Neutrinos are tiny, nearly massless particles that traverse long distances across the universe, interacting with matter only through the weak force. They come in three different types, or “flavors”—electron, muon, and tau—and during their journey through the atmosphere and the Earth can transform, or “oscillate,” from one flavor to another.
Over the last few decades, physicists have observed further anomalies in short baseline neutrino experiments that cannot be explained by the three-neutrino scheme but could be explained by the existence of “sterile” neutrinos. In this framework, the existence of a fourth, sterile neutrino can modify the oscillation of the Standard Model neutrinos.
Previous experiments have searched for sterile neutrinos by measuring their oscillations in vacuum-like conditions. But, in the presence of matter, neutrinos can oscillate differently, and they could undergo further modifications in the presence of sterile neutrinos.
In a study recently submitted to Physics Letters B, the IceCube Collaboration searched for a signature of sterile-neutrino-induced oscillations for muon neutrinos that traverse the Earth before arriving at the IceCube Neutrino Observatory at the South Pole. This is the first time an analysis used a joint three-parameter fit to the mass squared of the sterile mass state and its coupling to the muon neutrino and tau neutrino. The result is consistent with the nonsterile-neutrino hypothesis.
“In the past, our fits to the 3+1 model had set parameters that were known to be small to zero. This fits data from our previous analysis on its own, but leads to inconsistencies when comparing it to data from other experiments,” says Janet Conrad, a professor at the Massachusetts Institute of Technology (MIT). “Because of this tension, since we know that the 3+1 model is more complex, allowing these parameters to take on nonzero values is a first step in resolving the issues.”
“In reality, there is no reason to assume that the sterile neutrino would not also couple to the other neutrinos,” says Alejandro Diaz, who led the study while a PhD student at MIT. Diaz is now a postdoctoral research associate at the California Institute of Technology.
“In particular, coupling to the tau neutrino would have a non-negligible effect on the neutrino oscillations in matter and shouldn’t be ignored. This analysis incorporates this possible coupling into the sterile neutrino model, and we looked to see if this more accurate model yielded more information on the sterile neutrino question.”
Diaz and collaborators analyzed a little over seven years of data containing over 300,000 track-like IceCube events with reconstructed energies between 500 GeV and 10 TeV. They chose to look at upward-going tracks since only those neutrinos could have traversed a large amount of material (the Earth) and still reached IceCube to leave a signal.
The observed rate of upgoing muon tracks in the detector was used as a function of direction and energy and compared to the expected results with different sterile neutrino models. The team then produced a three-parameter fit over the sterile neutrino model and found the parameter point that would give a result closest to what they had observed.
The previous analysis found a preference for the sterile model over the nonsterile model with a confidence level of 90%. However, with the inclusion of the tau neutrino coupling, this preference has increased to 96%.
“While not indicative or showing proof of sterile neutrinos, this preference motivates continued exploration of the sterile neutrino and similar models,” says Diaz. “Few experiments are jointly sensitive to the combination of the three parameters we have fit for, but our results show the importance of this approach.”
+ info “Exploration of mass splitting and muon/tau mixing parameters for an eV-scale sterile neutrino with IceCube,” IceCube Collaboration: R. Abbasi et al. Submitted to Physics Letters B, arxiv.org/abs/2406.00905