tau (particle)

tau (particle)
Name	tau
Other names	tau lepton, tauon
Generation	third
Type	lepton
Electric charge	−1 e
Spin	1/2 ħ
Mass	1.77686 GeV/c^2
Lifetime	290.3×10^−15 s
Interactions	Electromagnetism, Weak interaction

tau (particle)

The tau is a third-generation charged lepton discovered as a heavy counterpart to the electron and the muon. It participates in electromagnetism and the Weak interaction and plays a central role in tests of the Standard Model and searches for physics beyond the Standard Model. Short-lived and relatively massive compared with first- and second-generation leptons, the tau's properties and decays probe CP violation, lepton flavour universality, and rare processes sensitive to supersymmetry and leptoquark scenarios.

Introduction

The tau is a fundamental fermion in the third family of elementary particles, together with the tau neutrino and third-generation quarks, the top quark and the bottom quark. As a charged lepton it is an SU(2) doublet component in the electroweak sector described by the Electroweak theory formulated by Sheldon Glashow, Abdus Salam, and Steven Weinberg. Its discovery required high-energy collisions and precision detectors developed at laboratories such as SLAC National Accelerator Laboratory and Fermilab. The tau's heavy mass leads to a rich decay spectrum that connects leptonic and hadronic final states and enables tests involving the CKM matrix indirectly through hadronic currents.

Properties

The tau has rest mass about 1.77686 GeV/c^2, spin 1/2, and electric charge −1 e. Its mean lifetime is approximately 290 femtoseconds, making it unstable compared with the electron and muon. The tau couples to the Z boson and the W boson via weak charged and neutral currents characterized by electroweak parameters measured at experiments like LEP and SLD. Radiative corrections involving the tau influence precision electroweak fits performed by collaborations such as ALEPH, DELPHI, L3, and OPAL. The tau's mass and lifetime provide constraints on the Higgs sector explored at CERN's Large Hadron Collider by collaborations including ATLAS and CMS.

Production and Decay

Taus are produced in high-energy processes such as e+e− annihilation at resonances like the Υ (4S) and continuum near B-factories like KEK's Belle and KEKB and SLAC's BaBar. At hadron colliders taus arise from W and Z boson decays, top quark decays, and heavy resonance decays probed by CDF and D0 at Tevatron and by ATLAS and CMS at the LHC. The tau decays leptonically into lighter leptons and neutrinos (τ→eντνe, τ→μντνμ) and hadronically into combinations of pions and kaons mediated by the strong interaction currents and resonances such as the ρ meson and the a1 meson. Branching fractions are precisely measured by collaborations like CLEO and provide inputs to determinations of the strong coupling constant and tests of lepton universality carried out by Heavy Flavor Averaging Group.

Experimental Detection and Measurements

Experimental identification of tau leptons relies on reconstructing short decay lengths, narrow collimated jets for hadronic tau decays, and isolated leptons with missing energy signatures from undetected neutrinos. Detectors with tracking systems, electromagnetic and hadronic calorimeters, and particle identification components—developed by teams at CERN, KEK, and SLAC—enable tau tagging algorithms used by ATLAS, CMS, Belle II, and LHCb. Precision measurements of the tau mass, lifetime, and branching ratios have been obtained by experiments including ARGUS, OPAL, ALEPH, and BaBar, while electroweak couplings and the number of light neutrino species were constrained by global fits incorporating results from LEP and SLC. Searches for lepton-flavor-violating tau decays such as τ→μγ and τ→3μ are carried out by Belle, BaBar, and LHCb; null results set limits that restrict models like supersymmetry and models with heavy Majorana neutrino mediators.

Role in the Standard Model and Beyond

Within the Standard Model, the tau tests universality of weak interactions across generations and contributes to loop corrections affecting precision observables measured by LEP and SLD. Its hadronic decays probe nonperturbative Quantum Chromodynamics phenomena and provide inputs to determinations of the hadronic vacuum polarization relevant to the anomalous magnetic moment of the muon measured by BNL and Fermilab Muon g-2. Beyond the Standard Model, tau-flavour-violating processes are sensitive to grand unified theories inspired by SU(5) or SO(10), to leptoquark exchanges motivated by flavor anomalies observed in B-meson decays by LHCb, and to two-Higgs-doublet models and minimal supersymmetric Standard Model contributions that alter branching fractions and CP asymmetries accessible at Belle II and at the High-Luminosity LHC.

History and Discovery

The tau was discovered in 1975 by Martin L. Perl and collaborators at SLAC and the Stanford Linear Accelerator Center experiment known as the Mark I detector, work that earned Perl the Nobel Prize in Physics in 1995 alongside contributions to heavy lepton physics. The initial observation of anomalous events with multiple charged particles and missing energy led to recognition of a new heavy lepton distinct from the muon and electron. Subsequent experiments at DESY, CERN, and Fermilab refined mass, lifetime, and decay-mode measurements, while the development of B-factories and high-energy colliders extended tau physics into precision tests and searches for rare processes. Continued exploration at facilities like KEK's Belle II and CERN's LHC aims to use the tau as a probe of subtle effects indicating new fundamental physics.

Category:Leptons