Leptons

 The leptons are fundamental particles that interact only through the weak and electromagnetic interactions; even though the strong force can exceed the weak or electromagnetic force in strength by many orders of magnitude, the leptons do not feel this force at all. The leptons are true fundamental particles; they have no internal structure and are not composed of other, still smaller particles. We can consider the leptons to be point particles with no finite dimensions. All known leptons have a spin of 1/2

The Lepton Family

Particle	Antiparticle	Particle Charge (e)	Spin	Rest Energy MeV	Mean Life (s)	Typical Decay Product
e-	e+	-1	½	0.511	ꝏ	___
ve	ve	0	½	<0.000015	ꝏ	___
µ-	µ+	-1	½	105.7	2.2 x 10-6	e- + v-e +vµ
vµ	V- µ	0	½	<0.19	ꝏ	___
Ƭ-	Ƭ+	-1	½	1777	2.9 x 10-13	µ- + v-µ+ vƬ
vƬ	vƬ	0	½	<18	ꝏ	___

Table shows the six leptons, which appear as three pairs of particles. Each pair includes a charged particle (e^-, µ^-, Ƭ^-) and an uncharged neutrino (v_e, v_µ, v_Ƭ). Both the charged leptons and the neutrinos have antiparticles.

Electron neutrinos and antineutrinos are produced in the beta decay radioactive elements. They are also produced in great quantity in solar fusion processes large underground detectors, have been constructed to observe solar neutrinos and measure their properties. Because neutrinos interact only very weakly with matter, the solar neutrinos come to us directly from the core of the Sun, where the fusion reactions take place. (The photons from the Sun, on the other hand, come to us from its surface and therefore carry no direct information about fusion processes in the core.) 

Experiments have been underway for the past 40 years to count these solar neutrinos; the results of these experiments indicate that only about one-third to one-half of the expected number are reaching the Earth. Neutrinos are also produced in intense bursts from supernovas; the first such observation of supernova neutrinos was made in 1987.

Some theories of the properties of the neutrinos require that the neutrinos are massless, like the photon. Other theories allow a small but definitely nonzero mass. Measuring a small neutrino mass, especially for the electron neutrino, is a challenging experimental problem. So far the best experimental upper limit on the rest energy of the electron neutrino, about 15 eV, comes from the beta decay of 7H. By comparing the arrival time of the neutrino burst from Supernova 1987AD with the arrival time of the light signal from the supernova, a similar upper limit of about 20eV was estimated. If neutrinos do have mass, then they are permitted to transform from one type to another, such as electron neutrinos to muon neutrinos. This transformation, called neutrino oscillation, has not yet been seen directly, but it has been suspected as an explanation of the reduction in the number of electron neutrinos reaching us from the Sun. 

In 1999, the detector shown in Fig.  produced the first evidence for neutrino oscillations the number of muon neutrinos (produced in the atmosphere by collisions of energetic cosmic rays with air molecules) coming down from above differed from the number coming up from below (that is, traveling through the Earth). This discrepancy could not be explained simply by absorption during passage through the Earth, because the neutrinos have a mean free path in matter that is measured in light years, so they have almost no chance of being absorbed in passing through the Earth. Currently there are many experiments underway to measure neutrino masses, partly to achieve a better understanding of the properties of the neutrinos, but also because of the implications of a nonzero neutrino mass for cosmology, as we discuss later in this chapter.

The electron is a stable particle, but the muon and tau decay to other leptons, according to

µ^- → e^- +v ̅_e + v_µ (mean life = 2.2 X10^-6 s),

Ƭ^- → µ^- +v ̅_µ + v_Ƭ  (mean life=2.9X10-13s)

These decays are caused by the weak interaction, as we can conclude from the presence of neutrinos (which always indicates a weak interaction process) among the decay products and as we infer from comparing the decay lifetimes to the characteristic times۔ The form of these decays can be understood based on a conservation law for leptons.