Super-Kamiokande confirms tau neutrino appearance in atmospheric neutrino oscillations


Atmospheric neutrinos are generated by the interaction of cosmic rays with the atmosphere above us. In these interactions two types of neutrinos are created, muon and electron neutrinos, but the third type, tau neutrinos, are almost never created. In 1998 the Super-Kamiokande collaboration announced that while the number of downward-going muon neutrinos generated above the detector agrees with expectations, the number of upward-going muon neutrinos originating on the other side of the earth is half of the predicted number. This observation implies that neutrinos have tiny masses and that muon neutrinos oscillate, or spontaneously change, into either tau neutrinos or other neutrinos that are difficult to detect.

Super-Kamiokande has recently released a new result confirming that indeed muon neutrinos transform into tau neutrinos (see figure) and has published these findings in the journal Physical Review Letters. The tau neutrino is difficult to detect experimentally because not only is its interaction probability small, but its interaction product, the tau particle, quickly decays into lighter particles. Despite these challenges, Super-Kamiokande has successfully extracted tau particles from more than 10 years of data using sophisticated analysis algorithms. The observed number of tau neutrinos (tau particles) was found to be larger than the expectation based on muon neutrino oscillations by a factor of 1.42±0.35 (statistical error) + 0.14-0.12 (systematic error). The statistical significance of this detection is 99.98% or 3.8σ.

Super-Kamiokande will continue observations to improve the precision of these measurements and also aims to further study the interactions of tau neutrinos in the future.



Upward-going muon neutrinos originating on the opposite side of the earth change into tau neutrinos before reaching the detector.


(1) Published paper in PRL

(2) Paper submitted to the preprint server arXiv