K2K experiment

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.

Experimental design

K2K is a neutrino experiment which directed a beam of muon neutrinos (
ν
μ) from the 6991192261178439999♠12 GeV proton synchrotron at the KEK, located in Tsukuba, Ibaraki, to the Kamioka Observatory, located in Kamioka, Gifu, about 250 km away.^[3] The muon neutrinos travelled through Earth, which allowed them to oscillate (change) into other flavours of neutrinos, namely into electron neutrinos (
ν
e) and tau neutrinos (
ν
τ). K2K however, focused only on
ν
μ →
ν
τ oscillations.^[4]

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 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.^[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.

Results

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 = 6997280000000000000♠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]

See also

• T2K experiment – the successor of the K2K experiment

References

External links

• K2K official website
• K2K publications