Measuring neutrino mixing above 1 TeV with astrophysical neutrinos

Today, the values of the neutrino mixing angles that govern flavor transitions are known to percent precision (the Dirac CP-violation phase is known much more poorly). However, these values are inferred exclusively from sub-TeV neutrino experiments. No measurement of the mixing parameters exists at the TeV scale and above. There, new-physics effects whose intensity grows with neutrino energy could modify the effective neutrino mixing. High-energy astrophysical neutrinos, with TeV-PeV energies, are primed for such measurements.

In a new paper with Qinrui Liu and Gabriela Barenboim, we have assessed in detail the power in these neutrinos to test mixing above 1 TeV, today and in the future. Concretely, we have extracted values of the four neutrino mixing angles (𝛉₁₂, 𝛉₂₃, 𝛉₁₃) and the CP-violation phase (δ_CP) from the flavor composition of high-energy astrophysical neutrinos, i.e., the proportion of electron, muon, and tau neutrinos in their diffuse flux.

We extract present bounds on the mixing parameters from the 11.4-year IceCube Medium Energy Starting Events (MESE) sample, published in 2025. We find that the uncertainty in the measurement is too large to claim meaningful sensitivity to the mixing parameter.

For our projections, we use multi-neutrino-telescope combinations using projected detection rates at existing (IceCube, Baikal-GVD, KM3NeT) and future (P-ONE, IceCube-Gen2, NEON, TRIDENT, HUNT) neutrino telescopes. For these, we combine High Energy Starting Events (HESE) and through-going muons. Our projections show clear sensitivity to 𝛉₂₃ and 𝛉₁₃ (and, if neutrino production occurs via muon-damped pion decay, to δ_CP). This establishes benchmarks for the minimum size that new-physics modifications to the mixing parameters must have in order to be detectable.

Read more at:

Measuring neutrino mixing above 1 TeV with astrophysical neutrinos
Mauricio Bustamante, Qinrui Liu, Gabriela Barenboim
2602.14308 hep-ph

GRAND prototypes paper

Our paper on the GRAND prototype arrays is finally out!

GRAND, the Giant Radio Array for Neutrino Detection, is an envisioned next-generation observatory for the detection of ultra-high-energy cosmic rays, gamma rays, and, especially, neutrinos.

For the past two years, three small-scale GRAND prototype arrays have been running to test the technology and detection principle of the experiment: GRAND@Nançay in France, GRAND@Auger in Argentina, and GRANDProto300 in China.

This paper was led by Beatriz de Errico, from the Federal University of Rio de Janeiro, and Shen Wang, from Purple Mountain Observatory.

ICRC 2025 Neutrino Rapporteur Talk

I recently gave the closing rapporteur track of the neutrino track at the 39th International Cosmic Ray Conference (ICRC 2025), held in Geneva during July 14-24, 2025. The goal was to summarize and highlight the contributions presented in the neutrino track throughout the conference, both in plenary and parallel talks.

It was plenty of work, but very rewarding. Here is my opening slide:

This is a slide summarizing the current state of the measurement of the TeV-PeV diffuse neutrino flux, including the new IceCube MESE measurement presented at the conference:

Finally, this was a good opportunity to look back to ten years ago, and to what I was thinking at the time, just a couple of years after the discovery of PeV neutrinos by IceCube:

You can find the full talk in the ICRC Indico page, or download it here, from this link.

TAMBO letter

TAMBO, the Tau Air-Shower Mountain-Based Observatory, is an envisioned detector targeting cosmic neutrinos in the 10-100 PeV energy range.

Within this energy range, kilometer-scale water-Cherenkov neutrino telescopes, like IceCube, KM3NeT, and Baikal-GVD, are too small to be sensitive to the falling flux of high-energy neutrinos. At the same time, within this energy range, neutrino energies are not high enough to efficiently trigger the emission of coherent radio signals that would allow them to be detected in envisioned neutrino radio telescopes.

Wrap-up of the 2025 NBI Neutrino School

The IV NBI PhD Summer School on Neutrinos has wrapped!

From July 7 to 11, 2025, we hosted 53 PhD, MSc, and BSc students from around the world. They received lectures on neutrino theory & phenomenology, neutrino astrophysics, and neutrino cosmology. All in all, we had 9 lectures, 5 topical seminars from NBI locals, and 24 student talks.

Here are a few photos (photo credit to co-organizer Markus Ahlers):

Like for the 2021, 2022, and 2023 schools, all the videos from the school are available on our YouTube channel.

Blurb on CERN Courier about KM3NeT ultra-high-energy neutrino

CERN Courier included a short quote of mine on their recent news bit about the observation by KM3NeT of the first ultra-high-energy neutrino: Cosmogenic candidate lights up KM3NeT.

To quote:

“Once KM3NeT and Baikal–GVD are fully constructed, we will have three large-scale neutrino telescopes of about the same size in operation around the world,” adds Mauricio Bustamante, theoretical astroparticle physicist at the Niels Bohr Institute of the University of Copenhagen. “This expanded network will monitor the full sky with nearly equal sensitivity in any direction, improving the chances of detecting new neutrino sources, including faint ones in new regions of the sky.”

KM3NeT discovery paper: Nature 638, 376 (2025) [open access]

Our PLEnuM paper about combining multiple neutrino telescopes:

Beyond first light: global monitoring for high-energy neutrino astronomy
Lisa Johanna Schumacher, Mauricio Bustamante, Matteo Agostini, Foteini Oikonomou, Elisa Resconi
2503.07549 astro-ph

No Flavor Anisotropy in the High-Energy Neutrino Sky Upholds Lorentz Invariance

Do neutrinos of different flavors have different preferred directions? If so, this would mean that Lorentz invariance is violated, something that is posited by some theories of quantum gravity. In them, Lorentz-invariance violation (LIV) would become more prominent the higher the energies involved.

Motivated by this, we look for signs of this flavor-dependent LIV using the high-energy astrophysical neutrinos seen by IceCube, with energies in the TeV-PeV range.

If LIV exists, the neutrinos would be affected by their interaction with a pervasive LIV field that couples differently to different neutrino flavors. As a result, the sky distributions of high-energy astrophysical electron, muon, and tau neutrinos arriving at Earth would be anisotropic.

In a new paper led by PhD student Bernanda Telalovic, we look for these high-energy neutrino flavor anisotropies in IceCube data, specifically, in the public 7.5-year sample of High-Energy Starting Events (HESE). We do this using the methods introduced in an earlier paper of ours (2310.15224).

We find no evidence for the patterns of flavor anisotropy expected from LIV, and so we place new upper limits on hundreds of parameters regulating Lorentz-invariance violation within the Standard Model Extension. We explore LIV operator dimensions from 2 to 8, each with a different dependence on neutrino energy and introducing different forms of flavor anisotropy.

For many of them, we improve upon existing limits—on account of using higher energies—or place limits for the first time ever:

Our new upper limits on the LIV parameters are available in digital form for download at 68%, 95%, and 99% C.L. GitHub, here.

Read more at:

No Flavor Anisotropy in the High-Energy Neutrino Sky Upholds Lorentz Invariance
Bernanda Telalovic, Mauricio Bustamante
2503.15468 astro-ph

Global monitoring for high-energy neutrino astronomy

Genuine high-energy neutrino astronomy needs many and varied astrophysical sources. But finding sources is hard, especially having only one km-scale neutrino telescope in operation. This is changing fast, though, thanks to the ongoing construction of KM3NeT and Baikal-GVD, but transformative progress will require us to think globally.

In a new paper, led by Lisa Schumacher, we show that, in the next 10-20 years, IceCube + Baikal-GVD, KM3NeT, IceCube-Gen2, P-ONE, TRIDENT, NEON, & HUNT, taken together in PLEnuM, could allow us to make global high-energy neutrino monitoring a reality. Together, they will increase the global rate of neutrino detection by up to 30 times and continuously monitor the entire sky.

To showcase this, we focus on one of the most prominent science cases in high-energy neutrino astronomy: finding steady-state sources. A combined analysis of global data will expedite source discovery—in some cases, by decades—and enable the detection of fainter sources anywhere in the sky, discovering up to tens of new neutrino sources.

This is seen, for example, in our forecasts for the evolution of the discovery potential of neutrino sources that have a soft spectrum (like NGC 1068), placed in different locations in the sky:

The PLEnuM tools used to obtain the results in our paper (plus more) are open-source and available on GitHub, here.

Read more at:

Beyond first light: global monitoring for high-energy neutrino astronomy
Lisa Johanna Schumacher, Mauricio Bustamante, Matteo Agostini, Foteini Oikonomou, Elisa Resconi
2503.07549 astro-ph

IV NBI Neutrino Summer School “Here, There & Everywhere”

Registration is now open for the fourth edition of our NBI Neutrino Summer School, “Here, There & Everywhere”.

Like in previous editions (2021, 2022, 2023), the school is aimed at PhD students and advanced MSc students from all over the world that work on neutrinos, are planning to work on them, or are simply interested in them! The school is a one-week series of lectures on neutrino phenomenology, astrophysics, and cosmology delivered by carefully chosen world experts on these subjects.

When? July 7-11, 2025

Where? Niels Bohr Institute, Copenhagen, Denmark

How to register? Go to our Indico page here

Registration deadline: March 31, 2025 (register early!)

Can you present a student talk? Accepted in-person participants are encouraged to submit an abstract

Is there a participation fee? Yes, there is a fee of 130 EUR for in-person participants

Questions? Find our contact information (Markus Ahlers, Mauricio Bustamante) on the Indico page

The school provides lunch (Monday to Friday), two coffee breaks a days, reception and a school dinner.

MITP Summer School 2024 Multi-Messenger Lecture Videos

The video recordings of my four-lecture mostly blackboard course on multi-messenger astrophysics at the 2024 MITP (Mainz Institute for Theoretical Physics) Summer School “Crosslinks of Early Universe Cosmology” are now on YouTube.

Part 1:

Part 2:

Part 3:

Part 4: