Solution to 1992 Problem 91

The proton (which consists of two up quarks and a down quark) has a strangeness of zero. The \pi^-, which consists of a \bar{u} quark and a d quark, also has a strangeness of 0. The \Lambda, which consists of an up quark, a down quark, and a strange quark, has a strangeness of -1. %(See Table 14.6 in Modern Physics)
Therefore, strangeness is not conservation in this reaction. Nonconservation of strangeness implies that the reaction proceeds via the weak interaction. %(See pages 523 and 524 of Modern Physics).
Therefore, answer (E) is correct.

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