Gromov-Witten theory of product stacks

Let ${\mathcal{X}}_1$ and ${\mathcal{X}}_2$ be smooth proper Deligne-Mumford stacks with projective coarse moduli spaces. We prove a formula for orbifold Gromov-Witten invariants of the product stack $$\mathcal{X}$_1\times $\mathcal{X}$_2$intermsofGromov-Witteninvariantsofthefactors$$\mathcal{X}$_1$and$$\mathcal{X}$_2$. As an application, we deduce a decomposition result for Gromov-Witten theory of trivial gerbes.