CMS ECAL monitoring and calibration in LHC Run 2

The CMS electromagnetic calorimeter (ECAL) plays a crucial role in measurements of electrons and photons produced in LHC collisions at CMS. LHC continues to operate a highly successful data-taking program, having already delivered an integrated luminosity of more than 125 $\mathrm{fb}^{-1}$ with {\it pp} collisions since the beginning of Run II. With the expected degradation of ECAL crystals and electronics due to aging and radiation damage, it is essential to regularly fine-tune ECAL calibration factors at the highest possible granularity in order to maintain excellent energy resolution. In addition, it is important to maintain a continuous program of monitoring various physics quantities that depend on ECAL measurements, in order to validate the calibrations and ensure long-term reliability of the measurements. We describe the three techniques currently used to set the calibration factors at the crystal level: using the known energy scales in $\pi^{0} \to \gamma \gamma$ and in $Z \to ee$ events, and comparing the energy measured by ECAL with an independent measurement of electron momentum from the Si tracker. We also describe the various physics quantities we monitor to validate the energy scale and shower shapes. Finally we provide an estimate of the improvement in the energy resolution after applying updated calibrations.