DONUT
|
Schematic overview of the DONUT detector |
DONUT (
Direct
Observation of the
NU Tau, E872) was an experiment at
Fermilab dedicated to the search for
tau neutrino interactions. Even though the detector operated only during a few months in the summer of 1997, it was largely successful. By detecting the tau neutrino, it confirmed the existence of the last lepton predicted by the
Standard Model.
In DONUT,
protons accelerated by the
Tevatron were used to produce tau neutrinos via decay of
charmed mesons. After eliminating as many unwanted background particles as possible by a system of magnets and bulk matter (mostly iron and concrete), the beam passed through several sheets of
nuclear emulsion. In very rare cases one of the neutrinos would interact in the detector, producing electrically charged particles which left visible tracks in the emulsion and could be electronically registered by a system of
scintillators and
drift chambers.
Using the electronic information, possible neutrino interactions were identified and selected for further analysis. This meant photographically developing the emulsion sheets so any traces left by particles passing through them would show up as a small black dot. By connecting these dots across subsequent sheets, the path that each particle had taken was reconstructed and likely neutrino interactions identified. The characteristic properties of tau neutrino interactions were that several tracks suddenly appeared without any leading up to them and that one of those tracks would show a "kink" after a few
millimeters, indicating decay of a
tau lepton.
In July 2000, the DONUT collaboration announced the first observation of tau neutrino interactions. Even though this result was based on only four events, the signal largely exceeded the expected background (< 0.2 events) and is therefore considered valid. Its significance lies in the fact that the tau neutrino had so far remained the only particle of the
Standard Model that had not been directly observed except for the
Higgs boson.
Other than the result itself, DONUT also allowed validation of new techniques for high energy neutrino detection, notably the
Emulsion Cloud Chamber, in which nuclear emulsion sheets are interspersed with layers of iron, leading to an increase in the number of interactions. Observation of tau neutrinos also plays an important role in some
neutrino oscillation experiments.
*
DONUT home page
