Leptons

Leptons and quarks are the basic building blocks of matter, i.e., they are seen as the "elementary particles". There are six leptons in the present structure, the electron, muon, and tau particles and their associated neutrinos. The different varieties of the elementary particles are commonly called "flavors", and the neutrinos here are considered to have distinctly different flavor.

Important principles for all particle interactions are the conservation of lepton number and the conservation of baryon number.

Now that we have experimental evidence for six leptons, a relevant question is "Are there more?". The present standard model assumes that there are no more than three generations. One of the pieces of experimental evidence for that is the measured hydrogen/helium abundance ratio in the universe. When the process of nucleosynthesis from the big bang is modeled, the number of types of neutrinos affects the abundance of helium. The observed abundance agrees with three types of neutrinos.

Electron and Positron

As one of the leptons, the electron is viewed as one of the fundamental particles. It is a fermion of spin 1/2 and therefore constrained by the Pauli exclusion principle, a fact that has key implications for the building up of the periodic table of elements.

The electron's antiparticle, the positron, is identical in mass but has a positive charge. If an electron and a positron encounter each other, they will annihilate with the production of two gamma-rays. On the other hand, one of the mechanisms for the interaction of radiation with matter is the pair production of an electron-positron pair. Associated with the electron is the electron neutrino.

Particle 质点	Symbol 符号	Anti- particle 反粒子	Rest mass MeV/c2 静止质量 MeV/c²	L(e)	L(muon) μ子的朗德因子	L(tau) L(τ)	Lifetime (seconds) 寿命（秒）
Electron 电子	e-	e+	0.511	+1	0	0	Stable 稳定
Neutrino (Electron) 中微子（电子）			0(<7 x 10-6)	+1	0	0	Stable 稳定

Electron spin 电子自旋

What evidence suggests that the electron is a fundamental particle? 什么证据表明电子是一个基本粒子？

Muon

The muon is a lepton which decays to form an electron or positron.

The fact that the above decay is a three-particle decay is an example of the conservation of lepton number; there must be one electron neutrino and one muon neutrino or antineutrino in the decay.

The lifetime of the muon is 2.20 microseconds. The muon is produced in the upper atmosphere by the decay of pions produced by cosmic rays:

Measuring the flux of muons of cosmic ray origin at different heights above the earth is an important time dilation experiment in relativity.

Muons make up more than half of the cosmic radiation at sea level, the remainder being mostly electrons, positrons and photons from cascade events. (Richtmyer) The average sea level muon flux is about 1 muon per square centimeter per minute.

Particle 质点	Symbol 符号	Anti- particle 反粒子	Rest mass MeV/c2 静止质量 MeV/c²	L(e)	L(muon) μ子的朗德因子	L(tau) L(τ)	Lifetime (seconds) 寿命（秒）
Muon 缪子	μ-	μ+	105.7	0	+1	0	2.20x10-6
Neutrino (Muon) 中微子（缪子）			0(<0.27)	0	+1	0	Stable 稳定

Some history 一些历史

Atmospheric muons 大气μ子

Tau

The tau is the most massive of the leptons, having a rest mass some 3490 times the mass of the electron, also a lepton. Its mass is some 17 times that of the muon, the other massive lepton.

Particle 质点	Symbol 符号	Anti- particle 反粒子	Rest mass MeV/c2 静止质量 MeV/c²	L(e)	L(muon) μ子的朗德因子	L(tau) L(τ)	Lifetime (seconds) 寿命（秒）
Tau 塔乌	τ-	τ+	1777	0	0	+1	2.96x10-13
Neutrino (Tau) 中微子（tau）			0(<31)	0	0	+1	Stable 稳定

The tau lepton has spin 1/2 like the electron and muon leptons and an antiparticle with equal mass but opposite charge.

The tau lepton was detected in experiments at SLAC and LBL between 1974 and 1977. The discovery contributed to the Nobel Prize awarded to Martin Lewis Perl in 1995. The tau is the only lepton massive enough to decay into hadrons, particularly into pions.

Positron Annihilation

The positron is the antiparticle of the electron, and when a positron enters any normal matter, it will find an abundant supply of electrons with which to annihilate. The energy released by the annihilation forms two highly energetic gamma rays, and if one assumes that the momenta of the positron and electron were equal before the annihilation, the two gamma ray photons must travel in opposite directions in order to conserve momentum.

These coincident gamma rays at 180 degrees provide a useful analysis tool. For one thing, eliminating all gamma events which are not coincident at 180 degrees improves the signal-to-noise ratio of experiments using positron annihilation. Another interesting application is the use of the coincident gammas to locate the source by back projecting. This is used in medical PET scans. 这些在180度处出现的伽马射线提供了一个有用的分析工具。一方面，消除所有在180度处不同时出现的伽马射线事件可以提高使用正电子湮灭的实验的信噪比。另一个有趣的应用是利用这些同时出现的伽马射线通过反投影来定位源。这在医学PET扫描中得到了应用。

Electron-Positron Pair Production

When a photon has quantum energy higher than the rest mass energy of an electron plus a positron, one of the ways that such a photon interacts with matter is by producing and electron-positron pair.

The rest mass energy of the electron is 0.511 MeV, so for photon energy above 1.022MeV, pair production is possible. For photon energies far above this threshold, pair production becomes the dominant mode for the interaction of x-rays and gamma-rays with matter. 电子的静止质量能量为0.511 MeV，因此当光子能量高于1.022 MeV时，产生对撞（pair production）是可能的。对于远高于此阈值的光子能量，X射线和伽马射线与物质相互作用时，产生对撞成为主导模式。

Properties of the Leptons

Particle 质点	Symbol 符号	Anti- particle 反粒子	Rest mass MeV/c2 静止质量 MeV/c²	L(e)	L(muon) μ子的朗德因子	L(tau) L(τ)	Lifetime (seconds) 寿命（秒）
Electron 电子	e-	e+	0.511	+1	0	0	Stable 稳定
Neutrino (Electron) 中微子（电子）			0(<7 x 10-6)	+1	0	0	Stable 稳定
Muon 缪子	μ-	μ+	105.7	0	+1	0	2.20x10-6
Neutrino (Muon) 中微子（缪子）			0(<0.27)	0	+1	0	Stable 稳定
Tau 塔乌	τ-	τ+	1777	0	0	+1	2.96x10-13
Neutrino (Tau) 中微子（tau）			0(<31)	0	0	+1	Stable 稳定

Numerical data from Giancoli

Lepton discussion 轻子讨论