Tau (particle)
The tau (), also called the tau lepton, tau particle, or tauon, is an elementary particle similar to the electron, with negative electric charge and a spin of. Like the electron, the muon, and the three neutrinos, the tau is a lepton, and like all elementary particles with half-integer spin, the tau has a corresponding antiparticle of opposite charge but equal mass and spin. In the tau's case, this is the "antitau" (also called the "positive tau"). Tau particles are denoted by the symbol and the antitaus by . Tau leptons have a lifetime of and a mass  of (compared to for muons and for electrons). Since their interactions are very similar to those of the electron, a tau can be thought of as a "much" heavier version of the electron. Because of their greater mass, tau particles do not emit as much bremsstrahlung radiation as electrons; consequently they are potentially much more highly penetrating than electrons. Because of its short lifetime, the range of the tau is mainly set by its decay length, which is too small for bremsstrahlung to be noticeable. Its penetrating power appears only at ultra-high velocity and energy (above petaelectronvolt energies), when  time dilation extends its otherwise very short path-length. As with the case of the other charged leptons, the tau has an associated tau neutrino, denoted by . 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 an 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: 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 pair is similar to the threshold for D meson production. The mass and spin of the tau was 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 was derived from the Greek ("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 experimental discovery of the neutrino. Tau decay. The tau is the only lepton that can decay into hadrons – the other leptons do not have the necessary mass. Like the other decay modes of the tau, the hadronic decay is through the weak interaction. The branching fractions of the dominant hadronic tau decays are: In total, the tau lepton  will decay hadronically approximately 64.79% of the time. The branching fractions of the common purely leptonic tau decays are: The similarity of values of the two branching fractions is a consequence of lepton universality. Exotic atoms. The tau lepton is predicted to form exotic atoms like other charged subatomic particles. One of such, called tauonium by the analogy to muonium, consists of an antitauon and an electron: . Another one is an onium atom called "true tauonium" and is difficult to detect due to tau's extremely short lifetime at low (non-relativistic) energies needed to form this atom. Its detection  would be an important test of quantum electrodynamics, if possible.