Beyond the Fock Vacuum in Finite-$N$ Bilocal Algebras of Vector Models

Abstract

We discuss the finite-$N$ Hilbert space of bilocal operators that underlies bilocal holography of vector models, beyond the sector built on the Fock vacuum. If the microscopic oscillator Fock space carries a multiplicity-free dual action of a compact color group $G$ and a bilocal algebra, Howe duality implies that projection to the derived subgroup $[G,G]$ selects precisely the characters of the abelianization $G^{\rm ab}=G/[G,G]$ that occur in the Fock space. Thus $O(N)/SO(N)=\mathbb Z_2$ gives two sectors, $U(N)/SU(N)=U(1)$ gives integer determinant-charge sectors, while $Sp(N)$ is perfect and gives no additional sector; Bose/Fermi statistics and the number of modes determine which sectors are actually realized. We construct the epsilon and determinant backgrounds that generate these sectors, together with their finite-$N$ constraints and Casimirs, and find that the two bosonic $O(N)$ sectors are not distinguished by any Casimir. Using group-integral projectors, we derive explicit sector-resolved characters for the $O(N)$ and $U(N)$ theories. Around each new background we solve the constraints with auxiliary oscillators, which form a Fermi sea for bosonic matter. In the free vector model on $\mathbb R\times S^2$ the new sectors consist of baryonic primaries; we determine their energies, degeneracies and spin content exactly at finite $N$, together with their lowest energy at fixed angular momentum.