An overview of the CMS High Granularity Calorimeter, and the engineering challenges in its construction

¹ CERN, European Organization for Nuclear Research, Geneva, Switzerland
^† On behalf of the CMS Collaboration

^* Corresponding author: thomas.french@cern.ch

Published online: 7 March 2025

Abstract

The CMS Collaboration is preparing to replace its current endcap calorimeters for the HL-LHC era with a high-granularity calorimeter (HGCAL), featuring a previously unrealized transverse and longitudinal segmentation, with 5D information (space-time-energy) read out. This design, an overview of which is presented in this article, uses silicon sensors for the electromagnetic section and high-irradiation regions (with fluences above 10¹⁴neq/cm²) of the hadronic section, while in the low-irradiation regions of the hadronic section plastic scintillator tiles with on-tile silicon photomultipliers are used. The active layers include copper cooling plates embedded with thin pipes carrying two-phase CO₂ coolant, front-end electronics and electrical/optical services. The scale and density of the calorimeter poses many engineering challenges, including: the design and production of stainless steel absorber plates to very high tolerances from 600 tonnes of raw material; the development of the CO₂ cooling system to maintain each 220-tonne endcap at -35°C with electronics dissipating up to 130kW; the need to cantilever the calorimeters from the existing CMS endcap disks, using titanium supports; the production of a thin but strong inner cylinder to take the full weight while minimising impact on physics performance; and the integration of on-detector services in a restricted height of only a few millimetres.