11.2 Fundamental particles

The six quark flavours are up, down, strange, charm, top and bottom. Quarks are treated as fundamental constituents of hadrons in the standard model.

Use the flavour names as a classification and distinguish quarks from composite protons and neutrons.

Ordinary nuclear matter is mainly built from up and down quarks, while strange, charm, top and bottom appear in higher-energy or unstable particles.

A quark is not a smaller proton; hadrons are composite states made from quarks.

Up, charm and top quarks carry charge +2/3e; down, strange and bottom carry −1/3e.

Add constituent charges to find a hadron’s total charge, remembering that antiquarks have opposite charge.

A proton uud has charge +2/3+2/3−1/3=+1e; a neutron udd has total charge zero.

Fractional quark charges are allowed inside hadrons even though isolated everyday matter has integral net charge.

A proton has composition uud and a neutron udd; neither is fundamental because each contains three valence quarks bound by the strong interaction.

Use quark composition to derive charge and distinguish valence content from the fluctuating sea of quantum field theory.

Replacing one up quark in uud with a down quark gives udd, changing a proton into a neutron and reducing charge by e.

“Three quarks” is the simple valence model, not a claim that no gluons or transient pairs are present.

Baryons are three-quark states such as protons and neutrons; mesons are quark–antiquark states. Both are hadrons and feel the strong interaction.

Check quark and antiquark charges when classifying a particle, and remember that the simple composition labels do not determine stability alone.

A proton is a baryon uud, while a pion is a meson made from a quark and an antiquark combination.

Leptons are not hadrons, and a meson is not “two baryons”; it contains one quark and one antiquark.

In β− decay a down quark changes to an up quark, turning a neutron udd into a proton uud and emitting an electron and antineutrino; β+ reverses the quark change with a positron and neutrino.

Track the quark charge change and the emitted lepton charges together. The weak interaction mediates the flavour change.

A neutron’s d→u raises hadron charge by +e, balanced by the emitted electron’s −e in β− decay.

The beta electron is not an orbital electron released from the atom; it is created in the weak decay process.

Leptons are fundamental particles including electrons, muons, taus and their neutrinos. Charged leptons have charge −e; neutrinos are neutral.

Pair each charged lepton with its neutrino flavour and distinguish leptons from hadrons, which are composite.

An electron can be emitted in β− decay, while the associated electron antineutrino carries away energy and lepton number.

A neutrino is not a neutral proton or a low-energy photon; it is a distinct fundamental lepton.