Calorimetry at very forward rapidity

The CMS experiment at CERN will undergo significant improvements during the so-called Phase-II Upgrade to cope with a 10-fold increase in integrated luminosity with the High Luminosity LHC (HL-LHC) era. A particularly challenging environment is the forward region, where the combination of extremely high radiation levels (fluence and ionising dose) coupled with up to 200 simultaneous pileup events necessitates new technologies and methodologies. The CMS collaboration is designing a High Granularity Calorimeter (HGCAL) to replace the existing endcap calorimeters. 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 silicon as active detector material. The sensors will be of hexagonal shape, maximising the available 8-inch circular wafer area. The lower-radiation environment will be instrumented with scintillator tiles with on-tile SiPM readout. In addition to the hardware aspects, the reconstruction of signals - both online for triggering and offline - is a quantum leap from existing detectors. We present the current status of the HGCAL, including its design and expected performance and the challenges ahead.