Quarks, leptons and their antiparticles

Book pg 295 - 298

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1. Leptons • Influenced by weak force

• Can travel on their own – they are not trapped inside larger particles

• 6 distinct types called flavour and their antiparticles

• 3 generations of leptons

• Identical spin of + 1

2

• Charge of +1, 0, -1

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1st generation – ordinary matter

• Electron and its antiparticle positron

• Neutral neutrino 𝑣 and its antiparticle antineutrino 𝑣

• Interact with - weak nuclear force - gravitational force - if charged, with EM force

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2nd generation

• Leptons existed in early moments of Big Bang

• They exist in cosmic rays and particle accelerator

• Muon

• Muon neutrino

• Both antiparticles, heavier than the electron

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3rd generation

• Found in particle collider, but not in nature

• Tau

• Tau neutrino

• Both particles are heavier than muon, ~𝑚𝑝

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Facts

• Muon: unstable

• Decays into electron

• Decays into electron antineutrino

• Decays into muon neutrino

• Decays about every 2.2𝜇𝑠

• Tau: unstable

• Decays every 3 × 10−13𝑠

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Summary

1st 2nd 3rd

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2. Quarks • They can never be found

in isolation - trapped inside hadrons - six quarks and six antiquarks

• Quarks are labeled by their flavour: - up (u) - down (d) - strange (s) - charm (c) - bottom (b) - top (t)

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Quarks • Quarks carry a charge of either +

2

3 e or -

1

3 e

• Antiquarks carry a charge of either - 2

3 e or +

1

3 e

• Quarks are split into 3 generations of increasing mass

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Quarks generations

1st generation

• Up and down quarks

• Lightest

• Form nucleons

3rd generation

• Bottom and top quarks

• heaviest

2nd generation

• Strange and charm quarks ©

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Remember: Charge of Quarks

Charge + 2

3 Charge -

1

3

Charge - 2

3

Charge + 1

3

u d 𝑢 𝑑

c s 𝑐 𝑠

t b 𝑡 𝑏

Quarks Antiquarks

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Quarks confinement • Quarks exist in hadrons

• Hadrons are formed from a combination of 2 or 3 quarks

• Mesons – 2 quarks

• Baryons – 3 quarks

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Quantum chromodynamics QCD • Explains quark confinement • To hold quarks in place they

exchange gluons (strong nuclear force)

• Moving quarks away from its neighbors stores more energy in the interaction between quarks

• To move a quark away from neighbor increasing amount of energy needed

• Instead of using energy to separate quarks, more quarks are produced

• Original quarks are left unchanged, but more mesons or baryons are created by 𝐸 = 𝑚𝑐2

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Collisions in particle accelerator produces…

• 3 colored baryons

• Color and anti-colour mesons

• Color is a new property for strong interaction

• A particle of 4 quarks has never been found

• Only colour neutral combinations can be found

• Quark – anti-quark pairs are formed

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The color force From Pauli exclusion principle

• Two identical particles with identical set of quantum numbers cannot occur together in the same atomic or subatomic system

Problem:

• Quarks all have a spin of + 1

2

• Quarks with identical flavors exist in the same particle

• Example: proton • Two u quarks, one d quark • Omega Ω – baryon has 3 s quarks

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Sheldon Glashow: colour

Colour

• Nothing to do with colour in the ordinary sense of the world

• Colour keeps quarks in different quantum states to avoid violation of Pauli’s exclusion principle

Glashow proposed

• Every quarks and antiquark had one of three possible values of this property, which he called red, green and blue

• 3 quarks making up baryons all have different colour, making the baryon colourless

• The 3 quarks in Ω all have different colour, making each baryon colourless

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Result:

• 9 quarks and 9 antiquarks

• Gluons are the exchange particles in quarks

• They are responsible for colour change

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Example • Quarks in neutron uud • Prior to gluon exchange • u quarks has exchanged

gluon with neighbouring d quark

• Colours are permuted • 1st and 2nd quark have

exchanged a gluon • Colours are permuted • One quark of each colour

present at all times

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