Hadrons (baryons and mesons), leptons, the conservation of baryon number, lepton number, strangeness and charge, the properties of the kaon and pion, and the decay of particles.

What this dot point is asking

AQA specification points 3.2.1.5 and 3.2.1.6 want you to classify particles as hadrons (baryons or mesons) or leptons, recall the properties of protons, neutrons, pions, kaons, electrons, muons and neutrinos, and apply the conservation laws for charge, baryon number, lepton number and strangeness to decide whether an interaction can happen.

Hadrons and leptons

The proton is the only stable baryon; all other baryons eventually decay into protons. Leptons are believed to be truly fundamental (not made of anything smaller), whereas hadrons are composite, made of quarks.

Mesons: pions and kaons

The pion ($\pi$) is the lightest meson and is the exchange particle modelled for the strong nuclear force between nucleons; it comes in three charge states ($\pi^+$, $\pi^0$, $\pi^-$). The kaon ($\text{K}$) is heavier and is a strange particle: it is produced through the strong interaction but decays through the weak interaction, often into pions or into a muon and a neutrino. Strange particles like the kaon have a non-zero strangeness quantum number.

Conservation laws

Strange particles are always created in pairs (so total strangeness is conserved at creation through the strong interaction) but decay individually through the weak interaction, which is why their decay can change the total strangeness by $\pm 1$.

Try this

Q1. State whether the muon is a hadron or a lepton, and give its lepton number. [2 marks]

• Cue. A lepton, with muon lepton number $+1$.

Q2. Explain why the decay $\text{K}^+ \rightarrow \mu^+ + \nu_\mu$ proceeds through the weak interaction. [2 marks]

• Cue. Strangeness changes (the kaon has $S = +1$, the products have $S = 0$), so only the weak interaction can do it.

Q3. State the quark content (in terms of number of quarks) of a baryon. [1 mark]

• Cue. Three quarks.