Abstract : In this thesis work, the first measurement of the WZ cross section with the CMS detector at the LHC using the leptonic modes is presented. The response of the electromagnetic calorimeter (ECAL) is studied through the measurement of the muon stopping power in the lead tungstate of the ECAL. Electron objects are commissioned with the first data so to be used for any analyses. With the cosmic ray data, the muon stopping power in the lead tungstate of the electromagnetic calorimeter was measured over the momentum range 5-1000 GeV/c and found consistent with expectations. Such a result allowed to extend the validity of the ECAL energy scale, previously set with 120 GeV/c electron beam, down to the sub-GeV region consistently with 1.004+0.002-0.003(stat.) ±0.019(syst.). With the LHC starting operation, commissioning studies were dedicated to the electron reconstruction, with particular attention to the very first stage of the tracking in the inner tracker layers. The algorithms were accurately optimized, with an electron sample from W boson decay and fully validated with order 14/pb of data. The physics measurement is the cross section of the WZ associated production in pp collisions at √s =7TeV. The clean leptonic signature allowed for an efficient signal extraction in each considered channel and a good background rejection. The first event was observed with 36/pb of data. With 1.09/fb, the WZ cross section was measured for the first time at √s =7TeV σ(WZ) = 19.11+3.30-2.53(stat.) ±1.10(syst.)±1.15(lumi.)pb and found consistent with the SM expectation (18.57+0.75−0.58pb NLO).