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Quark Composition (CIE AS Physics)

Revision Note

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Katie M

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Quark Composition: Protons & Neutrons

  • Protons and neutrons are not fundamental particles. They are each made up of three quarks
  • Protons are made up of two up quarks and a down quark
  • Neutrons are made up of two down quarks and an up quark

 

Quarks in proton and neutrons, downloadable AS & A Level Physics revision notes

Protons and neutrons are made up of three quarks

 

  • You will be expected to remember these quark combinations for exam questions

 

Worked example

WE - Quarks in an element question image, downloadable AS & A Level Physics revision notes

  • Step 1: Calculate number of protons:
    • The number of protons is from the proton number = 26 protons

  • Step 2: Calculate number of neutrons:
    • The number of neutrons = nucleon number - proton number = 56 - 26 = 30 neutrons

  • Step 3: Up quarks in a proton:
    • Protons are made up of uud quarks = 2 up quarks

  • Step 4: Up quarks in a neutron:
    • Neutrons are made up of udd quarks = 1 up quark

  • Step 5: Total number of up quarks:
    • 26 protons x 2 up quarks = 52 up quarks
    • 30 neutrons x 1 up quark = 30 up quarks
    • 52 + 30 = 82 up quarks

Baryons & Mesons

  • Hadrons are the group of subatomic particles that are made up of quarks
  • These may be either a:
    • Baryon (3 quarks)
    • Meson (quark and anti-quark pair)

 

Baryons & Mesons

Hadrons may be either a baryon or a meson

  • Quarks have never been discovered on their own, always in pairs or groups of three
  • Anti-hadrons can be either
    • Anti-baryons (3 anti-quarks)
    • Anti-meson (quark and anti-quark pair)

 

Anti-hadrons

Anti-hadrons may be either an anti-baryon or an anti-meson

  • Note that all baryons or mesons have integer (whole number) charges eg. +1e, -2e etc.
  • This means quarks in a baryon are either all quarks or all anti-quarks. Combination of quarks and anti-quarks don’t exist in a baryon
    • e.g.

      Wrong quark composition, downloadable AS & A Level Physics revision notes

  • The anti-particle of a meson is still a quark-antiquark pair. The difference being the quark becomes the anti-quark and vice versa

Worked example

The baryon Δ++ was discovered in a particle accelerator using accelerated positive pions on hydrogen targets.Which of the following is the quark combination of this particle?

Worked example - Baryons and mesons, downloadable AS & A Level Physics revision notes

Examiner Tip

  • Remembering quark combinations is useful for the exam. However, as long as you can remember the charges for each quark, it is possible to figure out the combination by making sure the combination of quarks add up to the charge of the particle (just like in the worked example)

Quark Composition: β– & β+ decay

  • Beta decay happens via the weak interaction
    • This is one of the four fundamental forces and it’s responsible for radioactive decays

Quark Composition: β- decay

  • Recall that β- decay is when a neutron turns into a proton emitting an electron and anti-electron neutrino
  • More specifically, a neutron turns into a proton because a down quark turning into an up quark

Beta minus decay quarks, downloadable AS & A Level Physics revision notes

Beta minus decay is when a down quark turns into an up quark

Quark Composition: β+ decay

  • Recall that β+ decay is when a proton turns into a neutron emitting an positron and an electron neutrino
  • More specifically, a proton turns into a neutron because an up quark turns into a down quark

 

Beta plus decay quarks, downloadable AS & A Level Physics revision notes

Beta minus decay is when an up quark turns into a down quark

Worked example

The equation for β decay is2.3.3 Beta Minus EquationUsing the quark model of beta decay, prove that the charge is conserved in this equation.

Worked example - beta decay quarks, downloadable AS & A Level Physics revision notes

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.