Quarks and Leptons

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2.1 The Particle Zoo

  • Cosmic rays are fast-moving protons or small nuclei. They collide with gas atoms in the atmosphere, creating showers of particles and antiparticles.
  • The Muon (heavy electron) is a negatively charged particle, with a rest mass 200x greater than an electron. 
  • The Pion (pi meson) is a particle that can be +ve,-ve or neutrally charged. It has a rest mass greater than a muon but less than a proton.
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2.1 The Particle Zoo (cont)

  • The Kaon (K meson) can also be +ve, -ve or neutral. It has a rest mass greater than a pion, but still less than a proton
  • Pions and Kaons are both produced through the strong interaction (collision), but decay through the weak interaction.
  • K mesons decay into pi mesons, muons and antineutrinos, and antimuons and neutrinos.
  • Charged pi mesons decay into muons and antineutrinos, or antimuons and neutrinos. The neutral pi mesons decay into high energy photons.
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2.1 The Particle Zoo (cont)

  • Muons and antimuons decay into electrons and antineutrinos, or positrons and neutrinos
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2.2 Particle Sorting

  • We clasify particles and antiparticles in two groups: Hadrons and Leptons.
  • Leptons interact through the weak interaction.
  • Hadrons interact though the strong interaction. Apart from the proton (stable), hadrons decay through the weak interaction.
  • All particles that are charged interact through the elecromagnetic interaction.
  • All particles with mass interact through the gravitational interaction. 
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Particle Sorting (cont)

  • When 2 particles collide, their combined energy before the collision = their rest energy + their kinetic energy.
  • The total energy of the particles/antiparticles created = their rest energy + their kinetic energy.
  • Therefore, the rest energy of products = total energy before - kinetic energy of products
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Particle Sorting (cont)

  • Baryons are protons and all other hadrons that decay into protons (directly or indirectly).
  • Mesons are hadrons that do not include protons in their decay products (eg K mesons and pi mesons).
  • Both baryons and mesons are hadrons because they are created through the strong interaction
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Leptons at Work

  • Neutrinos and antineutrinos produced by beta decays are different to those produced by muon decays.
  • Muon neutinos/antineutrinos only create muons/antimuons when they collide with hadrons (no electrons).
  • This is the opposite for electron neutrinos/antineutrinos. 
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Leptons at Work (cont)


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Leptons at Work (cont)

  • Leptons change into other leptons by the weak interaction.
  • They are the fundimental elements of matter (can't be broken down any further).
  • We have to ensure that the lepton number of an interaction is equal on both sides. However, we characterise muon leptons and electron leptons as different, and so count them seperately.
  • Electron lepton number and muon lepton number must be equal on both sides of an interaction. 
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Leptons at Work (cont)

  • In muon decay, a muon turns into a muon neutrino. In addition, an electron and an electron antineutrino are created to conserve charge and resulting lepton number.
  • A muon can only ever change into a muon neutrino, never a muon antineutino. 


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Quarks and Antiquarks

  • K mesons and sigma particles are strange particles.
  • They both decay through the weak interaction.
  • When these particles interact with other particles, they interact through the strong interaction
  • Certain reactions therefore cannot happen because strangeness is not conserved
  • Strangeness is always conserved in a strong interaction.
  • When a pi meson collides with a proton, a k meson and a sigma particle are created. 
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Quarks and Antiquarks (cont)

  • Hadrons are composed of smaller particles known as quarks and antiquarks. The quark composition of a hadron is what gives it its charge, strangeness and rest mass.
  • There are 3 different types of quark: 'up', 'down', and 'strange'
  • Each quark has its own charge and strangeness.
  • The antiquarks have the opposite charge and strangeness to the corresponding quarks. 
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Quarks and Antiquarks (cont)

  • Mesons consist of a quark and an antiquark.
  • Because K mesons are strange particles, they must contain a strange quark, and another quark to complete its charge.
  • Pi mesons consist of an up/down quark/antiquark composition. The neutrally charged pi meson can be a combination of any quark and its antiquark.
  • Baryons and antibaryons consist of 3 quarks or antiquarks respectively. Proton: uud, neutron: udd
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Quarks and Antiquarks (cont)


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Conservation Rules

  • The following must be conserved in all interactions and decays:
  • Baryon number
  • Electron lepton number
  • Muon lepton number
  • Charge
  • Energy and momentum
  • Remember in a strong interaction strangeness is always conserved.
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