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The K2K experiment (KEK to Kamioka) was a neutrino experiment that ran from June 1999 to November 2004. It used muon neutrinos from a well-controlled and well-understood beam to verify the oscillations previously observed by Super-Kamiokande using atmospheric neutrinos. This was the first positive measurement of neutrino oscillations in which both the source and detector were fully under experimenters' control.〔 〕〔 〕 Previous experiments relied on neutrinos from the Sun or from cosmic sources. The experiment found oscillation parameters which were consistent with those measured by Super-Kamiokande. ==Experimental design== K2K is a neutrino experiment which directed a beam of muon neutrinos () from the proton synchrotron at the KEK, located in Tsukuba, Ibaraki, to the Kamioka Observatory, located in Kamioka, Gifu, about 250 km away.〔 〕 The muon neutrinos travelled through Earth, which allowed them to oscillate (change) into other flavours of neutrinos, namely into electron neutrinos () and tau neutrinos (). K2K however, focused only on oscillations.〔 The proton beam from the synchrotron was directed onto an aluminium target, and the resulting collisions produced a copious amount of pions. These pions were then focused into a 200 m decay pipe, where they would decay into muons and muon neutrinos.〔 The muons were stopped at the end of the pipe, leaving a beam of muon neutrinos. The exact composition of the beam contained over 97% muon neutrinos, with the other 3% being made of electron neutrinos (), electron antineutrinos () and muon antineutrinos ().〔 After they exited the pipe, the neutrinos went through a 1-kiloton water Cherenkov neutrino detector ("near detector") located at about 300 m from the aluminium target to determine the neutrino beam characteristics. This 1-kiloton "near detector" was a scaled-down version of the 50-kiloton Super-Kamiokande "far detector" located at the Kamioka Observatory, which allowed scientists to eliminate certain systematic uncertainties that would be present if two different detector types were used.〔 〕 This dual-detector configuration allowed the comparison of the neutrino beam at the near detector with the neutrino beam at the far detector to determine if neutrinos had oscillated or not.〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「K2K experiment」の詳細全文を読む スポンサード リンク
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