K2K experiment
Encyclopedia
The K2K experiment was a neutrino experiment that ran from June 1999 to November 2004. It used muon neutrino
s 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.
K2K is a neutrino experiment which directed a beam of muon neutrino
s 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 neutrino
s, namely into electron neutrino
s 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 pion
s. These pions were then focused into a 200 m decay pipe, where they would decay
into muon
s and muon neutrino
s. 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.
) 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 (Δm)2 = . This result is in good agreement with the previous Super-Kamiokande
result, and the later MINOS
result.
Muon neutrino
The muon neutrino is a subatomic lepton elementary particle which has the symbol and no net electric charge. Together with the muon it forms the second generation of leptons, hence its name muon neutrino. It was first hypothesized in the early 1940s by several people, and was discovered in 1962 by...
s from a well-controlled and well-understood beam
Particle beam
A particle beam is a stream of charged or neutral particles which may be directed by magnets and focused by electrostatic lenses, although they may also be self-focusing ....
to verify the oscillations
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...
previously observed by Super-Kamiokande
Super-Kamiokande
Super-Kamiokande is a neutrino observatory which is under Mount Kamioka near the city of Hida, Gifu Prefecture, Japan...
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
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
. The experiment found oscillation parameters which were consistent with those measured by Super-Kamiokande.
Experimental design
Muon neutrino
The muon neutrino is a subatomic lepton elementary particle which has the symbol and no net electric charge. Together with the muon it forms the second generation of leptons, hence its name muon neutrino. It was first hypothesized in the early 1940s by several people, and was discovered in 1962 by...
s from the proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
synchrotron
Synchrotron
A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field and the electric field are carefully synchronised with the travelling particle beam. The proton synchrotron was originally conceived by Sir Marcus Oliphant...
at the KEK
KEK
, known as KEK, is a national organization whose purpose is to operate the largest particle physics laboratory in Japan, which is situated in Tsukuba of Ibaraki prefecture. Established in 1997. The term "KEK" is also used to refer to the laboratory itself, which employs approximately 900 employees...
, located in Tsukuba
Tsukuba, Ibaraki
is a city located in Ibaraki Prefecture, Japan. It is known as the location of the , a planned city developed in the 1960s.As of 2008, the city has an estimated population of 207,394 and a population density of 730 persons per km². Its total area is 284.07 km².Mount Tsukuba, particularly well-known...
, Ibaraki
Ibaraki Prefecture
is a prefecture of Japan, located in the Kantō region on the main island of Honshu. The capital is Mito.-History:Ibaraki Prefecture was previously known as Hitachi Province...
, to the Kamioka Observatory
Kamioka Observatory
The is a neutrino physics laboratory located underground in the Mozumi Mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida in Gifu Prefecture, Japan. A set of groundbreaking neutrino experiments have taken place at the observatory over the past two decades...
, located in Kamioka, Gifu
Gifu Prefecture
is a prefecture located in the Chūbu region of central Japan. Its capital is the city of Gifu.Located in the center of Japan, it has long played an important part as the crossroads of Japan, connecting the east to the west through such routes as the Nakasendō...
, about 250 km away. The muon neutrinos travelled through Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, which allowed them to oscillate
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...
(change) into other flavours
Flavour (particle physics)
In particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics, flavour is a global symmetry...
of neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s, namely into electron neutrino
Electron neutrino
The electron neutrino is a subatomic lepton elementary particle which has no net electric charge. Together with the electron it forms the first generation of leptons, hence its name electron neutrino...
s and tau neutrinos . K2K however, focused only on oscillations.
The proton beam from the synchrotron was directed onto an aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
target, and the resulting collisions produced a copious amount of pion
Pion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....
s. These pions were then focused into a 200 m decay pipe, where they would decay
Particle decay
Particle decay is the spontaneous process of one elementary particle transforming into other elementary particles. During this process, an elementary particle becomes a different particle with less mass and an intermediate particle such as W boson in muon decay. The intermediate particle then...
into muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
s and muon neutrino
Muon neutrino
The muon neutrino is a subatomic lepton elementary particle which has the symbol and no net electric charge. Together with the muon it forms the second generation of leptons, hence its name muon neutrino. It was first hypothesized in the early 1940s by several people, and was discovered in 1962 by...
s. 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
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
Cherenkov
Cherenkov detector
A Cherenkov detector is a particle detector using the mass-dependent threshold energy of Cherenkov radiation. This allows a discrimination between a lighter particle and a heavier particle ....
neutrino detector
Neutrino detector
A neutrino detector is a physics apparatus designed to study neutrinos. Because neutrinos are only weakly interacting with other particles of matter, neutrino detectors must be very large in order to detect a significant number of neutrinos. Neutrino detectors are often built underground to isolate...
("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
Super-Kamiokande
Super-Kamiokande is a neutrino observatory which is under Mount Kamioka near the city of Hida, Gifu Prefecture, Japan...
"far detector" located at the Kamioka Observatory
Kamioka Observatory
The is a neutrino physics laboratory located underground in the Mozumi Mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida in Gifu Prefecture, Japan. A set of groundbreaking neutrino experiments have taken place at the observatory over the past two decades...
, 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.
Collaboration
The K2K collaboration consisted of roughly 130 physicists from 27 universities and research institutes from all over the world, listed below. 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 σStandard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
) there had been a disappearance of muon neutrinos. Fitting the data under the oscillation hypothesis, the best fit
Curve fitting
Curve fitting is the process of constructing a curve, or mathematical function, that has the best fit to a series of data points, possibly subject to constraints. Curve fitting can involve either interpolation, where an exact fit to the data is required, or smoothing, in which a "smooth" function...
for the square of the mass difference between muon neutrinos and tau neutrinos was (Δm)2 = . This result is in good agreement with the previous Super-Kamiokande
Super-Kamiokande
Super-Kamiokande is a neutrino observatory which is under Mount Kamioka near the city of Hida, Gifu Prefecture, Japan...
result, and the later MINOS
MINOS
MINOS is a particle physics experiment designed to study the phenomena of neutrino oscillations, first discovered by a Super-Kamiokande experiment in 1998...
result.