Measurement of the diboson pp->WW+WZ production cross section and limits on the anomalous triple gauge couplings at \sqrt{s}=7 TeV with the ATLAS detector in the semileptonic decay channel

This work aims at describing the measurement of the WW+WZ production cross section performed with the ATLAS detector at the LHC using proton-proton collisions at \sqrt{s} = 7 TeV. The measurement is done in the semileptonic final state composed by a pair of jets, one lepton and one neutrino, the lepton being an electron or a muon. Although conceptually clear, it will be shown in the following that the measurement of this final state is delicate and difficult in the LHC environment: first of all, the signal-to-background ratio is of the order of percent, in fact the production cross section is about 4 times larger at the LHC than at the Tevatron, but the dominant background contribution, the W+jets process, is a factor of 10 larger; secondly, the background shape and the signal yield are such that the diboson signal extraction is critical. The importance of this study, other than the measurement of the production cross section using a non fully leptonic channel, is in demonstrating that the ATLAS experiment is able to reconstruct and identify dijet resonances with moderate-p_T jets starting with a percent signal-over-background ratio. The non-resonant diboson production is also an important background for the H->WW signal, therefore the proof that this background process is well understood is of importance for the Higgs searches, too. This work also reports limits on anomalous triple gauge couplings vertices, a portal for detecting very small effects due to non-Standard Model Physics. Furthermore, the semileptonic channel measurement involves the reconstruction of many physical object, such as jets, leptons, missing transverse energy. This requires a well-establish understanding of the detector and of the reconstruction performances. The results are based on the extraction of information both from data and Monte Carlo simulated events, as well as further selection, off-line calibrations and smearing techniques, good detector description and reliable event generation; additional emphasis has been given to the study of the systematic uncertainties and to their impact on the signal extraction. The analysis has been performed on 2011 data, about 5 fb-1 sample of proton-proton collisions at \sqrt{s} = 7 TeV; some final considerations about different approaches and the possibility of extending the study with new data and new tools are also discussed. The measurement of the diboson production cross section with the WW/WZ->lvjj channel and the anomalous triple gauge coupling limits obtained in this analysis have been published.