ASPhysicsCIERevision Notes11. Particle Physics11.2 Fundamental ParticlesThe Quark Model of Beta Decay

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First teaching 2023

First exams 2025

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The Quark Model of Beta Decay (CIE AS Physics)

Revision Note

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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 turns into an up quark

Beta minus decay

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

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 is

n space rightwards arrow space p space plus space e to the power of minus space plus space stack v subscript e with bar on top

Using the quark model of beta decay, prove that the charge is conserved in this equation.

Answer:

Step 1: Recall the process of beta minus decay

  • β− decay is when a down quark changes to an up quark
    • This changes a neutron into a proton

Step 2: Determine the charge on the left hand side of the equation

  • The quark composition of a neutron is udd
  • Adding the quark charges gives:

 plus 2 over 3 minus 1 third minus 1 third space equals space plus 0

  • The left side of the equation has a charge of 0

Step 3: Determine the charge on the right hand side of the equation

  • The quark composition of a proton is uud
  • Adding up the quark charges gives:

plus 2 over 3 plus 2 over 3 minus 1 third space equals space plus 1

  • The electron has a charge of −1
  • The anti-neutrino has a charge of 0
    • Therefore, the right hand side of the equation has a charge of:

plus 1 minus 1 plus 0 space equals space 0

Step 4: Prove that the charge is conserved

  • Since charge is equal on both sides, charge is conserved in the beta decay equation

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    Author: Leander

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    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.