DIRECT EVIDENCE FOR TAU NEUTRINOS will be reported tomorrow in a seminar at Fermilab. While the existence of neutrinos associated with the tau lepton was not in doubt, actually observing the particle interact had not occurred until now. This rounds out the program of experimental sightings of the truly fundamental building blocks prescribed by the standard model of particle physics.

This official alphabet consists of six quarks— known as up, down, strange, charm, top, and bottom—and six leptons—electron, electron neutrino, muon, muon neutrino, tau, and tau neutrino. All matter, according to the theory, should be made up from these most basic of constituents. Other particles, such as the anti-matter counterparts of the quarks and leptons, the force-carrying bosons (e.g., photons, gluons, etc.), and the Higgs boson (which confers mass upon some of the other particles) also appear in the theory. (Still other candidates, such as the "supersymmetric" particles, are not part of, but are expected to be compatible with, the standard model.)

The evidence for the tau neutrino is slim but impressive: five scattering events are being exhibited at the seminar by Fermilab physicist Byron Lundberg, leader of Experiment 872, the Direct Observation of Nu Tau (or DONUT) collaboration (http://fn872.fnal.gov/). Their experiment proceeds in the following manner. Fermilab's 900-GeV proton beam (the highest beam energy in the world) was steered onto a tungsten target, where some of the prodigious incoming energy is turned into new particles. Some of these quickly decay into taus and tau neutrinos. Next comes an obstacle course of magnets (meant to deflect charged particles away) and shielding material (meant to absorb most of the other particles except for rarely interacting neutrinos). Beyond this lies a sequence of emulsion targets in which the neutrinos can interact, leaving a characteristic signature.

Evidence for a tau neutrino in the emulsion is the creation of a tau lepton, which itself quickly decays (after traveling about 1 mm) into other particles. The E872 physicists estimate that about 10¹⁴ tau neutrinos entered the emulsion, of which perhaps 100 interacted therein. It is a carefully analyzed handful of such events that is now being presented to the public in evidence. The tau neutrino is the third neutrino type to be detected. The detection of the electron neutrino by Clyde Cowan and Frederick Reines garnered Reines the 1995 Nobel Prize for physics (Cowan had died some years before). For discovering the muon neutrino, Leon Lederman, Melvin Schwartz, and Jack Steinberger won the Nobel Prize in 1988.