Neutrinos have long been one of the most confusing and mysterious of all known subatomic particles — but they only got more confusing after two groups of scientists announced conflicting results.
Neutrinos are like ghostly chameleons
Neutrinos are often said to be the spirits of the physical world because they interact so little with matter, and easily pass through entire planets. But they’re also quantum chameleons, changing their identities, essentially as if a Chevrolet could turn into a Ford, then turn into a Chrysler, before turning back into the original car. The three different types of neutrinos are called the electron neutrino, muon neutrino, and tau neutrino.
This subatomic switcheroo, called neutrino oscillation, is unique in nature. And it has been observed in many different environments, including in neutrinos generated by nuclear reactors, from high-energy particle beams, and when cosmic rays from space enter Earth’s atmosphere.
A neutrino in the ointment
The leading theory of subatomic physics, known as the Standard Model, takes into account neutrino oscillation as well as a set of universal parameters that determine the rate at which the different types of neutrinos can change into each other. Neutrino experiments can then determine the true value of the parameters. Assuming the theory is correct and the experiments are accurate, all experiments should determine the same values for those parameters. Indeed, that is how it has turned out in many experiments.
In the 1990s, however, the Liquid Scintillator Neutrino Experiment (LSND), located at Los Alamos National Laboratory in New Mexico, found a fly in the ointment. LSND used a beam of muon neutrinos to look for instances where they had been transformed into electron neutrinos. As expected, the scientists observed electron neutrinos, but at a speed that was difficult to reconcile with the established theory.
It is interesting that they could reconcile their measurement with all other measurements if a fourth (as yet undiscovered) type of neutrino existed. This type of neutrino would be different from all other neutrinos. The known neutrinos interact via the weak nuclear force, while the fourth type would not. Because of this non-interaction, scientists have called this hypothetical fourth type of neutrino “sterile neutrinos.”
The existence or non-existence of sterile neutrinos has been the subject of fierce debate among particle physicists, with most experiments appearing to rule out the existence of a fourth neutrino, with a few supporting the conjecture. But there was always the LSND elephant in the room.
Confusion at the frontier of neutrino research
So, what’s the answer? There are two options. The first, and least exciting, is that the LSND result was just wrong. This does not imply incompetence of the researchers. Sometimes rare things happen and sometimes things are overlooked or mis-modelled. In what might be an epic understatement, science at the frontier is hard. On the other hand, if the disagreement between the predictions and the different experiments arises because the current theory is incomplete, then scientists will have to develop a new theory. This is of course more exciting, and sterile neutrinos are just the most popular possibility.
To determine whether the LSND result is the first harbinger of new physics, or just an unfortunate coincidence, it is necessary to replicate the experiment to see whether the second experiment confirms LSND or not. To this end, the MiniBooNE experiment at Fermilab in Illinois was designed and conducted. Like LSND, MiniBooNE also used a muon neutrino beam, looking for an inexplicable excess of electron neutrinos. In addition, MiniBooNE was a newer and more modern experiment, with improved capabilities. If the LSND sighting was correct, MiniBooNE would have confirmed it. However, that was not the case. MiniBooNE did not see the same surplus as LSND.
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Normally, this would be the end of the story, with the LSND reading being labeled a preliminary one that just turned out to be wrong. This reassuring conclusion would mean that the current theoretical framework was sufficient, and scientists could be satisfied that they understood neutrino interactions. But the history of neutrino research is full of surprises and rarely without mystery.
While MiniBooNE ruled out the LSND sighting, the new experiment discovered another anomaly. Because of the enhanced capabilities of the MiniBooNE experiment, they were able to search for neutrinos in more ways than LSND. When MiniBooNE searched for electron neutrinos in an energy range not explored by LSND, they found more electron neutrinos than the accepted theory could account for. And, like the LSND anomaly before it, the new MiniBooNE anomaly can be explained by the existence of a sterile neutrino (or possibly more than one). Although it would be different from the sterile neutrino proposed to explain the LSND abnormality, it was still sterile.
Since anomalies are often the first sign of discoveries, researchers couldn’t just give up. It was necessary that a third experiment, hopefully one that can definitively determine whether the current theory, with its three known types of neutrinos, is sufficient or not.
So the MicroBooNE experiment has been made. This experiment uses much more advanced detector technology and should give the final word on the MiniBooNE anomaly. In addition, there was one more loophole in the LSND/MiniBooNE comparison: they used different beams of neutrinos. Perhaps there were subtle differences in the beams that had not been properly considered. But in comparisons between MicroBooNE and MiniBooNE measurements, none of that would be. The two experiments used the exact same beamline, simplifying comparisons between the two experiments. MicroBooNE should be a final check on MiniBooNE.
An unsolved mystery
How is it all going? MicroBooNE does not detect the same deductible detect by MiniBooNE. However, the MicroBooNE paper does not conclude that MiniBooNE was wrong; it just says that the MiniBooNE anomaly remains unexplained† This is not surprising, as the two experiments are conducted by many of the same scientists.
To this day, the situation remains unsolved – an unsatisfactory state of affairs, to be sure, but this is often the case in groundbreaking scientific research, where the more confusing the mystery, the more interesting it is to researchers. And neutrinos are always confusing.
One possibility is that the MiniBooNE anomaly is not caused by sterile neutrinos, but by other possible forms of new physics, including dark matter† Only better and more accurate measurements, both underway at Fermilab, will unravel the mystery.