Fundamental particles
| English | Chinese | Pinyin |
|---|---|---|
| fundamental | 基频 | jī pín |
| quark | 夸克 | kuā kè |
| flavour | 味 | wèi |
| antiquark | 反夸克 | fǎn kuā kè |
| hadron | 强子 | qiáng zi |
| baryon | 重子 | zhòng zǐ |
| meson | 介子 | jiè zi |
| lepton | 轻子 | qīng zi |
Inside the proton
- Protons and neutrons feel solid — but they are not fundamental 基频.
- Each is built from smaller point-like particles called quarks 夸克.
- Some particles have no smaller parts; those are the truly fundamental ones.
Fundamental particle lab
Sort particles by the family or interaction that defines them.
Quarks
- Six flavours 味. Up, charm, top have charge $+\tfrac{2}{3}e$; down, strange, bottom have $-\tfrac{1}{3}e$.
- Each has an antiquark 反夸克 with the opposite charge.
The six quark flavours: up-type carry +⅔e, down-type carry −⅓e
What is the charge on an up quark?
Up-type quarks (up, charm, top) carry $+\tfrac{2}{3}e$; down-type (down, strange, bottom) carry $-\tfrac{1}{3}e$.
Hadrons 强子: baryons 重子 and mesons 介子
- Particles built from quarks are hadrons.
- Baryons = three quarks; mesons = a quark and an antiquark.
Match each hadron to what it is made of.
Protons and neutrons are baryons (three quarks); a pion is a meson (quark + antiquark).
Proton and neutron
- Proton = u u d: charge $\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$.
- Neutron = u d d: charge $\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$.
A proton is made of which quarks?
Proton = u u d, giving charge $\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$.
A neutron is u d d. What is its total charge, in units of $e$?
$\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$ — the neutron is uncharged.
β-decay in quark terms
- β$^{-}$: a neutron becomes a proton — a down quark turns into an up quark.
- β$^{+}$: a proton becomes a neutron — an up quark turns into a down quark.
β⁻ decay up close: one down quark flips to an up quark, so a neutron becomes a proton
In β$^{-}$ decay, which quark change happens?
A neutron (u d d) becomes a proton (u u d): one down → up, emitting a $\beta^{-}$ and an antineutrino.
Leptons 轻子 and what's fundamental
- Leptons (electrons, neutrinos) are fundamental and are not made of quarks.
- Fundamental: quarks, electrons, positrons, neutrinos. Not fundamental: protons, neutrons, mesons (all hadrons).
Particles sort into hadrons (made of quarks) and leptons (fundamental, not made of quarks)
Select all the particles that are fundamental.
Quarks and leptons (electron, neutrino) are fundamental. Protons and mesons are hadrons — made of quarks.
An electron is a lepton and is not made of quarks.
Leptons (electrons and neutrinos) are fundamental particles, not built from quarks.
You've got it
- quarks are fundamental (up-type $+\tfrac{2}{3}e$, down-type $-\tfrac{1}{3}e$)
- baryons = 3 quarks (proton uud, neutron udd); mesons = quark + antiquark
- in β$^{-}$ decay a down quark → up quark; leptons (electron, neutrino) are fundamental