Particle physics

• Proton


  • Charge of +1.6 x 10^-19 C
  • Mass of ~1.67 x 10^-27 kg
• Neutron

  • Charge of 0 C
  • Mass of ~1.67 x 10^-27 kg
• Electron


  • Charge of -1.6 x 10^-19 C
  • Mass of ~9.11 x 10^-31 kg
• Proton number (Z)


  Number of protons in the nucleus, defines the chemical element
• Nucleon number (A)


  Total number of protons and neutrons in the nucleus
• Isotopes have the same number of protons but different numbers of neutrons
• Specific charge


  Ratio of the charge of a particle divided by its mass, measured in C/kg
• The specific charge of the proton is ~9.6 x 10^7 C/kg
• Nuclear forces


  • Gravity (weak)
  • Electrostatic repulsion (large)
  • Strong nuclear force (glues nucleus together)
• Alpha decay


  1. Nucleus emits an alpha particle (2 protons, 2 neutrons)

  2. Decreases proton number by 2, decreases nucleon number by 4
• Beta minus decay


  1. Neutron turns into proton, emits electron and antineutrino

  2. Increases proton number by 1, nucleon number stays the same
• Antiparticle


  Has the same mass but opposite charge as the corresponding particle
• Antiparticles


  • Electron has positron as antiparticle
  • Proton has antiproton
  • Neutron has antineutron
• Rest energy


  Energy of a particle at rest, measured in MeV
• Electron/positron rest energy is 0.511 MeV, proton/antiproton is 938 MeV, neutron/antineutron is 939 MeV
• Photon


  Fundamental particle of electromagnetic radiation, with energy proportional to frequency
• Annihilation


  When a particle and antiparticle meet, their mass is converted to photon energy
• Photon energy


  Equals Planck's constant times the frequency
• The maximum wavelength of photons produced in annihilation is determined by the rest energy of the annihilating particles
• The minimum energy for pair production is that the energy of the photon has to be equal to at least twice the rest energy of the particles
• Minimum energy for pair production (e_Min)

  Equal to twice the rest energy of the particles (2*e_0)
• Minimum energy for pair production (e_Min)


  Can also be calculated as e_Min = hf = hc/λ
• Fundamental forces


  • Electromagnetic force
  • Weak nuclear force (responsible for nuclear decay)
  • Strong nuclear force (holds nucleus together)
  • Gravity
• Gravity is considerably weaker than the other three fundamental interactions and is often ignored in particle physics
• There is no quantum theory of gravity yet, which is one of the holy grails of physics
• Exchange particle/Gauge boson


  • Virtual photon for electromagnetic interaction
  • W+ and W- bosons for weak nuclear force
  • Pions for strong nuclear force
• The electromagnetic interaction is carried by the exchange of virtual photons
• The weak nuclear force is carried by the exchange of W+ and W- bosons
• The strong nuclear force is carried by the exchange of pions
• Gravity does not yet have a known exchange particle, the hypothetical particle is called the graviton
• Virtual particles


  Real particles that exist for a very short time
• Feynman diagram for electromagnetic repulsion

  1. Two electrons repel each other

  2. Mediated by exchange of virtual photon
• Feynman diagram for beta plus decay

  1. Proton turns into neutron, positron, and neutrino

  2. Mediated by exchange of W+ boson
• Feynman diagram for beta minus decay

  1. Neutron turns into proton, electron, and anti-neutrino

  2. Mediated by exchange of W- boson
• Feynman diagram for electron capture

  1. Proton captures electron, turns into neutron and neutrino

  2. Mediated by exchange of W+ boson
• Hadrons


  Particles affected by strong nuclear interaction
• Baryons


  Hadrons with 3 quarks
• Mesons


  Hadrons with quark-antiquark pair
• Baryon number


  Quantum number conserved in reactions, baryons have B=1, mesons have B=0
• Baryons are generally unstable, except for the proton