τ lepton

τ lepton
Name	Tau lepton
Other names	Tau, tauon
Generation	Third
Charge	−1 e
Spin	1/2
Mass	1776.86 MeV/c^2
Lifetime	2.903×10^−13 s

τ lepton

The τ lepton is a third-generation charged lepton with negative electric charge and fermionic spin, distinct from the electron and muon. It participates in electroweak interactions mediated by the W boson and Z boson and plays a central role in tests of lepton universality and searches for physics beyond the Standard Model. Precision measurements of the τ mass, lifetime, and branching fractions constrain models tested at facilities such as CERN, Fermilab, SLAC, and experiments like ALEPH, OPAL, L3, DELPHI, BaBar, Belle, and LHCb.

Introduction

The τ lepton belongs to the third family of charged leptons in the Standard Model alongside its associated neutrino, the tau neutrino. As a heavy lepton, the τ decays weakly and exhibits a rich decay topology that informs searches at colliders such as Large Hadron Collider, LEP, and KEKB. Its properties influence electroweak precision fits performed by collaborations including ATLAS, CMS, ALEPH, and CDF.

Properties

The τ has half-integer spin and negative unit electric charge, with measured mass approximately 1776.86 MeV/c^2 and a lifetime near 2.9×10^−13 s. Its weak interactions are described by the Cabibbo–Kobayashi–Maskawa matrix structure indirectly through charged-current processes and by the PMNS matrix via neutrino mixing of the tau neutrino. Electromagnetic form factors and anomalous magnetic moment contributions of the τ are constrained by experiments at Jefferson Lab, DESY, and Frascati. Radiative corrections calculated in Quantum Electrodynamics and Quantum Chromodynamics affect τ decay widths used in determinations of the strong coupling constant at low energies by collaborations like ALEPH and OPAL.

Production and decay

Tau leptons are produced in high-energy collisions via electroweak processes such as Z-boson decays at LEP and Drell–Yan production at Large Hadron Collider, through heavy-flavor decays studied at Belle II, and in cosmic-ray interactions observed by detectors like IceCube. Primary production channels include pair production in e+e− annihilation at facilities like SLAC and KEK and associated production in hadron colliders at CERN. The τ decays predominantly via charged-current weak interactions mediated by the W boson into leptonic modes (eν̄eντ, μν̄μντ) and hadronic modes involving resonances such as the ρ meson, a1 meson, and kaons measured by BaBar and Belle. Branching ratios and spectral functions from τ decays are inputs to determinations of the hadronic vacuum polarization and tests performed using data from CMD-3, SND, and radiative-return analyses at KLOE.

Experimental detection and measurements

Detection of τ leptons relies on reconstructing decay products in tracking systems, electromagnetic calorimeters, and muon spectrometers used by experiments like ALEPH, BaBar, Belle, ATLAS, and CMS. Techniques include one-prong and three-prong decay identification, impact-parameter measurements with silicon trackers developed at CERN and SLAC, and particle-identification systems employing ring-imaging Cherenkov detectors as in LHCb and Belle II. Precision mass and lifetime measurements were performed by collaborations at LEP and PEP-II; recent high-statistics studies at Belle II and LHCb continue to refine branching fractions and tests of lepton-flavor violation in searches influenced by theories from Supersymmetry and Grand Unified Theory model builders such as those at CERN and KEK.

Role in the Standard Model and beyond

Within the Standard Model, the τ serves as a probe of lepton universality and a laboratory for studying weak charged-current dynamics and hadronization through τ hadronic decays. Discrepancies in τ-involved observables motivate searches for new mediators like leptoquarks, additional gauge bosons often denoted Z′ in proposals by groups at Fermilab and CERN, and contributions from Supersymmetry scenarios studied by theorists at Institute for Advanced Study and Perimeter Institute. Measurements of τ-pair production and decay constrain parameters in Effective Field Theory fits performed by collaborations such as Global Electroweak Fit groups and inform neutrino oscillation experiments including Super-Kamiokande and IceCube through τ-neutrino appearance channels. Lepton-flavor-violating τ decays are sensitive to mechanisms explored in models proposed at Harvard University, MIT, and Stanford University.

Historical discovery and naming

The τ was discovered in 1975 in e+e− collisions by Martin L. Perl and colleagues at the SLAC using the SLAC-LBL Magnetic Detector (later experiments at SLAC and SLAC). The original observation was reported by the Mark I detector collaboration and subsequently confirmed by groups at DESY and CERN through analyses at PETRA and LEP. The particle was named "tau" (τ) by Perl to follow the Greek-letter convention used for leptons, and recognition of the discovery culminated in the award of the Nobel Prize in Physics to Martin Perl in 1995. Subsequent experimental programs at PEP-II, KEKB, and LEP expanded the understanding of τ properties and established its role within the Standard Model.

Category:Leptons