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The first part of this work describes how to use Z -> e+e- events in order to calibrate the CMS electromagnetic calorimeter, which makes use of scintillating crystals in order to precisely measure the energy of electrons and photons coming from the proton-proton interactions.
Using the very precise knowledge of the Z mass coming from LEP experiments, it is possible to set the absolute scale of the calorimeter as well as calibrating regions of the calorimeter with various topologies, and finely correct the calorimeter response to electrons. Focus is put on the first weeks of data taking.
The second part of this work concentrates on the misidentification of the electric charge of electrons/positrons in CMS. It will be shown how it is possible to extract the charge misidentification rate from the first CMS data, this time relying on the fact that electrons coming from the Z decay are always oppositely-charged.
Measuring this charge misidentification rate not only allows to perform a real-time check of the reconstruction quality during data taking, but also has an important role in the study of some physics channels. One of the studies where the charge misidentification has an important in influence is the W+/W- cross section ratio, that represent a test of the Standard Model which does not need a precise knowledge of the machine luminosity, that will be difficult to achieve with the first data.