The Chern character of the Laughlin vector bundle in the Fractional Quantum Hall Effect

Abstract: We begin by explaining how a physical problem of studying the quantum Hall effect on a closed surface $C$ leads, via Laughlin's approach, to a mathematical question of describing the rank and the first Chern class of a particular vector bundle on the Picard group ${\rm Pic}^g(C)$. Then we formulate and solve the problem mathematically, proving several important conjectures made by physicists, in particular the Wen-Niu topological degeneracy conjecture and the Wen-Zee shift formula. Let $C$ be a closed Riemann surface of genus~$g$ and $S^NC$ its $N$th symmetric power. The product $C \times {\rm Pic}^d(C)$ carries a universal line bundle. On the product $C^N \times {\rm Pic}^d(C)$ we consider the product of $N$ pull-backs of this universal line bundle and twist it by a power of the diagonal on $C^N$. The resulting line bundle descends onto $S^NC \times {\rm Pic}^d(C)$. Its push-forward (as a sheaf) to ${\rm Pic}^d(C)$ is a vector bundle that we call Laughlin's vector bundle. We determine all the Chern characters of the Laughlin vector bundle via a Grothendieck-Riemann-Roch calculation.