Quarks, Leptons and the Standard Model

$\text{Quarks}$ come in six flavours: up, down, strange, charm, top and bottom. Up, charm and top have charge $+\frac{2}{3}e$; down, strange and bottom have $-\frac{1}{3}e$. Quarks are never observed alone.

$\text{Leptons}$ are the electron, muon and tau, each with charge $-e$, plus a neutral neutrino for each. Leptons are not made of quarks and do not feel the strong nuclear force.

Every particle has an $\text{antiparticle}$ with the same mass and the opposite charge, baryon number and lepton number. The antiparticle of the electron is the positron; an anti-up quark has charge $-\frac{2}{3}e$.

Particles made of quarks are $\text{hadrons}$, and they feel the strong force. There are two families.

$\text{Baryons}$ contain three quarks and have baryon number $+1$. The proton is $uud$, charge $\frac{2}{3}+\frac{2}{3}-\frac{1}{3}=+1$; the neutron is $udd$, charge $\frac{2}{3}-\frac{1}{3}-\frac{1}{3}=0$. Antibaryons are three antiquarks with baryon number $-1$.

$\text{Mesons}$ contain one quark and one antiquark, so they have baryon number $0$. The positive pion is $u\bar{d}$, charge $\frac{2}{3}+\frac{1}{3}=+1$.

Each quark carries baryon number $+\frac{1}{3}$ and each antiquark $-\frac{1}{3}$, which is why three quarks give $+1$ and a quark–antiquark pair gives $0$.

In every particle interaction, these quantities are conserved: $\text{charge}$, $\text{baryon number}$, $\text{lepton number}$, and also energy and momentum.

Leptons have lepton number $+1$ and antileptons $-1$. Hadrons have lepton number $0$.

To test whether an interaction is possible, tabulate each quantity for every particle on both sides. If any total differs, the interaction is forbidden.

In ${\beta }^{-}$ decay, a neutron becomes a proton: $n\to p+e^{-}+{\bar{\nu }}_e$. At quark level, a $\text{down quark changes to an up quark}$, which only the weak interaction can do.

In ${\beta }^{+}$ decay, a proton in a nucleus becomes a neutron: $p\to n+e^{+}+{\nu }_e$, an up quark changing to a down.

The standard model includes three forces relevant here, each carried by exchange particles: the $\text{electromagnetic}$ force (photon), the $\text{strong}$ force binding quarks (gluons), and the $\text{weak}$ force responsible for changing quark flavour (W and Z bosons).

The existence of the neutrino was originally inferred from beta decay: electrons emerged with a range of energies instead of one fixed value, so an unseen particle had to be carrying the rest.