Neutrinos may help us understand how matter prevailed over antimatter

Everything we can see in the Universe is made of matter, and we wouldn't exist without it. For physicists, this is actually a problem. From the perspective of the physics that describes the behavior of particles, matter and antimatter are equivalent. As far as the Standard Model of particles is concerned, there's no reason we shouldn't see equal amounts of matter and antimatter—or, more correctly, just the photons left behind after they meet and annihilate each other.

For there to be the sort of difference between the two that makes our Universe possible, something has to break the apparent symmetry between them (technically termed charge-conjugation and parity-reversal symmetry, or simply CP symmetry). And we have identified some cases of CP symmetry violations; they're just too small to account for all the matter in the Universe.

Now, nearly a decade of data from the world's leading neutrino observatory has found an indication that these particles display hints of a CP symmetry violation that's potentially much larger. While the data doesn't reach the level where physicists are willing to call it a discovery, it definitely warrants follow-ups as additional detectors come online.

Identity issues

Neutrinos are the lightest particles we're aware of, and they barely interact with other matter. So how can they be the key to matter's existence? The answer is that they aren't really the key to the process of producing excess matter so much as they may display behaviors that can help us identify how other particles produced matter's excess. To understand why, we have to make a diversion into the world of an idea called leptogenesis.

The "lepto" in leptogenesis comes from the term lepton, which refers to an entire class of particles. These include the familiar electron, along with its heavier relatives, the muon and tau (each of which has antimatter counterparts), along with three corresponding types of neutrinos and anti-neutrinos. Leptogenesis is a theoretical idea that describes the behavior of leptons immediately following the Big Bang, and it includes a CP parity violation that's large enough to produce the Universe's excess of matter. Unfortunately, leptogenesis is also entirely theoretical and involves physics that goes beyond the well-supported Standard Model.

While the physics behind leptogenesis can only produce a Universe-worth of matter during the Big Bang, that physics should still govern particle behavior under some circumstances today. And that's where neutrinos come in.

Among the very long list of bizarre behaviors displayed by neutrinos, these particles appear to be unique in that they don't have a fixed identity. Instead, every neutrino appears to be a quantum superposition of all three types of neutrino—electron, muon, and tau. When we measure a neutrino, we only detect a single type, or flavor; the neutrino will be detected as an electron neutrino or a muon neutrino. But if you take a population of neutrinos, you'll find that its members oscillate among the three flavors. So if you take a group of muon neutrinos, wait a bit, and then measure those neutrinos, you may find some electron neutrinos in the mix.

How do these flavor oscillations relate to leptogenesis? It turns out that the CP symmetry violation would make for a slight difference between the oscillation behavior of neutrinos vs. that of antineutrinos. To see this difference, we simply need to measure a lot of flavor oscillations.

Super-K

And that's precisely what the T2K project in Japan is designed to do. It uses the Japan Proton Accelerator Research Complex (J-PARC) to smash protons into a stationary target, producing a stream of unstable particles. When these particles decay, they release some muon neutrinos, which inherit some of the momentum and move along the same pathway, creating a beam of neutrinos. By selecting the charge of the particles that produce the neutrino beam, it's possible to ensure that the beam is composed of either muon neutrinos or muon anti-neutrinos.

A portion of this beam is checked by a detector at the J-PARC facility to confirm the beam's composition. The rest is sent through the Earth to the Super-Kamiokande neutrino detector about 300 kilometers away. (This requires no special tunneling, as neutrinos rarely interact with matter, so they largely pass through the Earth unimpeded.) The time involved in this travel provides the neutrinos a chance to oscillate; in fact, the T2K matches the distance to the detector to the energy of the neutrinos to maximize the chance of oscillation.

Super-Kamiokande is a massive tank of ultra-pure water surrounded by photodetectors. When neutrinos interact with any of the particles in the nucleus of the water's atoms, it produces a corresponding particle, either an electron or muon (the detector isn't sensitive to tau neutrinos). The light produced as these particles travel through the water is picked up by the surrounding detectors, which can distinguish between the patterns produced by electrons and muons.

By comparing the beam at the source and destination, the researchers can measure the frequency of oscillations for both neutrinos and anti-neutrinos.

Unfortunately, the issue of neutrinos rarely interacting with matter applies to the matter in the detector itself. As a result, you need to gather a lot of data to observe a significant number of oscillations. In this case, the researchers took nearly a decade of data, spanning the period from 2009 to 2018.

Uncertainty

If matters were simple, the researchers could just generate the rates of oscillation for neutrinos and anti-neutrinos and compare the two. The only source of differences between these values should be the influence of CP parity violations, so any difference should allow us to calculate the magnitude of the CP parity violation.

But physics is rarely that simple. Oscillation rates depend primarily on neutrino mass, and we don't have very precise mass measurements for these particles. Picking neutrinos up at the detector requires that they interact with one of the neutrons in the nucleus of an atom, a process that involves multiple particle interactions. As the researchers put it, "Modeling the strong nuclear force in multi-body problems at these energies is not computationally tractable," meaning they have to do approximations of that.

All of these issues create a degree of uncertainty about the values we should expect for the oscillation values involved here. Even if the researchers saw a difference, it's possible that the difference would be small enough that it could be explained by the uncertainty rather than a CP parity violation.

Fortunately, that doesn't seem to be the case here; in fact, the most likely value indicated by the experimental results is the maximum CP parity violation predicted by leptogenesis. The uncertainties, however, mean that the data supports the existence of any parity violation at the level of three standard deviations—well short of physics standard for discovery. Put differently, however, the results eliminate most of the possible cases where these results occurred without CP parity violations existing.

To get more definitive results, we'll simply need to keep gathering data. Fortunately, that's probably going to get a bit easier. Super-Kamiokande is likely to get an upgrade to a larger detector. And a US-based project will see a neutrino beam generated at Fermilab sent to a detector in a South Dakota mine, providing a similar experiment. While it will still take years to generate the data to narrow the uncertainties further, having multiple large detectors will both boost the rate at which the data is gathered and ensure that any results we do get aren't just a fluke caused by a particular hardware configuration.

Nature, 2020. DOI: 10.1038/s41586-020-2177-0  (About DOIs).