Professor Martin Perl, discoverer of tau lepton.

The third family of fundamental particles of matter became more than just a theory during the 1970s, when a series of experiments at the Stanford Linear Accelerator Center led to discovery of the tau lepton. The tau was the first actual evidence of the third generation of quarks (the smallest particles of matter), leptons (weakly interacting particles), and their anti-particles. For this discovery, Martin L. Perl won the 1995 Nobel Prize in Physics, sharing the honor with Frederick Reines (the discoverer, much earlier while at Los Alamos National Laboratory, of a particle in the first family). Perl's early experiments failed, until a new particle collider gave him access to a previously inaccessible region of energy, about 5 billion electron volts. The higher the energy levels in a particle collider, the smaller the structures that can be probed. Even then, years of observations involving tens of thousands of pieces of data were needed before the researchers were certain they had discovered a new lepton. The final identification of the tau as a lepton was achieved by measuring its lifetime with the vertex detector method, an important technique that was new at the time.

Scientific Impact: Evidence of the third family of fundamental particles inspired confidence in the Standard Model, the theory then being developed by physicists to explain matter and the forces of nature. The vertex detector method has been used to measure the lifetime of other fundamental particles and is now used universally to measure particle properties.

Social Impact: These studies answer questions about the constituents and history of the universe, extending human understanding of nature and contributing to improvements in science education. In addition, although basic research is by definition a search for new knowledge without regard to its practical implications, such work often contributes to technologies with commercial value; examples include computers, lasers, and cancer treatments.

Reference: "Properties of the Proposed Charged Tau Lepton," M. L. Perl et al., Phys. Lett. 70B: 487 (1977).

"Measurement of the Tau Lifetime," G. J. Feldman et al., Phys. Rev. Lett. 48: 66-69 (1982).

"Precise Measurement of the Tau Lifetime," J. A. Jaros, et al. Phys. Rev. Lett. 51: 955-958 (1983).

URL: http://www.slac.stanford.edu/slac/hottopic/mperl95/tau.html

Technical Contact: Prof. Martin Perl, martin@slac.stanford.edu

Press Contact: Jeff Sherwood, DOE Office of Public Affairs, 202-586-5806

SC-Funding Office: Office of High Energy and Nuclear Physics