Chapter 2 - Quarks and leptons
2.1 - The particle zoo
Space Invaders
the muon or heavy electron, a negatively charged particle with rest mass of over 200 times the rest mass of the electron.
the pion, a particle which can be positively charged, negatively charged, or neutral, and has a rest mass greater than a muon but less than a proton.
the kaon, which also can be positively charged, negatively charged, or neutral, and has a rest mass greater than a pion but still less than a proton.
A strange puzzle
A kaon can decay into pions, or a muon and an antineutrino, or an antimuon and a neutrino
A charged pion can decay into a muon and an antineutrino, or an antimuon and a neutrino. A neutral pion decays into high-energy photons.
A muon decays into an electron and an antineutrino. An antimuon decays into a positron and a neutrino
2.2 - Particle sorting
Classifying particles and antiparticles
Hadrons can interact through all four fundamental interactions. They interact through the strong interaction and through the electromagnetic interaction if charged. Apart from the proton, which is stable, hadrons tend to decay through the weak interaction
Leptons interact through the weak interaction, the gravitational interaction, and through the electromagnetic interaction(if charged).
the rest energy of the products = total energy before - the kinetic energy of their products
Baryons and Mesons
Baryons are protons and all other hadrons that decay into protons, either directly or indirectly.
Mesons are hadrons that do not include protons in their decay products. In other words, kaons and pions are not baryons.
Baryons and mesons are composed of smaller particles called quarks and antiquarks.
2.3 - Leptons at work
Lepton collisions
Neutrinos interact very little, muons are very short-lived, and the electrons would repel each other. However, leptons and antileptons can interact to produce hadrons.
Neutrino types
Neutrinos and antineutrinos from muon and antimuon decays create only muons and no electrons when they interact with protons and neutrons. If there were only one type of neutrino and antineutrino, equal numbers of electrons and muons would be produced. So there is the muon neutrino and electron neutrino and their corresponding antineutrinos.
Lepton rules
In an interaction between a lepton and a hadron, a neutrino and an antineutrino can change into or from a corresponding charged lepton. An electron neutrino can interact with a neutron to produce a proton and an electron.
In muon decay, the muon changes into a muon neutrino. In addition, an electron is created to conserve charge and a corresponding antineutrino is created to conserve lepton number.
Lepton number must always be conserved in all interactions
2.4 - Quarks and antiquarks
Strangeness
All strange particles decay through to weak interaction, those that decay into pions only were referred to as kaons, the others such as sigma particle were found to have different rest masses and to decay either in sequence or directly into protons and pions. Strange particles are created in twos.
Quark combinations
Mesons are hadrons, each consisting of a quark and an antiquark
Baryons and antibaryons are hadrons that consist of three quarks for a baron or three anti quarks for an antibaryon.
A proton is uud
A neutron is udd
Quarks and beta decay
In a beta decay, a neutron in a neutron-rich nucleus changes into a proton, releasing an electron and an electron antineutrino. In quark terms, a down quark changes to an up quark.
In positron decay, a proton in a proton-rich nucleus changes into a neutron, releasing a positron and an electron neutrino. In quark terms, an up quark changes to a down quark.
2.5 - Conservation rules
Particles and properties
Conservation of energy and conservation of charge apply to all changes in science, not just to all particles and antiparticle interactions and decays.
Conservation rules used only for particle and antiparticle interactions and decays are essentially particle-counting rules, based on what reactions are observed and what reactions are not observed .
Conservation of lepton numbers. In any change, the total lepton number for each lepton branch before the change is equal to the total lepton number for that branch after the change.
Conservation of strangeness. In any strong interaction, strangeness is always conserved.
Are baryons and mesons conserved?
quarks are assigned a baryon number of 1/3, antiquarks are assigned a baryon number of -1/3 and leptons are assigned a baryon number of 0.
In any reaction , the total baryon number is conserved.