultra-high-energy γ-ray sources using the IceCube Neutrino Observatory,” IceCube Collaboration: R. Abbasi et al., The Astrophysical Journal Letters 945 (2023) 1, L, iopscience.iop.org, arxiv.org/abs/2211.14184

A good survey of the field, as of 2022, can be found at https://arxiv.org/abs/2203.08096