First observation of WZ events in leptonic channels with ATLAS

The LHC proton-proton collisions at a center-of-mass energy of 7 TeV now allow observation of Standard Model phenomena in an unexplored energy range, where windows towards new physics may be opened. Detection of boson pairs (W/Z+gamma, WW) were already done in the ATLAS Experiment in 2010. We present here the first observation of WZ diboson candidates in ATLAS, using the first few hundreds of pb-1 gathered in 2011. Leptonic decay channels, with muons and electrons plus a neutrino, have been considered. We discuss cut-based event selections, including lepton isolation, reconstruction of the Z boson, as well as missing transverse energy calibration and reconstruction of the W boson transverse mass. Electroweak, top and QCD backgrounds have been studied with Monte Carlo simulation. First cross-section measurements are then derived from the observed excess events and compared to the SM predictions. This will later allow searches for anomalous Triple Gauge Couplings, which may enhance those cross-sections.