Lepton

A lepton is an elementary fermion that does not experience the strong interaction. The six known leptons are the electron, muon, tau, and their three neutrinos.

Electron: A stable, negatively charged lepton found in atoms and electric currents. It is the lightest charged lepton and the familiar member of the electron-neutrino family.
Electroweak interaction: The unified description of electromagnetic and weak interactions in the Standard Model. Charged leptons couple electromagnetically; all leptons participate in weak interactions.
Electron discovery: The identification of the electron as a constituent of cathode rays by J. J. Thomson in 1897. The electron was the first known lepton and established that atoms contain smaller charged particles.
Positron emission tomography: A medical imaging technique that detects pairs of gamma rays produced when positrons annihilate with electrons. The positron is the electron's antiparticle, making this imaging method a direct application of lepton physics.
Muon: An unstable charged lepton about 207 times more massive than an electron. It is the second charged-lepton generation, heavier than the electron but otherwise similar in its interactions.
Weak interaction: A fundamental interaction responsible for processes such as beta decay and neutrino scattering. It enables lepton decays and reactions, including transformations between charged leptons and neutrinos.
Muon discovery: The 1936 identification of a penetrating cosmic-ray particle later recognized as a heavier electron-like lepton. Its discovery introduced a second charged lepton and prompted the question, “Who ordered that?”
Muon tomography: An imaging method that uses the absorption or scattering of cosmic-ray muons to map objects. Muons penetrate dense structures and provide images without conventional sources of radiation.
Tau: The heaviest charged lepton, an unstable particle that decays into lighter particles. It completes the three charged-lepton generations and can decay into hadrons as well as lighter leptons.
Electromagnetism: The interaction between electrically charged particles, mediated by photons. Only charged leptons couple directly to photons; neutrinos have no electric charge.

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