Fortuitous BMN Cohomology at Every $N$

Abstract

Finite-$N$ trace relations can produce $Q$-cohomology classes with no stable large-$N$ ancestor. The brane-fused construction at $SU(3)$ suggests that such states can retain an $SU(2)$ nonabelian core accompanied by scalar hair. We develop this idea into a general construction for every $SU(N)$, $N\geq3$. Starting from the $SU(2)$ threshold operator $O_{24}$, we select pairs of eigenvalues of one scalar and construct corrected two-colour letters whose $Q$-transformations obey the $SU(2)$ rules exactly. Substitution into $O_{24}$ gives a closed rational operator for each pair. Multiplying by eigenvalue-separation factors removes the denominators, and summing over all pairs produces a gauge-invariant polynomial. An exact block-restriction identity recovers a nonzero multiple of $O_{24}$. The restriction maps every stable class to a stable $SU(2)$ class, proving that the resulting operator is non-exact and fortuitous, even when non-Coulombic stable classes are included. This proves the existence of fortuitous BMN cohomology at every $N\geq3$. The construction is an exact algebraic realization of a nonabelian core with scalar dressing.