Leptons

The leptons exhibit the same familial structure as the quarks. Each lepton family contains a unit charged, massive lepton and a neutral, nearly massless³ neutrino. The first family of leptons consists of the electron () and the electron-neutrino ( $\nu_{\electron}$). As with the quarks, there are three families of leptons, named electron, muon, and tau (, $\mu$, and $\tau$). Each quark family and lepton family are related through symmetry. The $\nu_\tau$ remains unobserved to date,⁴ but its existence is not seriously doubted. The properties of the leptons are summarized in intro:leptons.

 

Table 1.2: Properties of the three families of leptons [4].

	Mass ()		Mass
	0.5110	$\nu_{\electron}$	$< 15~\evcc$
$\mu$	105.66	$\nu_\mu$	$< 0.17~\mevcc$
$\tau$	1777.1	$\nu_\tau$	$< 18.2~\mevcc$

 

Combining the families, or generations, of quarks and leptons, the generations of matter can be represented by

where the particles in each row differ only in mass.