THIS is the story of a particle that has refused to die. For 50 years, it has haunted particle physics, with hints of its presence appearing in maddeningly ambiguous ways. Some believe they have seen it. Others think it is a figment of our imagination. But every time we think it is definitely not there, a sudden gust of wind knocks over the furniture and once more there is room for doubt.
Elusive though it is, the sterile neutrino would be a real boon. It could make sense of experimental anomalies stretching back decades, and give us the first confirmed glimpse of physics beyond what we know. It could even explain the strangely low masses of the ordinary neutrinos, as well as offering a convincing candidate for dark matter.
The bad news is that the latest round of experiments set up to look for it claim that it can’t possibly be there. Some physicists think that’s the end of the story. Others still believe in a mystery particle, but conclude it is nothing like we imagined. However, for some, a combination of experimental uncertainties and flawed analyses means that maybe – just maybe – the sterile neutrino could be alive and kicking after all. The problem starts with the neutrino bit. Neutrinos are notoriously hard to pin down. They have no charge, almost no mass and can pass through matter by the millions without leaving a trace.
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The first inkling we had that they were out there came in 1930, when physicist Wolfgang Pauli was struggling to make sense of the decay of certain radioactive atoms. Atoms consist of a nucleus made out of protons and neutrons, orbited by much lighter electrons. When an atomic nucleus undergoes beta decay, it morphs into a daughter nucleus, emitting an electron in the process. The problem perplexing Pauli was that the two decay products went in directions that seemed to violate the cast-iron law of conservation of momentum – suggesting some third particle was needed to make sense of the results.
The tiny, chargeless particle Pauli dreamed up was subsequently called the neutrino, and was spotted in the wild in 1956. Over the following decades, experimentalists gradually worked out that neutrinos came in multiple types, or flavours. There were neutrinos that Pauli had predicted, with an intimate connection to the electron, but also those with similar relationships to the electron’s heavier cousins: the muon and the tau. These three neutrinos neatly slotted into the standard model, the grand blueprint of particle physics, and that appeared to be that.
But the neutrinos were only getting started. As it happens, we have a massive source of neutrinos right next door, in cosmic terms. The sun is a nuclear fusion reactor, powered by beta decays that pump out electron neutrinos by the billion – some of which come our way, passing straight through Earth on their journey out into the wider universe. As long ago as the 1960s, however, physicists measuring the quantity of electron neutrinos reaching Earth found a major shortfall, with one experiment detecting only 25 per cent of the expected number.
Solving the mystery involved conceding that the standard model of particle physics, which had guided the field for decades, was incomplete. Rather than being massless, each neutrino did in fact have a tiny amount of mass, no more than a millionth that of an electron. This mass gives neutrinos a remarkable ability to switch between flavours, morphing from one into another as they zoom along, in a process called neutrino oscillation. That meant electron neutrinos produced in the sun’s core could transform into either muon or tau neutrinos, evading our searches on Earth. “If we had not realised this could happen, then we would never have been able to figure out what was going on,” says Janet Conrad, an experimental physicist at the Massachusetts Institute of Technology.
Granting neutrinos the ability to transform into one another solved the solar neutrino problem, but other mysteries remained. The most vexing of these dates from the 1990s, when an experiment called the Liquid Scintillator Neutrino Detector (LSND) in Los Alamos, New Mexico, found that the antimatter versions of muon neutrinos were oscillating into electron antineutrinos faster than expected. The result was confirmed by a second experiment, MiniBooNE, which ran at Fermilab in Batavia, Illinois, from 2002 to 2017.
Either the experiments were both plagued by the same design flaw, or something deeper was going on. “The LSND/MiniBooNE anomaly is a genuine mystery,” says Raymond Volkas at the University of Melbourne.
Another set of confounding results comes from radioactive decays within nuclear reactors. Much like within the sun, these decays are a strong source of neutrinos. For some unexplained reason, though, nuclear reactors on Earth appear to produce about 6 per cent fewer electron antineutrinos than the standard model predicts.
So: too many electron antineutrinos being produced on the one hand, and not enough on the other. What on earth is going on?
That’s where the sterile neutrino comes in. Instead of the neat trio of neutrinos, the idea is to invent a fourth, “sterile” flavour of neutrino capable of shape-shifting into any of the other three. This seems like a big deal given that the notion of three “generations” is deeply baked into the standard model (see chart). But with the reasons for this magic number still a mystery, inventing a whole new generation of particles, or even assuming the sterile neutrino is a lone misfit, might be no more puzzling.
Unlike its siblings, though, which interact via the weak nuclear force, the sterile neutrino would only feel the pull of gravity. This idea was proposed in 1958 by Italian nuclear physicist Bruno Pontecorvo, as a way to allow neutrinos to morph into antineutrinos. But the sterility of his proposed particle meant it would be almost impossible to detect. Interest rapidly waned, until the anomalies revived it.
The logic goes that MiniBooNE and LSND saw more electron neutrinos than predicted because of all the extra sterile neutrinos that decayed into this kind. And the reason the reactor experiments saw all those electron neutrinos go missing was because they had decayed into their sterile counterpart. Genius!
Summoning the sterile neutrino into existence could exorcise a number of other problems as well. Neutrino masses are tiny, over a million times smaller than for the next lightest particle, the electron. The reason for this gap is unknown, but a heavier fourth neutrino could offer a solution. Via a process known as the seesaw mechanism, its increased mass would drive the masses of the others down, like toddlers going up and down on a seesaw.
The same particles have also been proposed as a plausible candidate for dark matter, the mysterious stuff known to make up 27 per cent of the universe. Others say sterile neutrinos could also solve the question of why antimatter, produced in equal quantities to matter at the start of the universe, has now all but vanished. If so, “three puzzles of modern physics would find their explanation within one theory,” says Oleg Ruchayskiy at the University of Copenhagen in Denmark.
Alas, nothing involving the neutrino is ever simple.
For each experiment suggesting hints of a sterile neutrino, there is another offering evidence that it does not exist. The most emphatic is the universe itself – the present-day arrangement of stars and galaxies, and the faint surviving echoes of the big bang. If sterile neutrinos had existed throughout the universe’s history, says Volkas, then their presence would have caused the cosmos to look different than it currently does.
What’s more, the reactor anomaly has recently been called into question. Several experiments now under way have put neutrino detectors closer to the nuclear reactors than before, allowing for a more accurate measurement of electron antineutrino disappearance. Although the statistical analysis of the data is not yet sufficiently advanced to say anything for certain, preliminary results suggest the anomaly might have disappeared.
MiniBooNE’s latest results, released earlier this year, muddied the waters still further: they showed a larger anomaly than before. Some hailed this as strong evidence for the existence of this hypothetical particle. But if a sterile neutrino exists, then muon neutrinos shouldn’t just transform into electron neutrinos on their way to a detector – some should disappear as well.
Yet muon neutrino disappearance has never been seen. “The recent strengthening of the MiniBooNE anomaly actually makes the situation worse, because the stronger appearance signal implies a stronger disappearance signal as well,” says Volkas.
During the 15 years MiniBooNE was ticking away, physicists set up a number of smaller experiments to probe for this disappearance explicitly. One of the biggest is called MINOS+, based at Fermilab. Over the 10 years it and its predecessor have been running, muon neutrinos have steadfastly refused to vanish. This places severe limits on what any potential sterile neutrino may look like, if it even exists.
For all the stakes hammered through the sterile neutrino’s heart, it still keeps lumbering on. Conrad says that experiments that conform to our expectations, like MINOS+, historically receive less scrutiny than those like LSND and MiniBooNE, which show signals we were not expecting. “Limits always get a lot less scrutiny than anomalies.”
Ghostbusters
This time, however, someone has stepped up to do the scrutinising. Bill Louis, a physicist at Fermilab, combed through the results from MINOS+ and how the team analysed them.
“The MINOS+ collaboration has worked very hard on the data analysis,” says Louis, who stresses the quality of the experiment and the complexity of the mathematics involved. That being said, he believes that errors in the data may have been overlooked, leading the team to rule out disappearances with greater certainty than is warranted.
The most recent paper from MINOS+ has been awaiting publication since October last year, held up because of issues including those raised by Louis, says Conrad. “Bill only touches on about half the obvious problems,” she says.
Karol Lang, spokesperson for the MINOS/MINOS+ collaboration, says the team has been in contact with Louis and others while auditing and dissecting its results to better explain the underlying trends, and the resulting constraints on the sterile neutrino.
So where do things stand? If the results from both MiniBooNE and MINOS+ survive further scrutiny, then a sterile neutrino on its own is just not going to cut it. We need something else to help explain why the same particle causes both muon and electron neutrinos to appear, but only electron neutrinos to vanish. Some models invent a new force that could resolve the discrepancy. Or perhaps, says Conrad, we don’t need to invent anything beyond the standard model. “It might be nuclear physics – we really don’t understand nuclear effects that well,” she says.
When experiments like MiniBooNE search for neutrino oscillations, the nature of the oscillation depends on the energy of the neutrinos observed at the detector. But neutrinos can interact with other particles on their way, changing their energy. We assume most neutrinos are interacting with free neutrons in atomic nuclei, but some neutrinos can hit a neutron-proton pair instead, and this changes the amount of energy they have. Conrad says we don’t fully understand this effect yet, so our analyses could make it look like there are oscillations when there aren’t. If this is the case, we might not need a sterile neutrino at all, just an improved calculation.
However, says Conrad, such effects could be working in the other direction, shielding a sterile neutrino signal that would otherwise be much stronger. This could be enough to rescue the sterile neutrino from its deathbed, she says.
While theorists develop ever more exotic ideas about what could be out there, experimentalists will keep on searching. Using detectors around the world, from Russia to the UK and even under the Antarctic ice, they hope to place ever tighter bounds on the disappearance of muon and electron neutrinos, as well as the rate at which one turns into the other.
Whether these experiments wind up discounting previous results or backing them up is yet to be seen, but it is an exciting time. Volkas believes current experiments could provide a definitive answer to these questions within the next few years. If Louis’ hunch is right, the zombie particle could rise from the grave once again.
The secret life of muons
Neutrinos have always been the black sheep of the particle family. They come in three “flavours”, each associated with heavier cousins called the electron, the muon and the tau. These heavier particles were long thought of as well-behaved, but now it seems the muon may have a dirty secret.
In 2001, an experiment at the Brookhaven National Lab in New York measured a quantum property of the particle known as its magnetic moment and found that it exceeded theoretical predictions. This could have been a statistical fluke, but we will have a better idea once more sensitive experiments start collecting data in February next year.
Meanwhile, experiments at the Large Hadron Collider near Geneva have also seen muons behaving oddly. Decays of particles known as B-mesons should produce muons in roughly the same quantities as electrons, but the detectors are spotting far fewer than expected.
This raises interesting parallels with the fact that muon neutrinos don’t disappear as often as electron neutrinos do (see main story), a major problem for neutrino physics. The two anomalies could be connected, says Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute for Advanced Studies in Germany. “You get the impression we are missing something here, but no one has a clear idea just what we are missing,” she says. “It’s as frustrating as exciting.”
This article appeared in print under the headline “Hunting the ghost particle”
