The Baksan Neutrino Observatory in Russia, home to a gallium-based detector deep underground where researchers have seen fewer neutrinos than expected

Maxim Babenko/New York Times/Redux/eyevine

Some missing neutrinos in experiments have been puzzling physicists for more than three decades, but a new theoretical study suggests there may be no mystery after all. Rather than a hint of a new kind of particle, the unexpected result may come down to a calculation oversight, which would reaffirm the laws of particle physics instead of challenging them.

Neutrinos are ghostly, slippery particles that are hard to detect and can spontaneously oscillate between their three types. Since the 1990s, several experiments with gallium-based detectors have found about 20 per cent fewer neutrinos than expected, kickstarting ideas about some neutrinos having turned into a new type of “sterile” neutrino that interacts with matter less and so eludes detection. If this were the case, the standard model of particle physics, which tabulates all existing particles and forces, would have to be amended.

Matteo Cadeddu at the University of Cagliari in Italy and his colleagues now propose that such a drastic intervention may not be necessary. Their calculations show that the problem might lie in the mathematical modelling of the experiments.

It all started when Cadeddu was preparing to teach a course on neutrino physics. Anticipating students’ questions, he retraced all the steps of calculations that went into establishing this “gallium anomaly” and found a detail interesting enough to bring to the attention of several colleagues.

“It gradually became a bit of an obsession,” he says. “What began as an attempt to understand an old formula well enough to teach it ended up opening a completely different way of looking at the anomaly.”

In the experiments, when an atom of gallium is hit by a neutrino, it is transmuted into an atom of germanium and an electron. To count the neutrinos in the gallium-based detector, researchers measure the interaction between the germanium and the electron, then work backwards.

Until now, they have routinely assumed that the quantum wave functions of the electron and the neutrino, which mathematically encode their properties and behaviour, don’t vary across the nucleus of the transmuting atom. This was the detail that Cadeddu and his colleagues focused on.

To their surprise, they discovered that dropping this assumption could account for the reported 20 per cent neutrino deficit, given certain properties of the atom’s nucleus. “We do not need to add a new particle or a new interaction,” says Cadeddu.

“It is an interesting lead that deserves further study,” says Joachim Kopp at the Johannes Gutenberg University of Mainz in Germany. In his view, attempts to extend the standard model to fit the experiments require “really bizarre and fine-tuned theoretical models”, so the new analysis is more promising. But details of the nuclear structure of gallium and germanium would have to be independently measured or calculated to fully resolve the neutrino mystery, he says.

Ante Ravlić at Michigan State University says the new study is a step in the right direction. “We have to wait for more sophisticated calculations based on microscopic nuclear models. However, the work does demonstrate a clear impact of treating the electron and neutrino wave function more rigorously in the possible resolution of the anomaly.”

Ravlić expects to see quite a few follow-up studies from nuclear physicists, especially as neutrinos don’t just test the standard model, but are also important for astrophysical processes such as explosions of massive stars.

The team is already working on follow-up studies with colleagues who specialise in understanding atoms’ nuclear structure, says Caddedu. The new work may also inform future neutrino experiments, helping them look for signs of different neutrinos more precisely.