DONUT (Direct observation of the nu tau, E872) was an experiment at
Fermilab dedicated to the search for tau neutrino interactions. The
detector operated during a few months in the summer of 1997, and
successfully detected the tau neutrino. It confirmed the existence
of the last lepton predicted by the Standard Model. The data from
the experiment was also used to put an upper limit on the tau neutrino
magnetic moment and measure its interaction cross section.
3 See also
5 External links
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
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 was far in excess of the expected
background (< 0.2 events) and is therefore 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.
^ a b c K. Kodama et al. (
DONUT Collaboration) (2001). "Observation of
tau neutrino interactions". Physics Letters B. 504 (3): 218.
arXiv:hep-ex/0012035 . Bibcode:2001PhLB..504..218D.
^ a b "Physicists Find First Direct Evidence for Tau Neutrino at
Fermilab" (Press release). Fermilab. 20 July 2000.
^ K. Kodama et al. (
DONUT Collaboration) (2008). "A first measurement
of the interaction cross section of the tau neutrino". Physical Review
D. 78 (5): 052002. arXiv:0711.0728 . Bibcode:2008PhRvD..78e2002K.
^ R. Schwienhorst et al. (
DONUT Collaboration) (2001). "A new upper
limit for the tau-neutrino magnetic moment". Physics Letters B. 513:
23. arXiv:hep-ex/0102026 . Bibcode:2001PhLB..513...23D.
DONUT home page
Neutrino detectors, experiments, and facilities
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