Neutrino oscillation

Laboratory experiments searching for neutrino oscillation

Electron antineutrino disappearance experiments / Reactor neutrino experiments

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 experiments

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.

Experiment	Location	Source	Baseline	Observation	Status	Years
BEBC	Cern	SPS		no osc	finished	- 1986?
CCFR	Fermilab	Tevatron	0.9 km to 1.4 km	no osc	taking data (?)	1990? -
E776	BNL	AGS	1 km	no osc	finished	85-86
LSND	Los Alamos	LAMPF proton beam	30 m	excess of : 40 ± 9 : 18 ± 7	completed	1994-1998
Karmen	Rutherford	ISIS proton beam	18 m	no osc	data taking ended	1994-2001
Nomad	Cern	SPS	820 m	no osc	analyzing data	1995-1998
Minos	Soudan mine, MS	Main Injector at Fermilab	730 km		taking data	2003-
miniBooNE	Fermilab	Fermilab Booster	0.5 km/ 1 km		in progress	2003-
Icarus	Gran Sasso	Cern	732 km		under construction	about 2005
Cosmos	Fermilab	Main Injector	1 km		rejected

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 disappearance

The K2K-experiment is running again after being stalled for about 2 years due to an accident at SuperK.

Tau neutrino appearance experiments

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.

Explanations

• O = observed
• E = expected
• ev = events
• no osc = no oscillation i.e. a signal consistent with no mixing
• AGS = Alternating Gradient Synchrotron
• SPS = Super Proton Synchrotron

Constraints for the mixing