Normality and Cohen–Macaulayness of parahoric local models

Abstract

We study the singularities of integral models of Shimura varieties and moduli stacks of shtukas with parahoric level structure. More generally, our results apply to the Pappas–Zhu and Levin mixed characteristic parahoric local models, and to their equal characteristic analogues. For any such local model we prove under minimal assumptions that the entire local model is normal with reduced special fiber and, if $p > 2$, it is also Cohen–Macaulay. This proves a conjecture of Pappas and Zhu, and shows that the integral models of Shimura varieties constructed by Kisin and Pappas are Cohen–Macaulay as well.