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Precision crystal calorimetry in LHC Run II with the CMS ECAL

The electromagnetic calorimeter (ECAL) of the Compact Muon Solenoid (CMS) Experiment, based on lead tungstate scintillating crystals, is crucial for achieving high-resolution measurements of electrons and photons. Maintaining and possibly improving the excellent performance achieved in Run I is vital for measurements of the Standard Model Higgs boson and searches for new higher mass resonances in final states with electrons and photons. In Spring 2015 the LHC started “Run II”, colliding protons at 13 TeV centre-of-mass energy and with 25 ns bunch spacing. This is very close to the original design specifications of the LHC (14 TeV at 25 ns). At this higher energy, and with the rapidly growing dataset, the performance for higher electron and photon energies becomes crucial. At the same time, the instantaneous luminosity has increased and, over the coming years, is expected to surpass the design value, possibly by a factor of two to about 2×1034 cm−2 s−1. The average number of concurrent proton-proton collisions per bunch crossing (pileup) is expected to reach about 40. This pileup is a major challenge, both for calibration and for ultimate energy reconstruction. The CMS ECAL design ensures that its superb performance extends over a very wide range of energies up to electron and photon energies of 1 TeV and beyond. We present new energy reconstruction algorithms and clustering techniques, developed to maintain the excellent performance of the CMS ECAL throughout Run II. We will show first performance results from 2015 and 2016 data, including triggering efficiency, event reconstruction and calibration precision. The latter has been achieved through the measurements of electrons from W and Z boson decays, photons from π0/η decays, and the azimuthally-symmetric energy distribution of minimum bias events. We also present an outlook on the expected Run II performance in the coming years, including the impact of the ECAL on resonance searches in the mass range up to 1 TeV.