The lambda particle $\Lambda^0$ has quark composition uds. It decays as follows:
$\Lambda^0 \rightarrow p + \pi^-$
(a) Show that charge and baryon number are conserved in this decay. [2]
(b) Explain why this decay must be a weak interaction. [2]
(c) State the quark composition of the $\pi^-$. [1]
- (a) charge: 0 → (+1) + (−1) = 0(1)
- (a) baryon number: 1 → 1 + 0 = 1(1)
- (b) strangeness of $\Lambda^0$ is −1 and strangeness of products is 0 (strangeness not conserved)(1)
- (b) strangeness can change (by 1) only in weak interactions(1)
- (c) $\mathrm{d}\bar{\mathrm{u}}$(1)
- Answer: (a) Q: 0 → 0; B: 1 → 1 (b) S changes from −1 to 0, only the weak interaction allows this (c) $\mathrm{d}\bar{\mathrm{u}}$
- Examiner tip: A strange quark has strangeness −1 (not +1) — the sign trips many students up.