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.
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.