Astrophysical bounds on the high-energy evolution of neutrino mixing

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.

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.

A plethora of long-range neutrino interactions probed by DUNE and T2HK

If there are new neutrino interactions with matter, and if they affect neutrinos of different flavor differently, then they could impact neutrino oscillations. Long-baseline neutrino experiments are well-suited to look for them, thanks to their use of intense, well-characterized neutrino beams.

If the new interactions have a long range—i.e., if they are mediated by a new, ultra-light mediator—then neutrinos on Earth may experience a matter potential sourced by the vast amount of faraway matter elsewhere inside the Earth, Moon, Sun, Milky Way, and in the cosmological matter distribution, as pointed out in 1808.02042 [Universe’s Worth of Electrons to Probe Long-Range Interactions of High-Energy Astrophysical Neutrinos, by MB & Sanjib Agarwalla, PRL 2019]. This boosts the chances of discovering the new interaction even if it is supremely feeble.

In a recent paper (2305.05184 [Flavor-dependent long-range neutrino interactions in DUNE & T2HK: alone they constrain, together they discover, by Masoom Singh, MB, and Sanjib Agarwalla, JHEP 2023]), we explored the prospects of constraining or discovering these new, long-range neutrino interactions in the upcoming long-baseline experiments DUNE and T2HK. We found promising prospects. However, we explored only three different possible forms of the interaction, introduced by gauging three of the accidental global lepton-number U(1) symmetries of the Standard Model.

In a new paper (2404.02775), led by PhD students Masoom Singh and Pragyanprasu Swain, we now extend this to many other symmetries—a plethora of them!—that introduce new neutrino interactions with electrons, neutrons, and protons. Each symmetry affects oscillations differently.

Our new results cement and extend our original findings: DUNE and T2HK should be able to probe the existence of new interactions—and possibly discover and distinguish between alternatives—regardless of which symmetry is responsible for inducing them. The reach of DUNE and T2HK to probe new neutrino interactions is not only deep, but also broad!

Mauricio Bustamante

Associate Professor – Niels Bohr Institute, University of Copenhagen

mixing

Astrophysical bounds on the high-energy evolution of neutrino mixing

Measuring neutrino mixing above 1 TeV with astrophysical neutrinos

A plethora of long-range neutrino interactions probed by DUNE and T2HK