Speaker
Description
The CMS Electromagnetic Calorimeter (ECAL) is made of 75848 lead-tungstate scintillating crystals. The excellent intrinsic energy resolution of the CMS ECAL is preserved with the aid of a precise light monitoring system. The high radiation doses recorded in the ECAL crystals and photodetectors from the LHC collisions radiation affects the light output. Crystal and photodetector response changes are monitored in real time by a sophisticated apparatus using lasers and LEDs. Soon after data are taken, a computer farm processes the laser and LED monitoring events and computes precise corrections to be used in the event reconstruction within 48 hours of data-taking. Similar corrections are also applied at trigger level. High-Luminosity running at the LHC, which is planned for 2030 and beyond, will provide an order of magnitude increase in radiation levels and particle fluences with respect to the present LHC running conditions. The mitigation of ageing is an important goal for the upgrade of the laser light monitoring system. This talk describes the key components of the ECAL monitoring system, discusses the evolution of the response of the ECAL crystals and photodetectors from Run 1 to Run 3 of the LHC (2011-2026), and presents the design and status of the monitoring system upgrade for HL-LHC.