The solar neutrino problem that lasted very long period in the latter half of the 20th century, has been solved rather quickly after entering the 21st century. Research of long baseline reactor neutrino oscillation has played a major role in the determination of the solution. The inverse beta decay reaction of hydrogen atom is essential in such research and consequently large and ultra-low radioactive liquid scintillator became indispensable for the experimental apparatus. Now the existence of the neutrino mass is established and a mechanism giving mass to neutrino is the next important issue. Ultra-low radioactive liquid scintillator apparatus is also playing an important role to investigate the mechanism. This note describes how research of long baseline reactor neutrino oscillation got to the solution of the solar neutrino problem and developing research there afterward.
An accelerator-based neutrino beam experiment with a baseline of several hundred kilometers is design to study neutrino oscillations discovered in atmospheric neutrinos. The K2K experiment in Japan is the first accelerator based long-baseline experiment and successfully confirmed atmospheric neutrino oscillations with an artificial neutrino beam. The T2K experiment has the higher power and higher quality of the muon neutrino beam, and finds the evidence of the third neutrino oscillation, appearance of electron neutrinos first in the world. We report the history, status and the future prospect of the accelerator neutrino oscillation experiments.