Following a successful 2026 Collaboration Meeting in Dunhuang, China, Zhang Yi (Purple Mountain Observatory) and I were elected as the co-spokespersons of the GRAND Collaboration!
All of the Collaboration is deeply grateful to our outgoing co-spokespersons, Kumiko Kotera, Olivier Martineau, and Xiangping Wu, for a decade of incredible dedication and leadership.
What’s next? GRANDProto300 is growing! As we commission our instrument, we look forward to our upcoming physics runs.
Today, the values of the neutrino mixing angles and mass differences that govern flavor transitions are known to quite high precision. However, we know them exclusively from sub-TeV terrestrial experiments. Because these experiments operate at low transferred momenta (typically, Q ~ few GeV), a fundamental question remains: are the mixing parameters universal constants, or do they evolve at high energies?
Quantum field theory dictates that these parameters should “run” with Q via renormalization group (RG) equations. While this running is negligible in the Standard Model, well-motivated new-physics frameworks, like the Standard Model Effective Field Theory (SMEFT), can accelerate this evolution. Detecting the high-energy running of neutrino mixing parameters would be a smoking-gun signature of new physics.
To access the high-Q regime, we turn to high-energy astrophysical neutrinos, with energies in the TeV–PeV and EeV ranges. They interact in neutrino telescopes via deep inelastic scattering and probe average momentum transfers of Q ~ 20-40 GeV, offering an unprecedented window into this high-Q evolution.
In a new paper with Qinrui Liu and Gabriela Barenboim, we evaluate the power of high-energy astrophysical neutrinos to test the evolution of neutrino mixing. We use the neutrino flavor composition at Earth, i.e., the relative proportions of electron, muon, and tau neutrinos in the diffuse flux. We execute a two-pronged analysis: first, extracting generic high-Q mixing parameters to remain model-agnostic, and second, constraining RG-inducing dimension-6 SMEFT coefficients.
We extract present bounds from the 11.4-year IceCube Medium Energy Starting Events (MESE) sample, published in 2025. Because the uncertainties in current flavor measurements are large, present data cannot yet meaningfully constrain the high-Q mixing parameters or SMEFT operators.
However, the future is promising. For our projections, we simulate the combined detection capabilities of existing (IceCube, Baikal-GVD, KM3NeT) and upcoming (P-ONE, IceCube-Gen2, NEON, TRIDENT, HUNT) optical-Cherenkov neutrino telescopes by the years 2040 and 2050, using both High-Energy Starting Events (HESE) and through-going muons. Crucially, we profile over the unknown astrophysical source flavor composition to ensure our bounds are robust and realistic.
Our results show that by 2040–2050, this global network will achieve the precision necessary to place unprecedented bounds on high-Q mixing (yielding particularly strong constraints in the 23 and 13 sectors). Furthermore, if the astrophysical neutrinos are produced via muon-damped pion decay, these telescopes will be capable of placing limits on individual SMEFT coefficients at a new-physics scale of 1 TeV, easily translatable to other energy scales.
We also forecasted the sensitivity of ultra-high-energy (UHE) radio arrays, such as the planned IceCube-Gen2 radio array. Counterintuitively, we found that despite UHE neutrinos possessing much higher energies, they yield drastically weaker constraints than their TeV–PeV counterparts. This is driven both by the logarithmic scaling of the RG evolution and by the vast experimental uncertainties inherent to radio-based flavor tagging.
This year’s April Fools’ Day arXiv paper haul was possibly the biggest one I have seen so far (as always, they are marked with a special symbol around April 1st on my daily arXiv picks). Thanks to John Beacom for pointing 2603.29912, which I missed on my first pass.
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 (δCP) 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 δCP). This establishes benchmarks for the minimum size that new-physics modifications to the mixing parameters must have in order to be detectable.
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.
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:
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.
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.
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.”
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.
Mauricio Bustamante 