tau lepton
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| tau lepton | |
|---|---|
| Name | Tau lepton |
| Caption | Computer reconstruction of a tau lepton decay event in a collider detector. |
| Statistics | Fermion |
| Generation | Third |
| Interaction | Electromagnetism, Weak interaction, Gravity |
| Status | Confirmed |
| Antiparticle | Antitau (τ+) |
| Theorized | Mid-1970s |
| Discovered | 1974–1977 |
| Discoverer | Martin Lewis Perl and colleagues |
| Experiment | SLAC–LBL collaboration |
| Mass | 1776.86, 0.12 |
| Decay time | 2.903, 0.005 |
| Electric charge | −1 ''e'' |
| Color charge | None |
| Spin | 1, 2 |
| Weak isospin | −1, 2 for τ−; +1, 2 for τ+ |
| Weak hypercharge | −1 |
| Chirality | Left-handed for particles |
tau lepton. The tau lepton is an elementary particle in the Standard Model of particle physics, classified as a third-generation charged lepton. It is the heaviest of the charged leptons, with a mass nearly twice that of a proton, and is the only lepton capable of decaying into hadrons. Its discovery provided crucial evidence for the existence of a third generation of fundamental fermions, completing the lepton family alongside the electron and the muon.
Discovery and properties
The tau lepton was discovered between 1974 and 1977 by a team led by Martin Lewis Perl at the SLAC National Accelerator Laboratory, using the SPEAR particle accelerator in collaboration with researchers from the Lawrence Berkeley National Laboratory. The discovery was made by analyzing annihilation events of electrons and positrons, which produced anomalous signatures inconsistent with known particles like the muon. This work earned Perl a share of the Nobel Prize in Physics in 1995. The tau has a mass of approximately 1777 MeV/c², a spin of ½, and a mean lifetime of about 290 femtoseconds. Like other charged leptons, it has a corresponding neutrino, the tau neutrino, which was directly observed later by the DONUT collaboration at Fermilab.
Production and decay
Tau leptons are typically produced in high-energy collisions, such as in electron–positron annihilation at facilities like CERN's Large Electron–Positron Collider or in hadron colliders like the Tevatron and the Large Hadron Collider. They are also created in decays of heavier particles, including W and Z bosons and Higgs bosons. Due to its large mass, the tau lepton decays via the weak interaction into lighter particles, with a distinctive branching ratio. Its decays are categorized as leptonic, involving a tau neutrino and either an electron or a muon, and hadronic, where it decays into a tau neutrino and a combination of mesons like pions or kaons. The Cabbibo–Kobayashi–Maskawa matrix elements influence these hadronic decay modes.
Role in particle physics
The tau lepton plays a fundamental role in testing the Standard Model and probing for physics beyond the Standard Model. Its relatively high mass makes it sensitive to potential new interactions and particles. Precision measurements of its properties, such as its magnetic moment and lifetime, provide stringent tests for quantum electrodynamics and the electroweak theory. The existence of the tau and its neutrino completed the three-generation structure of leptons, a pattern mirrored in the quark sector. Studies of CP violation in tau decays, conducted at experiments like Belle and BaBar, offer insights complementary to those from kaon and B meson systems.
Experimental studies
Major experiments dedicated to studying the tau lepton include the DELPHI and ALEPH detectors at the Large Electron–Positron Collider, which made precise measurements of its mass and branching ratios. The Belle experiment at KEK in Japan and the BaBar experiment at the SLAC National Accelerator Laboratory have extensively studied CP violation and lepton flavor violation in tau decays. At the Large Hadron Collider, general-purpose detectors like ATLAS and CMS study tau leptons as crucial signatures in decays of the Higgs boson and in searches for supersymmetry. Future facilities, such as the proposed Future Circular Collider, aim to produce tau leptons in even greater numbers for higher-precision studies.
See also
* Muon * Tau neutrino * Lepton flavor violation * Martin Lewis Perl * Standard Model
Category:Leptons Category:Elementary particles Category:Subatomic particles