Particles

• Matter is composed of atoms, but within the atoms there are sub atomic particles. Therefore, all matter is composed of quarks and leptons.
• Leptons

  • electron (e-)
  • electron neutrino (ve)
  • up (u)
  • down (d)
• Charge (e)

  • -1
  • 0
  • +
  • -
• This table of information will be given on the data booklet, but it is important to know how to use this information.
• Note that these are the first generation of leptons and quarks, there are three generations but questions will only be asked about the first.
• Antiparticles

  • Antielectron or positron (e+)
  • Antielectron neutrino (ve)
  • Antiup (u)
  • Antidown (d)
• Each antiparticle has the same properties as its equivalent particle except opposite charge.
• When a particle and its antiparticle meet, they annihilate each other, often energy in the form of photons, γ.
• Leptons always exist separately.
• Lepton properties

  • Charge: electrons have a charge of -1e, neutrinos have no charge.
  • Lepton number: leptons have lepton number = 1, antileptons = -1
• Fundamental forces

  • Gravitational (experienced by all matter, infinite range, very weak)
  • Weak (experienced by all leptons and quarks, very short range, only significant when e-m and strong do not operate)
  • Electromagnetic (e-m) (experienced by all charged particles, infinite range, also experienced by neutral hadrons)
  • Strong (experienced by all quarks, short range)
• In all interactions there are conservations laws that must be true: Conservation of charge, Conservation of baryon number, Conservation of lepton number.
• Strong interactions

  • Only involve hadrons
  • No change in quark flavour (the same number of u and d quarks before and after)
  • Typically involved in collision between particles
• Electromagnetic interactions

  • The particles must be charged or have charged components
  • No change in quark flavour
  • One or more photons may be emitted
• Weak interactions

  • Neutral leptons are involved
  • There may be a change in quark flavour
• Quarks don't exist in isolation; they are always bound into particles made up of more than one quark. They are known as hadrons and there are 3 types.
• Hadrons

  • Baryon – combination of 3 quarks. For example, a neutron.
  • Antibaryon – combination of 3 antiquarks. For example, an antiproton.
  • Meson – 1 quark – antiquark pair.
• All quarks have baryon number = + 1/3 and all antiquarks have baryon number = - 1/3. This means that all baryons must have baryon number = 1 and antibaryons = - 1.
• Baryons

  • proton (uud, charge +1e, baryon number 1)
  • neutron (udd, charge 0, baryon number 1)
• You must also be able to work out the quark made up of less familiar baryons, for example Δ++ = uuu.
• Mesons

  • π0 (uu, charge 0)
  • π- (ud, charge -1)
  • π+ (ud, charge +1)
  • π0 (dd, charge 0)
• As they are made of quark antiquark pairs their baryon number = 0.