New theory may solve 30-year neutrino mystery
A theoretical study suggests that missing neutrinos in gallium-based experiments may not indicate a new particle but could be due to a calculation oversight, potentially reaffirming the standard model of particle physics.
A long-standing mystery in particle physics may have a simpler explanation than previously thought. For over three decades, experiments using gallium-based detectors have reported about 20 per cent fewer neutrinos than expected, leading some to propose the existence of a new type of 'sterile' neutrino that interacts less with matter. However, a new theoretical study suggests this anomaly might stem from an oversight in the mathematical modelling of these experiments rather than a need to amend the standard model of particle physics.
The Gallium Anomaly Revisited
The issue, known as the 'gallium anomaly,' began in the 1990s when researchers observed fewer neutrinos than predicted in experiments involving gallium. When a neutrino strikes a gallium atom, it transforms into germanium and an electron. Scientists measure the interaction between germanium and the electron to count the neutrinos. Until now, calculations assumed that the quantum wave functions of the electron and neutrino did not vary across the nucleus of the transmuting atom.
Matteo Cadeddu at the University of Cagliari in Italy and his colleagues questioned this assumption. Their analysis found that accounting for variations in these wave functions could explain the 20 per cent deficit without invoking a new particle. 'We do not need to add a new particle or a new interaction,' says Cadeddu. The team’s findings were published in Physical Review Letters (DOI: 10.1103/5pvf-mcr1).
Expert Reactions and Future Steps
Joachim Kopp at Johannes Gutenberg University of Mainz in Germany calls the study an 'interesting lead that deserves further study.' He notes that extending the standard model to fit the anomaly requires 'really bizarre and fine-tuned theoretical models,' making the new analysis more promising. However, he stresses that independent measurements or calculations of the nuclear structure of gallium and germanium are needed to fully resolve the mystery.
Ante Ravlić at Michigan State University agrees, describing the work as a step in the right direction. 'We have to wait for more sophisticated calculations based on microscopic nuclear models,' he says. The study demonstrates the importance of rigorously treating the electron and neutrino wave functions in resolving the anomaly.
Ravlić also highlights the broader significance of neutrinos, which are crucial for understanding astrophysical processes like supernovae. Cadeddu and his team are already collaborating with nuclear structure specialists on follow-up studies. The new work may also improve future neutrino experiments by helping researchers look for different types of neutrinos more precisely.
