Calorimetry in Particle Physics, and the CMS High-Granularity Calorimeter

An overview of electromagnetic and hadronic shower development is presented, including how they lead to the design of modern calorimeters for energy measurements of high-energy particles. The differences between sampling and homogeneous calorimeters are explored, with the pros and cons of each being presented. Some real-world examples are given, focusing on state-of-the-art detectors designed and operating at the Large Hadron Collider. Methods to increase the information content from future calorimeters are given, and the challenges that these new calorimeters pose to detector physicists and engineers are discussed. Advances in large-area highly-segmented detectors are providing possibilities for high-granu-larity calorimetry. The CMS HGCAL, being designed to replace the existing CMS endcap calorimeters for the HL-LHC era, is one example. It is a sampling calorimeter, featuring unprecedented transverse and longitudinal readout segmentation for both electromagnetic (CE-E) and hadronic (CE-H) compartments. This will facilitate particle-flow calorimetry, where the fine structure of showers can be measured and used to enhance pileup rejection and particle identification, whilst still achieving good energy resolution. The CE-E and a large fraction of CE-H will use hexagonal silicon sensors as active detector material. The lower-radiation environment will be instrumented with scintillator tiles with on-tile SiPM readout. These concepts borrow heavily from designs produced by the CALICE collaboration but the design of such a detector at a hadron collider is considerably more challenging than at the linear colliders.