Tau Particles
In the standard model of particle physics the tau particles (e.g., tau leptons and tau neutrinos) are among the fundamental building blocks of matter.
The standard model attempts a comprehensive synthesis of fundamental particles and forces. According to the standard model, fermions (quarks and leptons) comprise all matter. There are twelve particles and twelve antiparticles in this class. In addition to the six known types of quarks (i.e., up, down, charm, strange, top, and bottom quarks) there are three charged leptons (the electron, the muon, and the tau particle), each with a corresponding neutral particle termed a neutrino. All of these fundamental particles have corresponding antiparticles that have exactly the same mass but which carry an opposite charge (i.e., the antiparticle of the electron is the positron and the antiparticle of the tau neutrino is the tau anti-neutrino). Fermions exist in three generations (mass and energy states). Protons, neutron and electrons are in the first generation of matter and the more massive and energetic particles of the second and third generations usually quickly decay into less-massive first generation particles.
By the early 1970s, there was a known symmetry between two generations of quarks and leptons. After a study of collisions of electrons and positrons, Martin Louis Perl (1927-) at the Stanford University linear acceleration (SLAC) accounted for energy enigmatically missing after such collisions with the discovery of a tau particle (t). In addition to finding a new lepton, Perl discovered evidence of a third generation of matter. The tau lepton, like the muon and electron, had a negative charge and had an antimatter particle version with a positive charge.
Because the tau particle carries a charge, it reacts via the electromagnetic force and is, of course, far less affected by the much weaker gravitational force. Interestingly, however, the tau was more massive than expected. The energy for producing the Tau +, Tau - pair was about 3.6 Gev. This energy implied a tau rest mass of approximately 1.8 Gev, (its actual mass is 1.784 Gev), about twice as heavy as a proton, 20 times as heavy as the muon, and an elephantine 4,000 times as heavy as an electron. Such a high mass made the tau unstable, with a lifetime of only 3x10-13 seconds.
The tau particle decays to produce a tau neutrino, an electron and an anti-electron-neutrino, while the anti-tau decays into an anti-tau-neutrino, an anti-mu and a mu-neutrino.
Neutrinos are the precursor particles to leptons (i.e., tau lepton results from the collision of tau neutrinos). Accordingly, the discovery of the tau lepton in the 1970s by the Direct Observation of the Nu Tau (DONUT) project spurred the quest for an experimental confirmation of the tau neutrino. The quest proved elusive because tau neutrinos can not be observed directly and their existence can only be inferred by the results of their interactions.
According to the standard model the tau neutrino is one of three neutrino particles. The other neutrinos, electron neutrinos and muon neutrinos, were discovered in the late 1950s and early 1960s. Like other neutrinos (Italian for little neutral one), tau neutrinos carry no electric charge and have such a small mass that they are able to travel near the speed of light. For many years scientists predicted that neutrinos would be massless. In 1998, however, a team of scientists working in Japan made an experimental determination that neutrinos possessed a very small mass (less than one-millionth that of an electron).
In 2000, an international team of scientists working at the Fermi National Accelerator Laboratory finally announced the experimental confirmation of the existence of the tau neutrino. The experimental observations fit perfectly with the predictions of the standard model.
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