The renormalization-group running of neutrino mixing is undetectable

Do neutrinos mix differently depending on how they are made or detected?

Today, solar, reactor, accelerator, and atmospheric neutrino experiments measure neutrino mixing at energies from MeV to TeV. Global fits combine all of them into a single set of mixing angles and a CP phase. That this works rests on an assumption so basic that it is rarely stated: every experiment sees the same mixing.

In quantum field theory, the parameters of a theory, like the strength of a force or the mixing angles, depend on the energy scale at which we choose to describe them. This “renormalization-group running” is a standard and powerful tool. It connects theories defined at very high energies, like grand unified theories, to the values we measure in the lab.

In recent years, a growing number of papers, including some of our own, went a step further. They assumed that the running is directly observable. In this view, a neutrino made or detected in a collision carries the mixing evaluated at the momentum transfer of that collision, roughly, at how hard the collision is. This would lead to striking signals:

neutrinos changing flavor right at their source;
different values of the CP phase in different experiments;
unusual flavor ratios of the high-energy astrophysical neutrinos seen by IceCube.

In a new paper with Gabriela Barenboim (IFIC, University of Valencia) and Qinrui Liu (Simon Fraser University), we show that this premise is incorrect. The mixing is a property of the neutrino itself. It is fixed by quantum corrections to the neutrino as it travels freely from source to detector.

For the mixing to depend on the momentum transfer, the momentum transfer would have to enter those corrections. It does not: in the collisions that create and detect the neutrino, the momentum is exchanged through a W boson, not through the traveling neutrino. One can still choose to evaluate the mixing at the momentum transfer, as earlier works did. But then the corrections to the traveling neutrino, which those works left out, undo the running. Apart from tiny corrections at the collision points, every experiment, at any energy, measures the same mixing.

Neutrinos are made and detected in collisions with different momentum transfers (top). Earlier works assumed that the mixing changes with it (red). In fact, the traveling neutrino fixes the mixing, and it is the same in every experiment (blue).

The running itself is real, and it remains useful for testing theories at high energies. What cannot be seen is a dependence of the mixing on the momentum transfer of the experiment. Even setting the theory aside, old data rule this out. The NuTeV and NOMAD experiments looked for neutrinos changing flavor close to their source. They limit any change in the mixing angles between production in pion decay and detection at a few GeV to below 0.7°–2.4°.

This tells us where not to look. If future measurements deviate from expectations, for example of the flavor composition of high-energy astrophysical neutrinos, the cause will be genuinely new physics, not the running of the mixing.

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.

Discovering Majorons from the neutrinos of the next galactic supernova

In the hot, dense cores of core-collapse supernova, neutrinos could coalesce to make new, heavy particles, like Majoron-like bosons, with masses from tens of MeV to more than 100 MeV. After escaping the supernova, these Majorons would decay into energies comparable to the parent Majoron mass, far more energetic than the standard supernova neutrinos emitted from the neutrinosphere, and arriving at Earth later than them.

Thanks to large upcoming neutrino detectors, we might observe these high-energy neutrinos from the next galactic core-collapse supernova. Combining all available detection channels provides us with information on the energy, flavor, and arrival times of these neutrinos.

In a new paper, led by NBI PhD student Bernanda Telalovic, we have shown for the first time that we can combine this information use to clearly distinguish neutrinos from Majoron decay from the standard neutrinos of the next galactic supernova.

And, on top of that, we fold in the large uncertainty that exists in supernova physics by using two sophisticated supernova simulations (hot and cold) from the Garching group, obtained for two different assumptions for the mass of the proto-neutron star. However, our results do not hinge on us knowing what the real model, since we marginalize over it.

Should we detect no high-energy neutrinos, we will be able to place upper bounds on the Majoron coupling to neutrinos that are more than an order of magnitude stronger than the ones inferred from the observation of neutrinos from SN 1987A:

We show explicitly how the bounds are different depending on the flavor texture of the Majoron.

Conversely, should we detect high-energy neutrinos with late arrival times (tens of seconds post-bounce), we will be able to measure the mass and flavor-universal coupling of the Majoron:

Mauricio Bustamante

Associate Professor – Niels Bohr Institute, University of Copenhagen

theory

The renormalization-group running of neutrino mixing is undetectable

Wrap-up of the 2025 NBI Neutrino School

Discovering Majorons from the neutrinos of the next galactic supernova