QCE Physics - Unit 4 - The Standard Model
Particle Interaction Diagrams | QCE Physics
Learn QCE Physics particle interaction diagrams for electron interactions, positron interactions and neutron beta decay with conservation checks.
Updated 2026-06-15 - 4 min read
QCAA official coverage - Physics 2025 v1.3
Exact syllabus points covered
- Describe the concepts of lepton number and baryon number.
- Solve problems relating to the conservation of lepton number and baryon number in particle interactions using $B = n_b - n_{\bar{b}}$, $B = \frac{1}{3}(n_q - n_{\bar{q}})$ and $L = n_l - n_{\bar{l}}$.
- Describe electron/electron, electron/positron and neutron decay interactions using particle interaction diagrams.
- Describe how symmetry in particle interactions occurs to maintain the principles of conservation.
Particle interaction diagrams are a compact way to represent how particles interact while obeying conservation rules. In QCE Physics, the important skill is not advanced Feynman-diagram calculation. The important skill is recognising the particles, the interaction, and what must be conserved.
Original Sylligence diagram for physics particle diagram structure.
Reading an interaction diagram
A particle interaction diagram usually shows incoming particles, outgoing particles and an exchange or decay process. The exact visual convention can vary, so read the labels carefully. Do not assume every line has the same meaning unless the question defines the diagram.
The core checks are:
- total charge before equals total charge after
- total baryon number before equals total baryon number after
- total lepton number before equals total lepton number after
- the named interaction is consistent with the particles shown
Charge conservation is familiar from earlier physics. Baryon and lepton conservation are new bookkeeping rules for the Standard Model topic.
Baryon number
Baryons have baryon number $+1$. Antibaryons have baryon number $-1$. Mesons have baryon number $0$ because they contain a quark and an antiquark.
At the quark level:
$ B=\frac{1}{3}(n_q-n_{\bar{q}}) $
A proton has three quarks, so $B=+1$. A neutron also has three quarks, so $B=+1$. A meson has one quark and one antiquark, so $B=0$.
Lepton number
Leptons have lepton number $+1$ and antileptons have lepton number $-1$. Electrons, muons, tau particles and neutrinos are leptons. Positrons and antineutrinos are antileptons.
The syllabus formula is:
$ L=n_l-n_{\bar{l}} $
Some interactions also conserve lepton family number, but the standard QCE starting point is total lepton number unless the question asks for more detail.
Electron and positron interactions
Electron/electron interactions can be represented as electromagnetic interactions where the particles exchange a photon. The electrons repel because they have the same charge. Charge and lepton number are conserved.
Electron/positron interactions can include annihilation. An electron and a positron may annihilate into photons:
$ e^-+e^+\rightarrow \gamma+\gamma $
Charge is conserved because $-1+1=0$ before and photons have charge $0$ after. Lepton number is conserved because the electron contributes $+1$ and the positron contributes $-1$, giving total lepton number $0$ before and after.
Neutron beta decay
A standard neutron beta decay is:
$ n\rightarrow p+e^-+\bar{\nu}_e $
At the quark level, a down quark changes into an up quark through the weak interaction. The neutron becomes a proton, and an electron plus electron antineutrino are emitted.
Check the conservation rules:
- charge: $0=+1+(-1)+0$
- baryon number: $1=1+0+0$
- lepton number: $0=0+1+(-1)$
This is why the antineutrino matters. Without it, lepton number would not balance.
Worked example
Symmetry and conservation
When the syllabus refers to symmetry in particle interactions, it is pointing to the idea that conservation laws reflect deeper symmetries. In practical exam terms, you usually show this by demonstrating that the totals before and after the interaction remain the same.