= Second-order strangeness-changing weak decay
{title2=$|\Delta S|=2$}
A <decay amplitude> changing <strangeness> by two units requires at least two insertions of the low-energy <weak interaction> in the <Standard Model>. Each charged-current insertion can remove only one net strange <quark> in such a process. Both two-<W boson> tree topologies and loop contributions can occur; a loop is not required for every exclusive final state. At hadronic energies, the extra power of the <Fermi constant>, with the needed powers of an energy scale to balance units, strongly suppresses a second-order amplitude relative to a first-order one. This is distinct from <Cabibbo suppression> within a fixed weak order.
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