Hints of a ‘sterile’ neutrino

Scientists may have caught a glimpse of a new breed of particle from an unseen side of the universe.

Researchers conducting an exercise known as the Mini Booster Neutrino Experiment (MiniBooNE) at Fermilab near Chicago in the US have painstakingly compiled measurements of neutrinos over the last 15 years.

The experiment has yielded only the three types of neutrinos described in the Standard model: electron neutrinos, muon neutrinos and tau neutrinos. But now the scientists have published a paper in the journal Physical Review Letters, reporting a possible trace of a fourth.

Neutrinos are subatomic particles less than a million times lighter than electrons. They are one of the three components of matter, along with electrons and quarks, which make up the nuclei in atoms. Each component has two heavier counterparts, which decay after fractions of seconds: this array of particles in threes is known as the Standard Model.

A fourth particle that bucks the threefold pattern could be big news says MiniBooNE spokesman Rex Tayloe, from Indiana University in the United States.

“If that is the correct explanation of the signal, it is an important and far-reaching result as it opens up the field of particle physics to a new set of particles – beyond the current Standard Model,” he says.

Neutrinos are already the most mysterious particle in the Standard Model. They are preposterously numerous – 100 million neutrinos pass through the human body every second, barely interacting.

And because they interact so weakly, only a tiny number are ever detected. Their mass is still uncertain. It is so small that for a long time it was thought to be zero.

Unlike quarks and electrons, which decay from unstable, heavy forms into lighter, stable ones, neutrinos continually change form, slipping between the three forms as they as they torpedo through space at close to the speed of light.

It is this shape-changing that MiniBooNE has been studying, using a 541-metre beam of neutrinos. The scientists create them by smashing high-energy protons into a target of the metal beryllium, which creates unstable particles called pions that quickly decay, creating neutrinos.

The process creates a type called muon neutrinos, which are directed to MiniBooNE’s detector, a 12.2-metre sphere filled with 818 tonnes of pure mineral oil, lined with 1520 photomultipliers that catch tiny flashes of light caused by the occasional neutrino interaction.

The Standard Model predicts a small percentage of muon neutrinos will change into electron neutrinos in the half-kilometre flight. But MiniBooNE found more of these than expected.

One possible explanation for this rapid oscillation is a fourth neutrino form – but because it has never been detected it must not even interact in the incredibly weak way that the other three forms do. The scientists term it a sterile neutrino.

The hint of a new, invisible particle raises scientists’ hopes for a whole new family that could help solve puzzles of dark matter, dark energy and the imbalance of matter and antimatter in the universe.

But the isuue is far from resolved. While MiniBooNE’s result is line with an experiment in the nineties at Los Alamos in New Mexico in the US, other experiments have failed to confirm the same effect, which has physicists scratching their heads.

Solutions could be found by new larger experiments that are coming online, such as DUNE, which tracks neutrinos over a 1300-kilometre path under the US.

There is also the huge Japanese detector Hyper-Kamiokande, and a larger scale version of MiniBooNE. It’s possible the new data will overturn the sterile neutrino theory as a systematic error of some sort. But even if so, given their history, the mysterious particles are still likely to have some surprises in store.