Historically, the measurement of KamLAND, together with that of SNO, closed the solar neutrino problem, showing unambiguously that also reactor antineutrinos undergo the oscillation phenomenon, while concurrently determining rather precisely the associated mass squared difference parameter [DELTA][m.sup.2.sub.21] and, jointlywith the outputs of all other solar experiments, the mixing angle [[theta].sub.12].
(1) The Solar Neutrino Problem: An Apparent Deficit in the Solar Neutrino Flux.
The solar neutrino problem raised by the radiochemical experiments has been confirmed in 1991 by a real-time experiment based on the water (Cerenkov technique, Kamiokande, detecting the v - e elastic scattering .
The KamLAND experiment had a big impact since it permitted to discriminate the possible solution of the solar neutrino problem. Its results , in fact, ruled out the LOW solution which was still allowed by the solar neutrino data only (see Figure 7) and restricted the LMA region.