Search for direct production of charginos and neutralinos in final states with three leptons and missing transverse momentum in proton-proton collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector

Search for direct production of charginos and neutralinos decaying via $W$, $Z$ or Higgs bosons is presented. The analysis is based on 20.3 fb$^{−1}$ of proton-proton collision data with the centre-of-mass energy of $\sqrt{s} = 8$ TeV delivered by the Large Hadron Collider and recorded with the ATLAS detector in 2012. This thesis discusses the search in the channels with three leptons ($\ell = e, \mu, \tau$) and missing transverse momentum in final states, in particular the detail of the setting of the optimal selections and a method of estimating fake lepton contributions in order to achieve the best sensitivity with the acquired data. The validation of the estimates of the Standard Model background works satisfactorily within the uncertainties. No significant deviation from the Standard Model expectation is observed. Exclusion limits for masses of the charginos and the neutralinos are set at 95% confidence level at simplified models for $W Z$ and $W h$ decay channels, as $m_{\tilde{\chi}_1^\pm,\tilde{\chi}_2^0} > 360$ GeV, if $m_{\tilde{\chi}_1^0} =0$ GeV (WZ channel), $m_{\tilde{\chi}_1^\pm,\tilde{\chi}_2^0} >150$ GeV, if $m_{\tilde{\chi}_1^0} =0$ GeV (Wh channel). The exclusion limits for the $W h$ channel has been set for the first time. The specific analysis for the scenario with the mass difference between the second lightest neutralino and the lightest neutralino of less than 50 GeV is also carried out. The region has leptons with low trans- verse momenta, hence it is difficult to suppress the fake contribution. In this thesis, the control method of the fakes are investigated and applied to the signal regions. The exclusion limit is set to the compressed mass scenario, as $m_{\tilde{\chi}_1^\pm, \tilde{\chi}_2^0} > 110$ GeV, if $\Delta m_{\tilde{\chi}_2^0,\tilde{\chi}_1^0} = 25$ GeV (WZ channel). These results are consistent with the CMS results. The interpretation for the scenario with the sleptons and the pMSSM are discussed. Discussions in terms of the dark matter limits and the deviation of the muon anomalous magnetic moment are given in this thesis. The prospects for the improvement of this analysis for next LHC Run-2 are also discussed. The suppressing and estimating precisely for fakes are important for upcoming experiments.