Abstract
The largest tau neutrino dataset to date is IceCube's atmospheric tau neutrino appearance dataset containing $>1,000$ tau neutrino and antineutrino events as determined by a fit to a standard three-flavor oscillation framework. On an event-by-event basis, however, it is impossible to know that any given event is a tau neutrino as they are identical to either an electron neutrino charged-current event or a neutral-current interaction of any active flavor. Nonetheless, we conclusively show that, using only the cascade sample even without knowledge of the oscillation parameters and without assuming that the lepton mixing matrix is unitary, tau neutrino identification is still possible and there is no viable scenario in which all of the tau neutrino candidates are actually electron neutrinos. This is primarily due to the matter effect and the tau lepton production threshold, as well as the fact that tau neutrinos are systematically reconstructed at a lower energy than electron neutrinos due to one or more outgoing neutrinos. This conclusively shows that it is possible for an atmospheric neutrino oscillation experiment to confirm that $U_{\tau1}$, $U_{\tau2}$, and $U_{\tau3}$ are not all zero even with limited particle identification.
Highlights
The tau neutrino is the least-studied particle in the Standard Model (SM), making it a crucial target for additional study
We assume that neutrinos oscillate, neutrinos experience the matter effect [17], the large tau lepton mass gives rise to a threshold effect, and tau neutrinos deposit systematically less energy than an electron neutrino with the same energy due to decays to undetected neutrinos
While the analysis presented here is focused on atmospheric neutrinos at IceCube, the story is equivalent for SK as well as future atmospheric neutrino experiments such as HyperKamiokaNDE [18], KM3NeT/ORCA [19], DUNE [20], 1Tau neutrino appearance can be confirmed if unitarity is confirmed; see e.g. [15,16]
Summary
The tau neutrino is the least-studied particle in the Standard Model (SM), making it a crucial target for additional study. This naturally leads one to investigate if IceCube can identify the presence of tau neutrinos in their detector without assuming knowledge of the oscillation parameters That is, while it is known from long-baseline accelerator experiments that since θ23 ∼ 45° [8,9,10], if one does not assume the 3 × 3 lepton mixing matrix is unitary, on an event-by-event basis every tau neutrino would appear to be indistinguishable from an electron neutrino. We assume that neutrinos oscillate, neutrinos experience the matter effect [17], the large tau lepton mass gives rise to a threshold effect, and tau neutrinos deposit systematically less energy than an electron neutrino with the same energy due to decays to undetected neutrinos The combination of these effects provides enough information to confirm that tau neutrinos are detected in IceCube’s atmospheric data without any prior knowledge on the oscillation parameters.
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