The tau neutrino
is the neutral
partner of the tau lepton
, and is the cousin
of the electron neutrino
and muon neutrino
It is the last neutrino to be directly
although its existence was not in doubt due to
a wealth of indirect
An experiment at Fermi National Accelerator Laborator,
or Fermilab, called DONUT (Direct Observation
of Nu Tau), reported their results on 21 July 2000.
They found four events showing a neutrino
interacting with an atomic nucleus to produce
a tau lepton. Since electron and muon neutrinos
produce longer lived electrons and muons, the
presence of the tau is a signature for a tau
It is interesting that these four events came from
a data sample of six million recorded interactions.
Demanding that various conditions be satisfied
in a candidate event, they applied cuts on the data,
producing 1000 candidate events. Finally four events
offer definitive evidence for the observation.
The delicate experiment made use of advanced
emulsion technology to track and tag
the tau lepton, which again is the signature
for having a tau neutrino in the initial state.
Source: Fermilab press release