A high granularity calorimeter for the CMS endcaps at the HL-LHC

Calorimetry at the High Luminosity LHC faces many technical challenges. In the forward direction the levels of radiation and the unprecedented in-time event pileup make detector design and operation particularly difficult. To meet these challenges, the CMS experiment has decided to construct a sampling calorimeter with silicon as the primary active medium. This calorimeter - the High Granularity Calorimeter - will have an unprecedented degree transverse and longitudinal segmentation for a collider detector, both for electromagnetic and hadronic compartments. In this paper we discuss the motivation for this choice of calorimeter and its design.