Thetau (τ), also called thetau lepton,tau particle ortauon, is anelementary particle similar to the electron, with negativeelectric charge and aspin of1/2. Like theelectron, themuon, and the threeneutrinos, the tau is alepton, and like all elementary particles with half-integer spin, the tau has a correspondingantiparticle of opposite charge but equalmass and spin. In the tau's case, this is the "antitau" (also called thepositive tau). Tau particles are denoted by the symbolτ− and the antitaus byτ+.
Tau leptons have a lifetime of2.9×10−13 s and amass of1776.9 MeV/c2 (compared to105.66 MeV/c2 for muons and0.511 MeV/c2 for electrons). Because their interactions are very similar to those of the electron, a tau can be thought of as amuch heavier version of the electron. Due to their greater mass, tau particles do not emit as muchbremsstrahlung (braking 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 (abovepetaelectronvolt energies), whentime dilation extends its otherwise very short path-length.[6]
As with the case of the other charged leptons, the tau has an associatedtau neutrino, denoted by ντ.
The search for tau started in 1960 atCERN by the Bologna–CERN–Frascati (BCF) group led byAntonino 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 theADONE facility in 1969 once its accelerator became operational; however, the accelerator he used did not have enough energy to search for the tau particle.[7][8][9]
The tau was independently anticipated in a 1971 article byYung-su Tsai.[10] Providing the theory for this discovery, the tau was detected in a series of experiments between 1974 and 1977 byMartin Lewis Perl with his and Tsai's colleagues at theStanford Linear Accelerator Center (SLAC) andLawrence Berkeley National Laboratory (LBL) group.[1] Their equipment consisted ofSLAC's then-new electron–positron colliding ring, calledSPEAR, and the LBL magnetic detector. They could detect and distinguish between leptons, hadrons, andphotons. They did not detect the tau directly, but rather discovered anomalous events:
"We have discovered 64 events of the form
e+ +e− →e± +μ∓ + at least two undetected particles
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:
e+ +e− →τ+ +τ− →e± +μ∓ + 4ν
This was difficult to verify, because the energy to produce theτ+τ− pair is similar to the threshold forD meson production. The mass and spin of the tau were subsequently established by work done atDESY-Hamburg with the Double Arm Spectrometer (DASP), and at SLAC-Stanford with theSPEAR Direct Electron Counter (DELCO),
The symbolτ was derived from the Greekτρίτον (triton, meaning "third" in English), since it was the third charged lepton discovered.[11]
The tau is the only lepton with enough mass to decay intohadrons. Like the leptonic decay modes of the tau, the hadronic decay is through theweak interaction.[12][a]
The tau lepton is predicted to formexotic atoms like other charged subatomic particles. One of such consists of an antitau and an electron:τ+ e− , calledtauonium.[citation needed]
Another one is anonium atomτ+ τ− calledditauonium ortrue tauonium, which is a challenge to detect due to the difficulty to form it from two (opposite-sign) short-lived tau leptons.[13]Its experimental detection would be an interesting test ofquantum electrodynamics.[14]
^Zichichi, A. (1996)."Foundations of sequential heavy lepton searches"(PDF). In Newman, H.B.; Ypsilantis, T. (eds.).History of Original Ideas and Basic Discoveries in Particle Physics. NATO ASI Series (Series B: Physics). Vol. 352. Boston, MA: Springer. pp. 227–275.
^Hooft, G. 't (1996).In search of the ultimate building blocks. Cambridge; New York, NY, USA: Cambridge University Press. p. 111.ISBN978-0-521-55083-3.
^Wu, C. S.; Barnabei, O., eds. (1998).The origin of the third family: in honour of A. Zichichi on the XXX anniversary of the proposal to search for the Third Lepton at Adone. World Scientific series in 20th century physics. Singapore; River Edge, N.J: World Scientific.ISBN978-981-02-3163-7.
^Perl, M.L. (6–18 March 1977)."Evidence for, and properties of, the new charged heavy lepton"(PDF). In Van, T. Thanh; Orsay, R.M.I.E.M. (eds.).Proceedings of the XII Rencontre de Moriond. XII Rencontre de Moriond. Flaine, France (published April 1977). SLAC-PUB-1923. Retrieved25 March 2021.
