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Section 9.5 Strangeness

We now come to the first conservation law that is partially violated: conservation of strangeness. Strangeness is a property carried only by hadrons, and it is conserved during strong and electromagnetic interactions, but not necessarily conserved in weak interactions. Its name arises from the strange behavior observed in the production and decays of certain hadrons. Consider the following sequence, a result of bombarding stationary protons with high energy pions:

\begin{equation} \pi^- + p \to \Lambda + K^0\text{,}\label{eq_lambda_k_production}\tag{9.12} \end{equation}

followed by

\begin{equation} \Lambda \to p + \pi^- \qquad \mbox{lifetime} \sim 3\times 10^{-10}\Xunits{s}\label{eq_lambda_decay}\tag{9.13} \end{equation}

and

\begin{equation} K^0 \to \pi^+ + \pi^- \qquad \mbox{lifetime} \sim 10^{-9}\Xunits{s}\text{.}\label{eq_k_decay}\tag{9.14} \end{equation}

Particles like \(\Lambda\) and \(K\) always seem to be produced in pairs. One never sees either of the following candidate reactions.

\begin{align} \pi^- + p \amp \to\amp n + K^0\notag\\ \pi^- + p \amp \to\amp \Lambda + \pi^0\text{.}\label{eq_strange_violation}\tag{9.15} \end{align}

Let's analyze these reactions. Since the \(\Lambda\) eventually decays to a proton, it must be a baryon, while \(K\) decays only to pions, marking the kaon as a meson. Then the reactions in both (9.12) and (9.15) conserve charge, spin, and baryon number, but (9.15) never occurs. Why not? The answer once again is that a new conservation law is being violated. Lambdas and kaons must carry a new property (in equal but opposite amounts) that pions and protons don't have. This new property, which particle physicists of the 1960's named strangeness, comes in integer amounts. The kaon, by convention, has \(S = +1\) and the lambda has \(S = -1\text{.}\) Strangeness then balances in (9.12) but not in (9.15). This, then, would “explain” why the reactions in (9.15) are never observed.

But what about the decays of (9.13) and (9.14)? Don't they violate strangeness conservation? The answer is yes, they do. But these decays proceed by the weak interaction. The clue that these are weak decays is the particles' relatively long lifetimes, a topic we discuss further in Chapter 11. Suffice it to say that \(\Lambda\) and \(K^0\) are produced together by the strong interaction, conserving strangeness, but they decay separately by the weak interaction, which does not conserve strangeness.