First results from the CMS SiPM-based hadronic endcap calorimeter

The CMS hadronic calorimeter employs a plastic-scintillator-based endcap detector. In early 2017, a 20 degrees wedge of the endcap was upgraded with silicon photomultipliers (SiPMs) and readout electronics based on the QIE11 digitizer. Based on the excellent experience with this 20 degrees pilot system in 2017, the entire endcap detector was upgraded with SiPMs in early 2018. We report on the first ever operation of SiPMs in a high-rate collider detector. We compare SiPM performance to that of the previous hybrid photodiode / QIE8-based readout and describe how the factor three improvement in photon-detection efficiency, increased longitudinal segmentation, and improved response stability allow for mitigation of scintillator radiation damage and simplified calibration. We report in situ measurements of radiation-induced SiPM dark current. Overall, we show that the upgraded SiPM-based system brings more than 50% improvement in radiation-induced response degradation.