P2 quarks + leptons

• the muon is a heavy electron, represented by $\mu$
• a muon is negatively charged, an antimuon is positively charged
• muons decay into an electron and an antineutrino

  antimuons decay into a positron and a neutrino
• the pion can be positive, negative or neutral, represented by $\pi$
• charged pions decay into a muon and an antineutrino, or an antimuon and a neutrino

  neutral pions decay into high-energy photons
• the kaon can be positive, negative or neutral, represented by $K$
• kaons decay into two pions, or a muon and an antineutrino, or an antimuon and a neutrino
• all particles are either hadrons or leptons, depending on whether they are fundamental
• hadrons are made up of quarks, and can interact through all four fundamental interactions

  example of hadrons include protons, neutrons, pions and kaons
• leptons are fundamental, and can interact through three fundamental interactions but not the strong nuclear interaction

  example of leptons include electrons, neutrinos, and muons
• the proton is stable, but apart from that the hadrons tend to decay through the weak nuclear interaction
• hadrons can be divided into two groups, baryons and mesons
• baryons are protons and all other hadrons that decay into protons, directly or indirectly

  examples of baryons include neutrons and protons
• mesons are hadrons that do not decay into protons, directly or indirectly

  examples of mesons include pions and kaons
• none of the leptons have many important reactions with each other because neutrinos interact very little, muons are very short-lived and electrons repel each other
• leptons and antileptons can interact to produce hadrons
• a muon neutrino is represented by $v_\mu$

  an electron neutrino is represented by $v_e$
• lepton number is assigned to all particles:

  • hadrons have a lepton number of 0
  • leptons have a lepton number of 1
  • antileptons have a lepton number of -1
• in an interaction between a lepton and a hadron, a neutrino or antineutrino can change into or from a charged lepton
• in muon decay, the muon changes into a muon neutrino, an electron is created to conserve charge, and an electron antineutrino is created to conserve lepton number
• the equation for muon decay is:

  muon -> electron + electron antineutrino + muon neutrino
• baryons are formed from 3 quarks

  antibaryons are formed from 3 antiquarks

  mesons are formed from a quark and an antiquark
• strangeness is a property possessed by some particles

  strange particles are produced through the strong interaction but decay through the weak nuclear interaction
• kaons are strange particles, they decay into pions through the weak interaction
• strangeness must be conserved in particle interactions which involves the strong nuclear interaction, so strange particles must be created in pairs
• strangeness does not need to be conserved in particle interactions which involve the weak nuclear interaction, it can change by 0, -1 or +1
• in order to investigate particle physics, we build particle accelerators, these are very expensive to build and run, and produce huge amounts of data, so rely on collaboration of scientists internationally
• there are 3 types of quark that you need to know: up, down and strange
• up quarks have the following properties:

  • charge of 2/3 e
  • baryon number of 1/3
  • strangeness of 0
• down quarks have the following properties:

  • charge of -1/3 e
  • baryon number of 1/3
  • strangeness of 0
• strange quarks have the following properties:

  • charge of -1/3 e
  • baryon number of 1/3
  • strangeness of -1
• the quark combination for a proton is $uud$
• the quark combination for an antiproton is $\overline {uu} \overline {d}$
• the quark combination for a neutron is $udd$
• the quark combination for an antineutron is $\overline {u} \overline {dd}$
• the quark combination for a $\pi ^0$ meson is $u \overline {u}$or $d \overline {d}$or $s \overline {s}$
• the quark combination for a $\pi ^+$meson is $u \overline {d}$
• the quark combination for a $\pi^-$meson is $d \overline{u}$
• the quark combination for a $K^0$meson is $d \overline {s}$
• the quark combination for a $K^+$meson is $u \overline {s}$