The Quark Model of Beta Decay | CIE A Level Physics Revision Notes 2025

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

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

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

The equation for β^– decay is

$n \to p + e^{-} + \bar{v_{e}}$

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

Answer:

β− decay is when a down quark changes to an up quark
- This changes a neutron into a proton
The charge on the left hand side of the equation:
- The quark composition of a neutron is $udd$
- Adding the quark charges gives $+ \frac{2}{3} - \frac{1}{3} - \frac{1}{3} = + 0$
  - The left side of the equation has a charge of 0
The charge on the right hand side of the equation:
- The quark composition of a proton is $uud$
- Adding up the quark charges gives $+ \frac{2}{3} + \frac{2}{3} - \frac{1}{3} = + 1$
- 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 $+ 1 - 1 + 0 = 0$
Since charge is equal on both sides, charge is conserved in the beta decay equation