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.
K2K is a
neutrino experiment which directed a
beam of
muon neutrinos (
ν
μ) from the 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]
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]
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.
K2K is a
neutrino experiment which directed a
beam of
muon neutrinos (
ν
μ) from the 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]
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]