DONUT (Direct Observation of Nu-Tau) is the first experiment to directly observe interactions of the tau neutrino.
The figure shows how this is achieved:
Protons from the Fermilab Tevatron hit a high-density beam-dump target in which many secondaries were produced. Virtually all long-lived particles (pions and kaons) interacted before they could decay, only short-lived particles decayed. Of these, the D mesons had the highest abundance, and their decay produced the electron-neutrino and muon-neutrino component of the prompt neutrino beam. Tau-neutrinos were produced in the decay of the Ds meson (quark content cs). Since the tau lepton itself is short-lived, its decay produced another tau-neutrino.
Shielding concrete was used to range out charged secondaries and magnets were used to deflect muons away from the neutrino interaction region. This was necessary because the emulsion detector functioned as an integrating particle detector.
Neutrino interactions were recorded in emulsion targets consisting of thin emulsion sheets interleaved with thin (1mm) steel plates. Each emulsion sheet provided a measurement of track angle and position with a resolution of better than on micron. Since the decay length of the tau lepton is about 2mm in this experiment, the tau track was recorded at least once for every tau-neutrino interaction.
About 85% of the tau decays are single-prong, producing a charged lepton or meson and neutrinos. The kink between the tau and the lepton is reconstructed in the emulsion analysis. A tau-neutrino interaction is positively identified by the reconstructed kink and the requirement of no reconstructed leptons from the primary interaction vertex.
Data was taken during the 1996-1997 Fermilab fixed-target run. Since then we selected neutrino interactions, scanned them, and analyzed them in detail.
This page was last modified on 07/21/00 by Reinhard Schwienhorst.