Quarks and Leptons

Quarks and Leptons

1. Particle zoo

   • Cosmic rays are high energy particles that travel through space
     • When they enter the atmosphere, they collide with gas particles creating short-lived particles and antiparticles
       • Using cloud chambers and other detectors, particles such as the muon, pion and kaon were discovered
     • Most cosmic rays are fast-moving protons or small nuclei
   • Decays
     • 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 muon decays into an electron and an antineutrino
     • Decays alwas conserve energy, momentum and charge

2. Particle sorting

   • The new particles were sorted into two groups, hadrons and leptons
     • Hadrons are particles that can interact via the strong interaction
       • Can decay through all 4 fundamental interactions
       • Hadrons can be further divided into two groups, baryons and mesons
         • Baryons are protons and all other hadrons that decay into protons
         • Mesons are hadrons that do not include protons in their decay products
         • Both Baryons and mesons are composed of smaller particles called quarks and antiquarks
     • Leptons are particles that do not interact via the strong interaction
       • Can decay through all of the fundamental interactions except for the strong interaction

3. Leptons at work

   • When a lepton and its corresponding antilepton collide, they can produce a quark and its corresponding antiquark producing a shower of hadrons
   • Neutrinos and antineutrinos in beta decay are different to those in muon decays as different amounts of electrons and muons are produced
     • We call the types muon neutrinos and electron neutrinos
   • In an interaction between a lepton and a hadron, a neutrino or antineutrino can change into a corresponding charged lepton
   • In muon decay, a muon neutrino is formed, an electron is formed to conserve charge and a antineutrino is formed to conserve lepton number

4. Quarks and antiquarks

   • When kaons were first discovered, they were called V particles because of the tracks. They were later changed to strange particle because they decay into just pions, or pions and protons
     • Those that decay into pions are referred to as kaons
     • The others were found to have different rest masses which was always greater than the protons rest mass and decay into protons and pions
   • Mesons consist of a quark and an antiquark
   • Baryons consist of three quarks and three antiquarks for an antibaryon

5. Conservation rules

   • Energy must be conserved
   • In any change, the total lepton number in each branch must be conserved
   • Strangeness must be conserved in any strong interaction
   • Baryon number must be conserved