Discussion
Which one of the following is an allowed process?
Answer: A
The allowed process is \(\pi^{-} + p \to \pi^{0} + n\).
This charge exchange reaction conserves charge (−1 + 1 = 0 + 0), baryon number (1 = 1), and isospin and strangeness, so it proceeds by the strong interaction and is observed.
\(\pi^{0} \to 3\gamma\) is forbidden by charge conjugation: the \(\pi^{0}\) has C = +1, and each photon carries C = −1, so three photons have C = −1. \(p + \bar{p} \to \Lambda^{0} + \Lambda^{0}\) has baryon number 0 before and 2 after (and strangeness 0 before, −2 after). \(p + \bar{p} \to \gamma\) cannot conserve both energy and momentum: in the centre of mass frame the total momentum is zero, but a single photon always carries momentum.
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