Fundamental particles
Inside the proton
- Protons and neutrons feel solid — but they are not fundamental.
- Each is built from smaller point-like particles called quarks.
- Some particles have no smaller parts; those are the truly fundamental ones.
Quarks
- Six flavours. Up, charm, top have charge $+\tfrac{2}{3}e$; down, strange, bottom have $-\tfrac{1}{3}e$.
- Each has an antiquark with the opposite charge.
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: baryons and mesons
- 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.
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 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).
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