Laboratory experiments searching for neutrino oscillation
Electron antineutrino disappearance experiments / Reactor neutrino experimentsThese experiments are done close to powerful nuclear reactors. Each fission produces on average six electron antineutrinos, with a mean energy of a few MeV. A conventional uranium reactor produces a neutrino flux of 1.9 1020 /s /GW. Typically neutrinos are detected in liquid scintillators.
To measure the disappearance the neutrino flux must be known
very well. In practice, to get reliable results one uses two or more detectors at
different distances, one very close to the reactor. However,
this kind of experiments are not sensitive to small mixing angles,
despite a relatively large amount of events.
The results of Kamland after 766 ton-year exposure are consistent with the solar neutrino LMA-solution with delta m2 ≈ 8 10-5 eV2 and tan2t ≈ 0.40.
Kamland has also reported evidence of spectral distortion, which is a clear evidence for oscillation effects.
To see the reactor antineutrino background flux throughout the world calculated with the parameter values above click here.
Electron neutrino appearance experimentsMuon neutrinos are copiously produced in spallation reactions. Typically a proton with energy more than O(200 MeV) is shot to a fixed target, made of water or something else. The collision produces a large amount of pions, with all charges. For higher energies other mesons (e.g. K) are also produced.
A charged pion decays to a muon and a muon neutrino, with a lifetime 2.6 10-8 s. Positive pions typically decay at rest, since they are stopped in the matter. Negative pions, however, are quickly absorbed in the matter. Therefore, only decay in flight may be observable.
The muon subsequently decays to an electron, and two neutrinos, with a lifetime 2.2 10-6 s. The decay is practically isotropic.
The above decay scenario leads to muon neutrinos
with energy spectrum of some tens of MeV, and
electron neutrinos and muon antineutrinos
with smaller mean energy.
The appearance of high energy electron neutrinos
is therefore a signal of neutrino oscillation.
All experiments, except LSND, are consistent with no oscillation.
The results of LSND can be interpreted as a signal of oscillation
of muon neutrinos to electron neutrinos. Most of the parameter
range explaining the LSND results are in disagreement with
other experiments, particularly Karmen. However, there
still seems to be a small area allowed by all experiments.
Muon neutrino disappearanceK2K experiment (KEK to Kamioka, 250 km) has reported results of neutrino oscillation. If there were no oscillation, they would expect 150.9 (+11.6, -10.0) muon events inside the fiducial volume of SuperKamiokande. However, they have only see 108 muon neutrino events. The result is consistent with the results from atmospheric neutrino measurements. K2K has also reported the first significant evidence for the energy dependence of the oscillation effect.
The K2K-experiment is running
again after being stalled for about 2 years due to an accident at SuperK.
Tau neutrino appearance experimentsThese experiments use a high energy proton beam to produce muon neutrinos with high energy and high luminosity. As the neutrino beam does not contain any significant amount of tau neutrinos, an appearance of a charged tau lepton in a nearby detector is a signal of neutrino oscillation.
Taus cannot be detected in normal lepton detectors,
because of their short lifetime. One uses two different
techniques: Chorus has a very fine grained emulsion
of AgBr, in which the charged particles leave a track.
The tracks are identified by scanning the emulsion
with a microscope, nowadays done automatically.
Another way is to measure very accurately the
tracks of other leptons, and conclude the
existence of tau kinematically. This tehnique
is used e.g. in Nomad.
No signals of oscillation have been seen, so far.
Constraints for the mixingChooz:
[Fogli et al 98]
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