Physicists May Have Solved the 30-Year Mystery of Missing Neutrinos

A 30-Year Puzzle

For three decades, physicists have been puzzled by a persistent discrepancy in experiments involving neutrinos—the ghostly particles that barely interact with matter. The so-called 'gallium anomaly' first emerged in the 1990s when detectors using gallium, a soft silvery metal, consistently observed fewer neutrinos than theoretical predictions. This shortfall hinted at the possibility of a new type of particle, perhaps a sterile neutrino, which would have profound implications for our understanding of the universe. But a new study suggests that the anomaly may be explained by a more mundane cause: errors in the calculated reaction rates used to calibrate these experiments.

The Gallium Anomaly Explained

Gallium-based neutrino detectors work by exploiting a specific nuclear reaction: when a neutrino interacts with a gallium-71 nucleus, it can transform into germanium-71, emitting an electron. By measuring the rate of germanium production, scientists can infer the neutrino flux. However, since the 1990s, experiments like GALLEX and SAGE have reported a deficit of about 10-20% in the expected number of neutrinos. This deficit was initially attributed to the possible existence of sterile neutrinos—hypothetical particles that interact only via gravity and not through the weak nuclear force. If sterile neutrinos exist, they could oscillate with regular neutrinos, leading to a reduced detection rate.

But the new study, published by a team of physicists, offers a different explanation. They argue that the theoretical calculations for the neutrino capture rate on gallium, which are based on nuclear physics models, might be slightly off. By revising these calculations, the anomaly could disappear entirely, without the need for new physics.

Revisiting Nuclear Physics

The team focused on the so-called 'Bethe-Heitler' process, which describes how neutrinos interact with nuclei. In particular, they re-evaluated the cross-section—the probability of a neutrino interacting with a gallium nucleus—using more precise data from recent nuclear experiments. They found that the previously used cross-section values were overestimated, leading to an expected neutrino count that was too high. When they corrected these values, the predicted number of neutrinos matched the observed counts within experimental uncertainties.

This finding is significant because it suggests that the gallium anomaly may not be evidence for sterile neutrinos after all. Instead, it highlights the importance of accurate nuclear data in interpreting neutrino experiments. The team's calculations were based on a detailed analysis of the nuclear structure of gallium and germanium isotopes, using state-of-the-art theoretical models and experimental data from particle accelerators.

Implications for Neutrino Physics

If the new interpretation is correct, it would resolve a long-standing discrepancy and rule out one of the few hints for sterile neutrinos. Sterile neutrinos are a popular candidate for dark matter and could explain various anomalies in particle physics, but their existence remains unconfirmed. The new study does not definitively exclude sterile neutrinos, but it removes the gallium anomaly as evidence for them.

However, not all physicists are convinced. Some argue that the revised calculations may introduce new uncertainties, and that the anomaly could still be a sign of new physics. Others point out that other experiments, such as the MiniBooNE and LSND, have also reported anomalies that could be explained by sterile neutrinos. Nevertheless, the new study provides a plausible and testable alternative explanation.

Future Experiments

The debate is likely to continue, but future experiments may help settle it. For instance, the BEST (Baksan Experiment on Sterile Transitions) experiment, which uses a powerful artificial neutrino source, has already provided data that could be reanalyzed with the new calculations. Additionally, upcoming experiments like JUNO and Hyper-Kamiokande, which are designed to precisely measure neutrino properties, could offer further insights.

The new study also underscores the need for more precise nuclear data. As neutrino experiments become more sensitive, the accuracy of the underlying nuclear physics becomes increasingly critical. This work demonstrates that even seemingly well-understood processes may harbor subtle errors that can lead to major misinterpretations.

Conclusion

The 30-year mystery of missing neutrinos may finally be solved, not by discovering a new particle, but by refining our understanding of known physics. While the existence of sterile neutrinos remains an open question, this study shows that careful attention to nuclear details can resolve apparent anomalies. As the field moves forward, the interplay between nuclear physics and particle physics will continue to be a rich source of discovery.

This article is based on reporting by New Scientist. Read the original article.