Cambridge A-Level Physics 9702 – Fundamental Particles
Fundamental Particles
In the Standard Model of particle physics, matter is built from a small set of indivisible particles called fundamental particles. These particles are not known to have any sub‑structure.
Leptons
Leptons form one of the two families of fundamental fermions (the other family being quarks). They do not experience the strong nuclear force. The six leptons are grouped into three generations, each containing a charged lepton and a neutral neutrino.
Lepton
Charge (\$e\$)
Lepton Number (\$L\$)
Mass (MeV/\$c^2\$)
Electron (\$e^-\$)
\$-1\$
\$+1\$
0.511
Electron neutrino (\$\nu_e\$)
\$0\$
\$+1\$
\$<10^{-6}\$ (very small)
Muon (\$\mu^-\$)
\$-1\$
\$+1\$
105.7
Muon neutrino (\$\nu_\mu\$)
\$0\$
\$+1\$
\$<10^{-4}\$ (very small)
Tau (\$\tau^-\$)
\$-1\$
\$+1\$
1776.9
Tau neutrino (\$\nu_\tau\$)
\$0\$
\$+1\$
\$<10^{-3}\$ (very small)
Key Points to Remember
Electrons (\$e^-\$) and neutrinos (\$\nu\$) are both fundamental particles; they are not composed of smaller parts.
Electrons carry a negative electric charge of \$-1e\$, whereas neutrinos are electrically neutral.
Both electrons and neutrinos belong to the lepton family and have a lepton number of \$+1\$.
Neutrinos interact only via the weak nuclear force and gravity, making them extremely difficult to detect.
The three generations of leptons differ mainly in mass; the charge and lepton number are the same within each generation.
Why Electrons and Neutrinos Are Classified as Leptons
The classification is based on the following criteria:
No colour charge: Leptons do not carry the colour charge required for the strong interaction.
Spin \$\\frac{1}{2}\$: All leptons are fermions with half‑integer spin.
Conserved lepton number: In any reaction, the total lepton number (sum of lepton numbers of all particles) remains constant.
Family grouping: Each charged lepton is paired with its corresponding neutrino, forming a generation.
Suggested diagram: A schematic showing the three generations of leptons, with electrons and neutrinos highlighted.
Typical Exam Question
Explain why the electron and the electron neutrino are both considered fundamental particles, and describe one key difference in their interactions.
Answer outline:
Both have no known sub‑structure; they are point‑like in experiments.
Both belong to the lepton family with lepton number \$+1\$.
Key difference: The electron participates in electromagnetic interactions because it carries charge, while the neutrino does not interact electromagnetically and only participates in weak interactions.