Multiple choiceQuestion 1 · Recall
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By the end you can state that electrons, positrons and neutrinos are leptons, explain that leptons contain no quarks, and classify a given particle as a lepton or a hadron.
Lepton
A lepton is a fundamental particle that is not made of quarks and is not affected by the strong interaction that binds quarks together.
The leptons you need are:
| particle | symbol | charge | notes |
|---|---|---|---|
| electron | fundamental, carries no quarks | ||
| positron | the antilepton of the electron | ||
| electron neutrino | almost massless | ||
| electron antineutrino | almost massless |
Electrons and neutrinos are companions in every beta decay: beta-minus emits , and beta-plus emits .
Ask: does it contain quarks?
Since an electron and a neutrino contain no quarks at all, neither can be a baryon or a meson. That is the whole distinction, and it is worth stating explicitly in an answer.
A proton is a busy object: three quarks bound together by a strong force, with a rich set of excited states. An electron has none of that complexity. There is nothing inside it to rearrange, which is why it behaves as a single, permanent point-like particle.
Neutrinos are simpler still: no charge, almost no mass, and so little interaction with matter that a vast number of them pass through the Earth every second without stopping.
Worked example: Example: classifying a mixed list
Classify each of these as a baryon, a meson or a lepton: , , (a muon), , .