Searching for neutrino oscillations in NOMAD
The main aim of NOMAD was to search for muon-neutrino to tau-neutrino oscillations by the decay of the tau. The tau particle typically travels less than a millimetre after the interaction so NOMAD was not able to see this short lived state. Instead it searched for the kinematic characteristics from the different decay modes of the tau. The following diagram illustrates the number of interactions we expected to see in NOMAD and the different tau decay modes that NOMAD can identify (86% of the branching fraction).
The most sensitive channel for NOMAD was the electronic channel, since the background from electron-neutrino interactions was the lowest. NOMAD used kinematic criteria in the transverse plane to the direction of the neutrino to recognize the tau particle. In a normal electron-neutrino interaction the spray of hadrons from the nucleus is in the opposite direction to the electron in the transverse plane. For a tau decay, since there are two extra neutrinos in the deacy that are not observed by NOMAD, the electron is not always back-to-back to the hadronic jet in the transverse plane.
Below, one can see two examples of interactions observed inside the NOMAD volume, one of them is the common muon-neutrino charged current interaction and the other is a relatively rare electron-antineutrino charged current interaction (only 0.3% of the neutrinos in the beam are of this type).
No tau candidates were found with the analyses carried out by NOMAD. The limit obtained from the first year of data-taking in 1995 was:
Alternatively, we performed a search for muon to electron-neutrino oscillations by searching for an excess in electron-neutrino events from what we would expect from our knowledge of the neutrino beam. Improvements in our understanding of the beam would come from the knowledge of the SPY results which would reduce the systematic error of this result. This result rules out a large portion of the parameter space claimed by the LSND experiment to be due to muon-antineutrino to electron-antineutrino oscillations.