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 open. Detection of boson pairs (W/Z+gamma, WW) has thus already been possible in the ATLAS Experiment with 2010 data. We present here the first observation of WZ diboson candidates in ATLAS, using the first 205 pb-1 gathered in 2011. Leptonic decay channels, with muons and electrons plus a neutrino, have been considered. We discuss cut-based event selections, based on lepton isolation, reconstruction of the Z boson, as well as missing transverse energy calibration and reconstruction of the W boson transverse mass. Electroweak and QCD backgrounds have been studied with Monte Carlo simulation and data-driven methods. 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.