# 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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## 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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