Probing Fundamental Physics

Neutrinos offer rare clues that a more complete theory of particles must supersede the 50-year-old set of equations known as the Standard Model. This model describes elementary particles and forces with near-perfect precision, but it errs when it comes to neutrinos: It predicts that the neutral particles are massless, but they aren’t — not quite.

Physicists discovered in 1998 that neutrinos can shape-shift between their three different types; an electron neutrino emitted by the sun can turn into a muon neutrino by the time it reaches Earth, for example. And in order to shape-shift, neutrinos must have mass — the oscillations only make sense if each neutrino species is a quantum mixture of three different (all very tiny) masses.

Dozens of experiments have allowed particle physicists to gradually build up a picture of the oscillation patterns of various neutrinos — solar, atmospheric, laboratory-made. But cosmic neutrinos originating from AGNs offer a look at the particles’ oscillatory behavior across vastly bigger distances and energies. This makes them “a very sensitive probe to physics that is beyond the Standard Model,” said Carlos Argüelles–Delgado, a neutrino physicist at Harvard University who is also part of the sprawling IceCube collaboration.

Cosmic neutrino sources are so far away that the neutrino oscillations should get blurred out — wherever astrophysicists look, they expect to see a constant fraction of each of the three neutrino types. Any fluctuation in these fractions would indicate that neutrino oscillation models need rethinking.

Another possibility is that cosmic neutrinos interact with dark matter as they travel, as predicted by many dark-sector models. These models propose that the universe’s invisible matter consists of multiple types of nonluminous particles. Interactions with these dark matter particles would scatter neutrinos with specific energies and create a gap in the spectrum of cosmic neutrinos that we see.

Or the quantum structure of space-time itself can drag on the neutrinos, slowing them down. A group based in Italy recently argued in Nature Astronomy that IceCube data shows hints of this happening, but other physicists have been skeptical of these claims.

Effects such as these would be minute, but intergalactic distances could magnify them to detectable levels. “That’s definitely something that’s worth exploring,” said Scholberg.

Already, Argüelles–Delgado and collaborators have used the diffuse background of cosmic neutrinos — rather than specific sources like NGC 1068 — to look for evidence of the quantum structure of space-time. As they reported in Nature Physics in October, they didn’t find anything, but their search was hampered by the difficulty of distinguishing the third variety of neutrino — tau — from an electron neutrino in the IceCube detector. What’s needed is “better particle identification,” said co-author Teppei Katori of King’s College London. Research is underway to disentangle the two types.

Katori says knowing specific locations and mechanisms of cosmic neutrino sources would offer a “big jump” in the sensitivity of these searches for new physics. The exact fraction of each neutrino type depends on the source model, and the most popular models, by chance, predict that equal numbers of the three neutrino species will arrive on Earth. But cosmic neutrinos are still so poorly understood that any observed imbalance in the fractions of the three types could be misinterpreted. The result could be a consequence of quantum gravity, dark matter or a broken neutrino oscillation model — or just the still-blurry physics of cosmic neutrino production. (However, some ratios would be a “smoking gun” signature of new physics, said Argüelles–Delgado.)

Ultimately, we need to detect many more cosmic neutrinos, Katori said. And it looks as though we will. IceCube is being upgraded and expanded to 10 cubic kilometers over the next few years, and in October, a neutrino detector under Lake Baikal in Siberia posted its first observation of cosmic neutrinos from TXS.

And deep in the Mediterranean, dozens of strings of neutrino detectors collectively called KM3NeT are being fastened on the seafloor by a robot submersible to offer a complementary view of the cosmic-neutrino sky. “The pressures are enormous; the sea is very unforgiving,” said Paschal Coyle, a director of research at the Marseille Particle Physics Center and the experiment’s spokesperson. But “we need more telescopes scrutinizing the sky and more shared observations, which is coming now.”