Particle physics

• Atom


  Has a small nucleus located in the center containing protons and neutrons, with electrons orbiting the nucleus
• Proton


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


  • No charge
  • 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 (binds nucleus)
• Alpha decay


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

  2. Decreases proton and nucleon numbers by 2
• Beta minus decay


  1. Neutron turns into proton, emits electron and antineutrino

  2. Proton number increases by 1, nucleon number stays constant
• Antiparticle


  Particle with same mass but opposite charge as the corresponding particle
• Antiparticle properties


  • Same mass as corresponding particle
  • Opposite charge
• 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
• Photon energy


  Proportional to frequency, given by E = hf
• Annihilation


  Particle and antiparticle meet and their mass is converted to photon energy
• Photon energy from annihilation

  Equals twice the rest energy of the annihilating particles
• The maximum wavelength of the annihilation photons is given by the rest energy of the annihilating particles
• The minimum energy we can use is the energy conservation and we can estimate the wavelength of these photons because this is the minimum energy, the wavelength will be a maximum
• Rest energy of the electron and positron

  Typically given in the question, around 0.511 Mega electron volts
• Calculating the maximum wavelength


  1. Input energy = 2 x 0.511 Mega electron volts

  2. Energy afterwards = 2 x energy of a photon

  3. Rearrange for wavelength

  4. Plug in values for H and C
• The opposite of annihilation is known as pair production
• Minimum energy for pair production

  Energy of the photon must be at least twice the rest energy of the particles
• Fundamental forces


  • Electromagnetic force
  • Weak nuclear force (responsible for nuclear decay)
  • Strong nuclear force (holds the nucleus together)
  • Gravity
• Gravity is considerably weaker than the other three fundamental interactions and is often ignored in particle physics
• Virtual photon


  The exchange particle that carries the electromagnetic interaction
• Exchange particles for the fundamental forces are known as gauge bosons
• Virtual particles


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

  1. Two electrons repelling

  2. Virtual photon exchanged between them
• Feynman diagram for beta plus decay

  1. Proton turns into neutron, positron, and neutrino

  2. W+ boson emitted and decays
• Feynman diagram for beta minus decay

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

  2. W- boson emitted and decays
• Feynman diagram for electron capture

  1. Proton captures electron, turns into neutron and neutrino

  2. W+ boson emitted and decays
• Hadrons


  Particles affected by the strong nuclear interaction
• Baryons


  Hadrons with three quarks
• Mesons


  Hadrons with a quark and an antiquark
• Baryon number


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