Gerbes over Orbifolds and Twisted K-theory

In this paper we construct an explicit geometric model for the group of gerbes over an orbifold $X$. We show how from its curvature we can obtain its characteristic class in $H^3(X)$ via Chern-Weil theory. For an arbitrary gerbe $\LL$, a twisting $\Korb{\LL}(X)$ of the orbifold $K$-theory of $X$ is constructed, and shown to generalize previous twisting by Rosenberg, Witten, Atiyah-Segal and Bowknegt et. al. in the smooth case and by Adem-Ruan for discrete torsion on an orbifold.