Astroparticle Physics

6.2 Neutrino Astronomy 99nos (or into so far undiscovered sterile neutrinos which arenot even subject to weak interactions). If this scenario werecorrect, one could have m νµ ≈ 10 meV and m ντ ≈ 50 meV.Recently the Sudbury Neutrino Observatory (SNO) hasconvincingly confirmed the oscillation picture. The SNO SNO experimentCherenkov detector installed at a depth of 2000 m undergroundin a nickel mine in Ontario, Canada, consists of a1000 ton heavy water target (D 2 O) contained in a 12 m diameteracrylic vessel. The interaction target is viewed by9600 phototubes. This central detector is immersed in a 30 mbarrel-shaped cavity containing 7000 tons of normal lightwater to suppress background reactions from cosmic raysor terrestrial radiation from radioisotopes in the surroundingrock or the mine dust. The SNO experiment can distinguishthe charged-current interaction (CC)charged currents(a) ν e + d → p + p + e − ,which can only be initiated by electron neutrinos, from theneutral-current reaction (NC)neutral currents(b) ν x + d → p + n + ν x ′ (x = e, µ, τ) ,where an incoming neutrino of any flavour interacts with adeuteron. The neutrons produced in this reaction are capturedby deuterons giving rise to the emission of 6.25 MeVphotons, which signal the NC interaction. While the ν e fluxas obtained by the CC reaction is only 1/3 of the predictedsolar neutrino flux, the total neutrino flux measured by theNC reaction is in agreement with the expectation of solarmodels, thereby providing evidence for a non-ν e component.This result solves the long-standing neutrino problem. Itdoes not, however, resolve the underlying mechanism of theoscillation process. It is not at all clear, whether the ν e oscillateinto ν µ or ν τ . It is considered very likely that matteroscillations via the MSW effect in the Sun is the most likelymechanism for the transmutation of the solar electron neutrinosinto other neutrino flavors, which unfortunately cannotdirectly be measured in a light-water Cherenkov counter.The oscillation mechanism suggested by the differentsolar experiments (ν e → ν µ ) was confirmed at the end of2002 by the KamLAND 2 reactor neutrino detector, which KamLANDremoved all doubts about possible uncertainties of the standardsolar modelreactor experimentpredictions.2 KamLAND – Kamioka Liquid-scintillator Anti-Neutrino Detector