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$^{\circ}$ 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$^{\circ}$ 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.