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.[1][2] 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.
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.[3] 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 ( ν e), electron antineutrinos ( ν e) and muon antineutrinos ( ν μ).[4]
After they exited the pipe, the neutrinos went through a 1-kiloton
waterCherenkovneutrino 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.[5] 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.[6]
Collaboration
The K2K collaboration consisted of roughly 130 physicists from 27 universities and research institutes from all over the world, listed below.[7] The full list of scientists and their countries of origin is available on the
K2K website.
The final K2K results found that at 99.9985% confidence (4.3
σ) there had been a disappearance of muon neutrinos. Fitting the data under the oscillation hypothesis, the
best fit for the square of the mass difference between muon neutrinos and tau neutrinos was Δm2 = 2.8×10−3 eV2.[4] This result is in good agreement with the previous
Super-Kamiokande result,[8] and the later
MINOS result.[9]