The IceCube DeepCore experiment has utilized eight years of atmospheric neutrino data to set the most stringent limits to date on the possible non-unitarity of the neutrino mixing matrix. While the Standard Model describes the mixing between flavor and mass eigenstates of active neutrinos using a 3x3 unitary matrix, the existence of additional heavy sterile neutrinos could introduce deviations, causing this matrix to become non-unitary. These deviations would manifest in neutrino oscillations, particularly through Earth matter effects, which contribute non-trivially in the presence of non-unitarity.

Researchers analyzed a high-purity dataset of muon neutrino charged-current (νμ CC) interactions collected by IceCube DeepCore. This detector, located in Antarctica, is particularly sensitive to non-unitary parameters appearing in the νμ → νμ channel due to the wide range of energies and propagation baselines of atmospheric neutrinos. The data analysis revealed no significant deviation from the standard unitary framework, reinforcing the validity of the current model.

The study has established the most restrictive limit to date for the non-unitary parameter α33, determining that α33 > -0.027 at a 90% confidence level. Other non-unitary parameters were also constrained at competitive levels. These results are crucial for particle physics, as they restrict the parameter space for models beyond the Standard Model that postulate the existence of sterile neutrinos or other exotic interactions. The absence of a non-unitarity signal suggests that, if sterile neutrinos exist, their coupling to active neutrinos is extremely weak.