It needs more sigmas, but Fermilab boffins in America are carefully speculating that they may have seen evidence of a new fundamental particle: the sterile neutrino.
The suggestion follows tests conducted by the MiniBooNE (Mini Booster Neutrino Experiment) instrument, located near Chicago. Its mission is to detect neutrino mass through their oscillations. In the Standard Model of physics, neutrinos, like all particles, are initially assumed to be massless, but some observations, like neutrino oscillation, suggest there's mass there.
Simply put, neutrinos switch between three flavors: electron, muon, and tau neutrino types. MiniBooNE detected stronger than expected neutrino oscillations, which are now thought to be evidence of a fourth flavor: a sterile, or inert, neutrino. Sterile neutrinos are considered one among several candidates for dark matter, which is thought to account for around 80 per cent of the matter in the universe.
Particle physics is hard: the experiment that possibly detected sterile neutrinos collected 15 years of data from its commissioning in 2002, and the results have only now reached pre-press outlet arXiv.
And to the non-expert, the results look indeed tiny: over 15 years, MiniBooNE detected a few hundred more electron neutrinos than were predicted in Standard Model theory. The extra particles suggests there is a fourth, heavier flavor. The findings bring the MiniBooNE team tantalizingly close to a “result” – it's a 4.8 sigma result, when “discovery” demands 5 sigma.
What's going on?
The lack of interaction makes the sterile neutrino hard to detect – so much so that a single 1990s observation by the now-decommissioned Los Alamos National Lab's Liquid Scintillator Neutrino Detector (LANL LSND) has never been replicated.
The MiniBooNE experiment is straightforward: proton collisions (12.84 x 1020 protons, to be precise) emit neutrinos, and the instrument fired muon neutrinos at an oil tank. Some of those oscillated into electron neutrinos, so their interaction with the oil produce flashes that instruments can detect. The oscillation rate is predictable, so even a few hundred extra electron neutrinos are a result.
Physicist and blogger Sabine Hossenfelder explained the significance in this tweet thread, in which she noted:
The new data from MiniBooNE confirms that this tension in the data is real. This data can (to my best knowledge) NOT be fitted with the standard framework. It requires either new particles (sterile neutrinos) or some kind of symmetry violation.
She added: “Now it's time for theoretical physicists to come up with an explanation.”
The known neutrino flavors all interact via the electroweak force as well as gravity, which makes them identifiable by scintillators. The hints that a sterile flavor might exist arise because of neutrino oscillations – the little blighters like to flip between different flavors.
As Quanta Magazine explained, one possible explanation for the excess is that some of the muons oscillated into sterile neutrinos on the way, and then into electron neutrinos.
When the MiniBooNE experiment is combined with the LANL experiment, the crucial sigma level is reached – the significance is rated in the paper at 6.1σ. That, however, doesn't complete the picture: the tiny 1 electron-volt sterile neutrino discovered in this experiment wouldn't account for dark matter, and as the Quanta article added, cosmologists studying the Big Bang have only identified evidence for three neutrino flavours in light from the early universe. ®