= Wall of marginal stability
{title2=$\arg Z_1=\arg Z_2$}
A wall of marginal stability is a locus in the parameter or vacuum space where the complex <central charges in supersymmetry> of possible decay products align in phase. The <triangle inequality> then becomes an equality, $|Z_1+Z_2|=|Z_1|+|Z_2|$, permitting a <BPS state> to reach the threshold for decay into other <BPS states>. Away from such alignment, the strict triangle inequality can prohibit the decay. Supersymmetry protects the mass-charge relation of an existing short state; it does not guarantee that the state exists or remains stable everywhere in parameter space.
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