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Leptons are the another class of elementary particles besides the quarks. Like quarks there are three families of leptons. Each lepton family consists of negatively charged particle and extremely light particle. They are affected by the gravitational, electromagnetic and weak forces but not by a strong force. They must be either created or destroyed in the particle-anti-particle pairs, the total number of leptons is conserved in all processes. Let's see more about them.

What is a Lepton?

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A lepton is the major elementary particles like quarks and bosons. These leptons are believed to be fundamental units of matter, i.e., the particle do have no more smaller units. These six leptons come in pairs, each pair consisting of one charged lepton and one neutral lepton. The neutral lepton is called a neutrino. Each pair is called a generation and in each generation the mass of the neutrino is much less than the mass of the charged lepton. Electrons, muons and taus have a negative electric charge. Muons and taus have a much larger mass than that of electrons. A muon is about 207 times as heavy as electrons and tau is about 3490 times as heavy as an electron.

Particle Charge Mass
1 Electron (e)
Electron neutrino ($\nu_e$)
0.51 Mev
Less than 50 ev
2 Muon ($\mu$)
Muon neutrino ($\nu_\mu$)
106 Mev
Less than 0.5 Mev
3 tau ($\tau$)
tau neutrino ($\nu_r$)
1784 Mev
Less than 160 Mev

Lepton Number

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Each lepton generation has its own lepton number. The lepton number for electron is electronic number for the electron and electron neutrino denoted as $L_{e}$. The lepton number for muon is muonic number for muon and muon neutrino denoted as $L_{\mu}$. The lepton number for tau is tauonic number for tau and tau neutrino denoted as $L_{\tau}$.

Lepton number is the number of leptons minus number of antileptons. It is given as
L = $n_{\iota}$ - $n_{\bar{\iota}}$
$n_{\iota}$ is the lepton,
$n_{\bar{\iota}}$ is the antilepton.

Unstable Lepton

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Its known that leptons are having three generations. Each with two flavors (Lepton and antilepton). The electron is the most stable lepton in nature whereas tau and muons are unstable leptons showing a very small mean life time. Among the two the tau is most unstable and produce enough collisions with a higher speed and greater energy. Muons being unstable particle decay into lighter particles. A muon decays into an electron, a mu-neutrino and an electron anti neutrino, in about 2 millionths of a second. The life time of a tau is less than 2 trillionths of a second. A tau because of its large mass can decay into many different combinations of lighter particles.


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Just as six quarks and antiquarks combine to form hadron family, six leptons and six antileptons form the lepton family. All leptons have anti matter counter parts that are called antileptons. It is nothing but the positron and the antiparticle. These antileptons have same mass as leptons but all other properties are reversed. Because neutrinos have no charge their anti particles are also neutral.