Tau (particle)
History

The search for tau started in 1960 at CERN by the Bologna-CERN-Frascati (BCF) group led by Antonino Zichichi. Zichichi came up with the idea of a new sequential heavy lepton, now called tau, and invented a method of search. He performed the experiment at the ADONE facility in 1969 once its accelerator became operational; however, the accelerator he used did not have enough energy to search for the tau particle.

The tau was independently anticipated in a 1971 article by Yung-su Tsai. Providing the theory for this discovery, the tau was detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl with his and Tsai's colleagues at the Stanford Linear Accelerator Center (SLAC) and Lawrence Berkeley National Laboratory (LBL) group. Their equipment consisted of SLAC 's then-new electron–positron colliding ring, called SPEAR, and the LBL magnetic detector. They could detect and distinguish between leptons, hadrons, and photons. They did not detect the tau directly, but rather discovered anomalous events:

"We have discovered 64 events of the form

for which we have no conventional explanation."

The need for at least two undetected particles was shown by the inability to conserve energy and momentum with only one. However, no other muons, electrons, photons, or hadrons were detected. It was proposed that this event was the production and subsequent decay of a new particle pair:

This was difficult to verify, because the energy to produce the τ τ pair is similar to the threshold for D meson production. The mass and spin of the tau were subsequently established by work done at DESY -Hamburg with the Double Arm Spectrometer (DASP), and at SLAC-Stanford with the SPEAR Direct Electron Counter (DELCO),

The symbol τ was derived from the Greek τρίτον (triton, meaning "third" in English), since it was the third charged lepton discovered.

Martin Lewis Perl shared the 1995 Nobel Prize in Physics with Frederick Reines. The latter was awarded his share of the prize for the experimental discovery of the neutrino.
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