Anomalous experiment result suggests physicists may need to rethink the humble neutrino

Every now and then an experiment throws everything we think we know about the universe into a tailspin. It makes particle physicists cranky and excited all at once. It makes the non-physicists scratch their noggins as they wonder what all the fuss is about.

Just this year, there was hype around the mass measurement of the W boson which refused to adhere to our accepted theories. Could it have happened again?

A collaboration between the US-based Los Alamos National Laboratory and the Baksan Experiment on Sterile Transitions (BEST) experiment in Russia have reported an anomaly which may be suggestive of a new fundamental particle – a sterile neutrino. This theoretical particle is “sterile” because it would carry no charge and only interact through the gravitational force and none of the other fundamental forces of nature (electromagnetism, nuclear weak force and nuclear strong force).

The results of the experiment were published in Physical Review Letters and Physical Review C.

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BEST builds on the Soviet-American Gallium Experiment (SAGE) begun in the late 1980s, to which Los Alamos also contributed. Both SAGE and BEST experiments involve firing high-intensity neutrinos into the soft, silvery metal gallium.

Now known as the “gallium anomaly”, both experiments, and others, have shown a “discrepancy” between the predicted number of electron neutrinos (neutrinos with zero charge which do interact with the nuclear weak force) and the number observed in the gallium tank.

Over 2km underground, BEST is in the Baksan Neutrino Observatory in Russia’s Caucasus Mountains. The experiment used 26 irradiated disks of chromium-51 – a synthetic radioisotope of chromium and the 3.4 megacurie (measurement of radioactive intensity) source of electron neutrinos – to irradiate a tank of gallium.

A set of 26 irradiated disks of chromium-51 are the source of electron neutrinos that react with gallium and produce germanium-71 at rates which can be measured against predicted rates. Credit: A.A. Shikhin.

The reaction between electron neutrinos and gallium produces the isotope germanium-71. In line with previous experiments, germanium-71 production was 20-24% lower than expected based on theoretical modelling.

But the discovery of a new fundamental particle – namely sterile neutrinos – is only one possible explanation.

“The results are very exciting,” says Steve Elliott, lead analyst and member of Los Alamos’ physics division. “This definitely reaffirms the anomaly we’ve seen in previous experiments. But what this means is not obvious. There are now conflicting results about sterile neutrinos. If the results indicate fundamental nuclear or atomic physics are misunderstood, that would be very interesting, too.”

Martin White, physics professor at the University of Adelaide, tells Cosmos the anomaly “can come from the neutrinos oscillating into another sort of neutrino”.

“We have a model of neutrino production in the sun,” says White. “Anything that could be causing neutrinos to behave over these distances, you should see something in solar neutrinos.”

What else could the anomaly represent?

Maybe the theories are wrong. In particular, maybe our theoretical prediction of the neutrino cross section is incorrect.

In particle physics, “cross sections” are the probabilities associated with certain processes taking place when you fire one kind of object (eg a stream of photons or neutrinos) into another kind of object (usually a particle or atom).

“Here they looked at uncertainties on the cross section. What are the different calculations of the cross section? Do they affect the results?” asks White. “The answer is not really, you still see something dramatic, even if you change the cross-section calculation. But there could be some other uncertainty on something that you’re picking up in gallium.”

Whether or not the gallium experiments have picked up signs of sterile neutrinos, White believes they’re almost certainly there. “When we look at the table of particles in the Standard Model, we have these things called leptons (including the electron), which don’t feel the strong force, so they’re not quarks,” he says. “And we have an electron and an electron neutrino. But what that table hides is the fact that you can get particles of different-handedness.”

The “handedness” of particles, or chirality, refers to their orientation. Right-handed particles are the mirror image of left-handed particles, and can exhibit quite different behaviour.

“There’s a left-handed and right-handed electron. In the neutrino sector of the Standard Model, there are only left-handed neutrinos, and if they were right-handed neutrinos, they would be sterile,” explains White.

“The difference between a right-handed electron and a right-handed neutrino is the right-handed electron is electrically charged, so it interacts via electromagnetism. But the right-handed neutrino wouldn’t interact with anything,” White adds.

White says that the presence of sterile neutrinos could plug a number of holes in particle physics. They may, he says, be a component of dark matter – the mysterious invisible substance five times more plentiful than ordinary matter.

“There are lots of different visible particles, right?” he poses. “The dark sector of the universe are things that don’t really interact strongly with light. It could be equally complicated, right?”

Sterile neutrinos can also help explain neutrino masses, says White. But has the BEST experiment definitively shown evidence for these new particles?

“There’s enough there that we don’t understand that it’s an intriguing possibility. What’s great about this kind of gallium experiment is, compared to a hadron collider or something, it’s pretty cheap. I think there’s enough evidence here that it could be (evidence for new particles). But further theoretical and experimental challenges need to be solved,” says White.

White also believes that experiments like BEST are “a very rich source of potential discoveries.

“If you look at how long it takes to build a major collider, you’re talking 20 years minimum. I think that it’s a great advert for some of these small-scale experiments that are relatively low investments. They could be game-changing. The BEST experiment gallium anomaly is “clearly a result that’s been taken very seriously in the literature,” according to White. “It builds on previous experiments carried out in Russia with Los Alamos. There’s been a history of this, and I’m just intrigued to see where it goes next.”

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