CMS-PRF-21-001
Development of the CMS detector for the CERN LHC Run 3
Schematic view of the HCAL as of 2016, showing the positions of its four major components: HB, HE, HO, and HF. The layers marked in blue are grouped together as ``depth 1,'' i.e., the signals from these layers of a given tower are optically summed and read out by a single photodetector. Similarly, the layers shown in yellow and green are combined as depths 2 and 3, respectively, and the layers shown in purple are combined for HO. The notation ``FEE'' denotes the locations of the HB and HE frontend electronics readout boxes. The solid black lines, roughly projective with the interaction point, denote the η divisions in the tower η-ϕ segmentation, and the numbers at the edge of the tower denote the ieta index.