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DONUT comes to neutrino town

17 August 2000

A neutrino experiment at Fermilab has seen the first direct evidence for the tau
neutrino, the most elusive of the 12 particles that make up the Standard Model
picture of the fundamental structure of matter.

Using the intense
neutrino beam from Fermilab’s Tevatron, the DONUT (Direct Observation of
the Nu Tau) experiment has seen four examples of neutrinos producing slightly
kinked tracks, the tell-tale sign that an unstable tau particle has been
produced.

According to the Standard Model, all of the matter we know
in nature can be explained in terms of six quarks – the ultimate constituents of
nuclear matter – and six other particles (leptons). The quarks are arranged in
three pairs – up and down, heavy strange and charm, and still heavier beauty
and top. The six leptons are also arranged in three pairs – three electron-like
particles; the electron, the muon and the tau; and three ghostly neutrinos – each
associated with one of the electron-like particles.

The Standard Model
quarks and leptons can thus be arranged in three “families” of four: the first
contains the up and down quarks, the electron and the electron neutrino; the
second contains the strange and charm quarks, the muon and the muon
neutrino; and the third contains the beauty and top quarks and the tau and tau
neutrino.

It has been known for a long time that the Standard Model
contains these 12 particles, but initially not all of them had been seen. In 1995,
experiments at Fermilab’s Tevatron collider saw evidence for particles
containing the long-awaited sixth “top” quark. Now, with the evidence for the
tau neutrino, all of the direct evidence for the 12 particles is finally in
place.

For the DONUT experiment, Fermilab’s 800 GeV proton beam
(effectively the highest energy in the world) is slammed into a huge target or
“beam dump”, which produces a dense fog of highly unstable secondary
particles. One of these is a D meson, containing both strange and charmed
quarks (the Ds particle), which can decay to produce tau
neutrinos. (Conventionally, neutrinos are produced by the decay of secondary
pions and kaons. However, with a beam dump, many of these are otherwise
absorbed by the surrounding material before they have a chance to decay and
produce neutrinos. The fraction of the neutrino content produced via other
decays is therefore increased.)

After the beam dump, an obstacle course
of magnets sweeps away charged particles, while thick shielding absorbs many
of the rest. However, the ethereal neutrinos continue almost
unaffected.

Downstream of the magnets and shielding is the DONUT
detector, a sandwich of iron plates and photographic emulsion. In this target,
one in a trillion tau neutrinos hits an iron plate, releasing an unstable tau
lepton.

The tau leptons (which like the electron carry electric charge)
leave a sub-millimetre track in the emulsion before decaying. The DONUT
experiment set out to look for these tiny track stubs. Of the 100 or so tau
neutrino collisions, just four track stubs have been unearthed so far. Isolating
these signals from the mass of accumulated data is a triumph of painstaking
analysis. Emulsion technology developed at Nagoya plays a major role in this
work, and the Nagoya team handles DONUT’s crucial emulsion
analysis.

When CERN’s LEP electron-positron collider came into
operation in 1989, one of its first results was to show that particle decays allow
for three, and only three, kinds of neutrino. The first of these had been seen by
Clyde Cowan and Fred Reines in a reactor experiment in 1955, and for this the
latter received the Nobel Prize for Physics in 1995 (Cowan died in 1974). In
the 1950s, seeing the neutrino (in this case the electron-type particle) was
considered a major accomplishment.

Soon the decay patterns of the
muon suggested that the neutrino had to come in two different kinds, one
preferring to associate with electrons, the other with muons. In 1962 an
experimental team led by Leon Lederman, Mel Schwartz and Jack Steinberger
at Brookhaven revealed muon tracks emerging from neutrino interactions. For
this discovery the trio received the 1988 Nobel Prize.

In 1975 Martin
Perl at the SPEAR electron-positron collider at SLAC, Stanford, discovered
the third lepton, the tau. Before this discovery only two families of fundamental
particles had been known. Perl’s breakthrough suggested that there are three.
For the tau discovery he was awarded the 1995 Nobel Prize, sharing it with
neutrino pioneer Reines.

For the tau to fit into the picture it also had to
be accompanied by its own neutrino. Physicists learned to live with the
elusiveness of this particle, and could infer its existence directly. For example,
in 1987 the UA1 experiment at CERN’s proton-antiproton collider studied
decays of the W particle, the electrically charged carrier of weak interactions,
which was discovered at CERN four years previously. Setting to one side the
W decays producing electrons and muons, they found 29 decays that could be
designated as candidate decays producing a tau (and a tau neutrino). Although
the neutrino could not be seen, energy-momentum accounting revealed
“missing energy, showing that an invisible particle – the tau neutrino – had
escaped in the W decays”.

Tau physics, with the tau neutrino playing an
essential but invisible role, went on to become a precision science in the hands
of experiments at electron-positron colliders – LEP at CERN and CESR at
Cornell (January p20).

The recent Chorus neutrino experiment at CERN
also used Nagoya emulsion technology. This study (and the companion Nomad
experiment) explicitly set out to look for the transformation of muon neutrinos
into tau neutrinos (neutrino oscillations). These experiments used a
conventional neutrino target rather than a beam dump. At the lower proton
energies available at CERN, few Ds particles containing heavy
quarks are produced directly. The experiments did not see any tau neutrinos,
either through oscillations or via direct production.

DONUT is a
collaboration between the US, Greece, Japan and Korea. For further DONUT
information, see “http://fn872.fnal.gov/”.

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