Leptons & Fundamental Particles
Leptons & Fundamental Particles
- Not every particle is made of quarks. The other family is the leptons. And some particles cannot be split at all.
Fundamental and composite particles
Definition
1 markWhat is meant by fundamental particle?
Model answer: A particle that cannot be subdivided into smaller particles.
- A cannot be split into anything smaller. As far as we know, quarks and electrons are fundamental.
- A proton is not fundamental, because it is made of three quarks. We say it is composite. The mark scheme wants the exact idea: a fundamental particle cannot be subdivided. (9702/22/O/N/25 Q6(a))
Hadrons and leptons
Definition
1 markWhat is meant by lepton?
Model answer: A fundamental particle that is not made of quarks.
- Particles split into two families, by whether they are built from quarks.
- Hadrons are made of quarks. Protons and neutrons are hadrons.
- are fundamental and are not made of quarks. The electron is a lepton, and so is the neutrino (the small, chargeless particle in beta decay). (9702/22/F/M/24 Q8)
- So the electron and the proton are in different families. The electron is a fundamental lepton. The proton is a composite hadron. A positron is also a lepton (it is the antiparticle of the electron). (9702/11/O/N/25 Q40)
Made of quarks → hadron (proton, neutron). Fundamental, no quarks → lepton (electron, positron, neutrino). Fundamental means it cannot be split.
Your turn9702/22/O/N/25 Q6(a)
State what is meant by a fundamental particle and explain why a proton is not fundamental. [2] (9702/22/O/N/25 Q6(a))
Show worked answer
A fundamental particle cannot be subdivided into smaller particles. A proton is composite because it contains three quarks.