Quarks and Conservation Laws

At A-Level you need three types (flavours) of quark: $\text{up}$ ($u$), $\text{down}$ ($d$) and $\text{strange}$ ($s$), each with a corresponding antiquark ($\bar{u}$, $\bar{d}$, $\bar{s}$) of opposite charge. Quarks carry fractional charge — a genuine surprise when first discovered, since every other known charge is a whole multiple of $e$.

Baryon number for a single quark is $+\frac{1}{3}$ (and $-\frac{1}{3}$ for an antiquark), so that three quarks always combine to give a baryon number of exactly $+1$, matching what you already know about baryons.

A baryon is three quarks; the proton is $uud$ and the neutron is $udd$. Checking the proton’s charge: $\left(+\frac{2}{3}\right)+\left(+\frac{2}{3}\right)+\left(-\frac{1}{3}\right)=+1$, exactly matching its known charge of $+e$. The neutron: $\left(+\frac{2}{3}\right)+\left(-\frac{1}{3}\right)+\left(-\frac{1}{3}\right)=0$.

A meson is a quark and an antiquark; the positive pion is $u\bar{d}$ and the negative pion is $\bar{u}d$. A kaon such as $K^{+}$ ($u\bar{s}$) contains a strange antiquark, which is why kaons carry nonzero strangeness while pions do not.

The strange quark carries a quantum number called $\text{strangeness}$, $S=-1$ (its antiquark $\bar{s}$ has $S=+1$); up and down quarks have $S=0$. A particle’s strangeness is just the sum over its constituent quarks.

Strangeness is conserved in $\text{strong}$ and $\text{electromagnetic}$ interactions but can change (by 0 or $\pm 1$) in $\text{weak}$ interactions. This is why strange particles are always produced in pairs by the strong force — for example ${\pi }^{-}+p\to K^0+{\Lambda }^0$ creates a strange meson ($S=+1$) and a strange baryon ($S=-1$) together, so the total strangeness stays at $0$ — but they then decay individually via the (strangeness-violating) weak interaction, since nothing else around is strange enough to conserve $S$ in a strong decay.

Zoom into ${\beta }^{-}$ decay and it is really a single down quark inside a neutron converting into an up quark, emitting a virtual $W^{-}$ boson which itself decays into an electron and an antineutrino: $d\to u+e^{-}+{\bar{\nu }}_e$. At the level of the whole nucleon this turns a neutron ($udd$) into a proton ($uud$) — exactly one $d$ has become a $u$, and everything else is unchanged.

This single-quark picture is why beta decay is classified as a weak interaction: only the weak force can change a quark’s flavour (up into down, or vice versa), which is precisely what has to happen for a neutron to become a proton.

A proposed interaction is only allowed if $\text{every}$ conservation law holds simultaneously: charge, baryon number, lepton number, and — for strong and electromagnetic interactions — strangeness. Checking a reaction is a routine four-point test: add up each quantity on both sides and compare.