Atmospheric Neutrinos

Particle shower generated by a cosmic ray

Cosmic rays (protons or nuclei) interact in the upper atmosphere of the Earth. The resulting pions and kaons decay into muons and muon-neutrinos, many of the muons then decay into electrons and a muon-neutrino electron-neutrino pair. All these neutrinos are called atmospheric neutrinos and have been predicted and described in the early 60’s (Markov, Greisen, Reines, Zatsepin,…) [Mar60,Rei60,Gre60]. A crude estimation is that there are two times more muon-neutrinos than electron-neutrinos. In 1965, these atmospheric neutrinos were first detected by two experiments, installed in deep mines in India (Kolar Gold) [Ach65b] and in South Africa [Rei65b].

Interest on these neutrinos renewed in the early 80’s with the building of large underground detectors for the observation of the proton decay which was then predicted by the grand unified theories. Indeed atmospheric neutrino interactions were a background for the rare signal expected from the proton decay. At the end of the 80’s, the first results of the Cerenkov detectors Kamiokande [Hir88] and IMB [Cas91] were showing a small deficit of muon-neutrinos coming from the antipodes. On the contrary, fine resolution tracking detectors like Frejus [Ber90] or NUSEX [Agl89], found results in agreement with the predictions of atmospheric neutrino models [Bar88], though with a limited statistics. The deficit was not statistically convincing and the way to separate the muons and the electrons in Cerenkov detectors was seriously discussed. The muon-neutrino deficit could be interpreted in terms of neutrino oscillation, but such an announcement was clearly too premature (as soon as 1984, at the Neutrino’84 Conference [Kle84], there was a talk by John Learned entitled : « Search for oscillations with atmospheric neutrinos », but no written version…).

Inside SuperKamiokande

The debate has been solved by the SuperKamiokande experiment that started in 1996. After two years of data taking, SuperKamiokande announced in June 1998 the clear observation of a deficit of muon-neutrinos coming from the antipodes [Fuk98b]; this deficit has been almost immediately interpreted as an oscillation between muon-neutrino and tau-neutrino.

At the same time, the calorimetric MACRO [Amb98] and Soudan 2 [All97] experiments had observed the atmospheric neutrino anomaly, but with a statistical significance insufficient to claim that neutrinos would oscillate.

The oscillation between νμ and ντ has been confirmed by the K2K long-baseline experiment [Ali05].

Calculations of the atmospheric neutrino fluxes have been refined with time. A complete review is in [Gai02].

Further information

During the conference on the History of the Neutrino (Sept. 5-7, 2018 in Paris) the Atmospheric Neutrinos were reviewed by :


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