What does a lepton do

Particle physics

Particle physics looks inside the atoms and atomic nuclei. It deals with the structure of matter.

Matter and radiation
The content of our universe can be divided into two groups: matter and radiation. Matter is the particles we are made of, i.e. the components of atoms. Radiation is e.g. light. These particles also mediate forces between particles of matter. Photons convey the electrical force and so-called gluons generate the nuclear forces.

Structure of matter
Ordinary matter is made up of atoms. Atoms are made up of electrons and atomic nuclei. According to the current state of science, electrons are real elementary particles, i.e. they have no structure, they cannot be distinguished from points. The electrons are the most important representatives of the family of matter called leptons.
Atomic nuclei consist of protons and neutrons, both of which consist of quarks, in this case up and down quarks. The quarks form the other large families of matter.

The lepton family consists of three generations, each with a massive particle and a neutrino.

Electron neutrino
Muon neutrino
Tau neutrino
There is also an antiparticle for every lepton.

The leptons only respond to gravity, weak force, and electrical force if they are charged. The leptons do not combine to form more massive particles and they have no excited states.
Muon and tauon are unstable and decay into electrons and neutrini. Because there is no lighter, more charged lepton, the electron is stable.

Neutrinos are electrically neutral and hardly interact with ordinary matter. They can easily cross the whole earth, so they are difficult to detect. They have very little, if any, mass.

Electron and electron neutrino are carriers of the property "electron number". The number of electrons is a conserved quantity, analogous to the number of muons and tauons. When a muon decays into an electron, the number of muons must be given to a muon neutrino, and since a new electron is created, an electron antineutrino must be created at the same time so that the number of electrons remains constant.

The quark family consists of three generations with two members each.

q = 2e / 3
q = 2e / 3
q = 2e / 3
q = -e / 3
q = -e / 3
q = -e / 3
Each quark comes in three different "color charges" and as antiparticles.

Quarks have never been observed individually. Quarks must always group themselves in such a way that the charge of the resulting particle has an integral multiple of the elementary charge and is color-neutral ("white").
Quarks are bound to each other by the strong force. The strong force acts on the so-called color charge, which is available in red, green and blue, as well as anti-red, anti-green and anti-blue. One can make such an analogy to the theory of colors, accordingly the theory of the strong force is called quantum chromodynamics.

Particle zoo
Those particles that are composed of quarks and therefore react to the strong force are called hadrons. There are two subgroups of hadrons: mesons and baryons.

Mesons are made up of two quarks: a quark and an antiquark. The lightest mesons are called pions. They consist of an up and an antidown (or vice versa or both). There are three pions: π0, π+ and π-. The π- is the antiparticle of π+, the neutral π0 is its own antiparticle. Pions are unstable and disintegrate after a short time.

Baryons are made up of three quarks. The lightest baryons are the proton and the neutron. The proton consists of two up and one down quark: p = uud. For the neutron we have n = udd. The neutron is stable against the strong force, but the weak interaction can convert a down quark into an up; then the neutron becomes a proton.

Both mesons and baryons have excited states, so-called resonances (as with atoms).

Radiation particles convey forces. There are four of them in the standard model:

ParticleGluonsZ0, W±photonGraviton (?)

There is also (?) The Higgs boson, which gives the particles mass.

The graviton is purely hypothetical, one has not even found gravitational waves, let alone particles.
(Virtual) photons convey electrical power. Because photons have no rest mass, the range of the electric force is infinite.
The particles W and Z convey the weak force. Because these particles are very heavy, the range of the weak force is small.
Gluons themselves carry a color charge, i.e. the force particles act on themselves. This makes the laws of force mathematically difficult and it prevents free gluons from being observed.
The search for the Higgs particle is still ongoing.

Supplements: addition

first version: October 12, 2008 / Lie.

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