An Introduction to the Standard Model of Particle Physics

1.3 Leptons

The known leptons are listed in Table 1.2. The Dirac equation for a charged massive fermion predicts, correctly, the existence of an antiparticle of the same mass and spin but opposite charge, and opposite magnetic moment relative to the direction of the spin. The Dirac equation for a massless neutrino predicts the existence of an antineutrino nubar .

**Table 1.2.** Leptons

	Mass	Mean life	Electric
	(MeV / c²)	(s)	charge

Electron e^-	0.5110		-e
Electron neutrino _e	< 15 x 10^-6	?	0
Muon µ^-	105.658	2.197 x 10^-6	-e
Muon neutrino _µ	< 0.17	?	0
Tau ^-	1777	(291.0 ± 1.5) x 10^-15	-e
Tau neutrino	< 24	?	0

Of the charged leptons, only the electron e^- carrying charge -e and its antiparticle e⁺, are stable. The muon µ^- and tau ^- and their antiparticles, the µ⁺ and ⁺ differ from the electron and positron only in their masses and their finite lifetimes. They appear to be elementary particles. In this book we take the neutrinos to have zero mass. The experimental situation regarding small neutrino masses has not yet been clarified. Non-zero neutrino masses would have dramatic implications for cosmology. There is good experimental evidence that the e, µ and have different neutrinos _e, _µ and associated with them.

It is believed to be true of all interactions that they preserve electric charge. Leaving aside the possibility of neutrino masses, it seems that in its interactions a lepton can change only to another of the same type, and a lepton and an antilepton of the same type can only be created or destroyed together. These laws are exemplified in the decay

µ^- -> _µ + e^- + _e.

This conservation of lepton number, antileptons being counted negatively, which holds for each separate type of lepton, along with the conservation of electric charge, will be apparent (aside from the considerations of Chapter 19) in the Standard Model.