J. Shirai: KamLAND
Junpei Shirai (see picture below, right) gave a review of the KamLAND experiment.
The technique of detecting electron antineutrinos from reactors has not changed since the times of Reines and Cowan first detection of a neutrino signal: one detects a prompt signal of the charged-current neutrino-proton interaction, followed by a delayed signal of neutron absorption. Reactors can provide intense fluxes of electron antineutrinos, in the 10^20 Hz range.
For the study of neutrino oscillations, one needs to study the flux as a function of the baseline – the distance between source and detector. No oscillation was observed in the 1990s for baselines up to a kilometer. KamLAND, with a LS mass of 1000 tons and a baseline of the order of 100km, could probe deeper to challenge the solar neutrino paradigm using reactor neutrinos, being exposed to fluxes of neutrinos from many different reactors, located from 50 to 1000 km from the detector. Located at 2700m of water equivalent depth, it suffered a cosmic ray flux five orders of magnitude smaller than at sea level.
The detector went online in November 2001. The results brought the first evidence of reactor anti-neutrino disappearance. All the solutions to the solar neutrino paradigm except for large-mixing-angle matter oscillation were excluded. The survival probability could also be shown to have an oscillatory pattern as a function of baseline. The solar neutrino problem was thus completely solved under the assumption of CPT invariance, and neutrino oscillation parameters were determined precisely; the DeltaM^2_12 parameter was determined with 2.5% accuracy.