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Quarks and Leptons (Conservation rules (• Particles and antiparticles…
Quarks and Leptons
Cosmic rays
Space invaders
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• Cosmic rays are fast moving protons or small nuclei which collide with gas atoms creating showers of particles and antiparticles
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Strange particles
• Pions and kaons produced in twos through the strong interaction, however decay of kaons took longer than expected and also included pions in products
• Means that kaons decay through weak interaction – one of the reasons why they are called strange particles
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Decays of muons, pions and kaons
• Kaon decays into pions, or muon and antineutrino, or antimuon and neutrino
• Charged pion decays into muon and antineutrino, or antimuon and neutrino
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Particle sorting
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Baryons and mesons
• When kaons created, short-lived particles with greater rest masses than protons produced
• These particles created through strong interaction, so are hadrons
• However, they decay into protons as well as pions, which kaons cannot
• Baryons are protons and all other hadrons (including neutrons) which decay into proton, directly or indirectly
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• Baryons and mesons composed of smaller particles called quarks and antiquarks, so hadrons aren’t fundamental
Leptons
Lepton collisions
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• These move away in opposite directions, producing a shower of hadrons in each direction
Neutrino types
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• Neutrinos and antineutrinos produced by muon decay only create muons and not electrons when they interact with protons and neutrons
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Lepton rules
• Leptons can change into other leptons through the weak interaction and can be produced in annihilation interactions
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• In interaction between lepton and hadron, neutrino or antineutrino can change into corresponding charged lepton
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• Lepton number must always be conserved (antileptons have lepton number of -1), so electron neutrino and neutron cannot become proton and electron and lepton number not conserved
• Muon decay, muon decays into muon neutrino + electron (conserve charge) + muon antineutrino (conserve lepton number)
• Lepton number is 1 for any lepton, -1 for any antilepton and 0 for any non-lepton
Quarks and antiquarks
Strangeness
• Although strange particles all decay through weak interaction,
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• Others, such as Σ found to have different rest masses which always greater than protons rest mass, and decay either in sequence or directly into protons and pions
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Quark model
• Properties of hadrons such as charge, strangeness and rest mass cam be explained by assuming they are composed of smaller particles – quarks and antiquarks
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• 3 types we need to know – up (u), down (d) and strange (s)
Types of quarks
• Up – charge + 2/3, strangeness 0, baryon number + 2/3
• Down – charge – 1/3, strangeness 0, baryon number + 1/3
• Strange – charge – 1/3, strangeness -1, baryon number + 1/3
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Conservation rules
• Particles and antiparticles possess energy, can be charged or uncharged, may have non-zero strangeness and may not be stable
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