Precision Alignment of the CMS Electromagnetic Calorimeter

The electromagnetic calorimeter (ECAL) of the CMS detector is a homogenous, fine-grained detector made of scintillating lead-tungstate crystals. It was designed to provide excellent energy resolution for electrons and photons and was crucial in the discovery of the Higgs boson. The performance of ECAL is essential for CMS physics analyses. The precise alignment of ECAL is necessary to achieve the high-level position resolution required for photon and electron reconstruction in CMS and their identification. The position of each ECAL module is determined to an accuracy of 1 mm using photogrammetry measurements in CMS; however, the calorimeter is placed within a 4 Tesla superconducting solenoid and to the effect of its strong magnetic field can slightly displace it. Further alignment of ECAL with respect to the CMS Tracker is performed using a dedicated data analysis. This poster will describe the CMS ECAL alignment procedure, present the alignment accuracy reached at the LHC Run 3, and introduce the alignment process automation put in place.