Search for a heavy particle decaying into electron-muon in $pp$ collisions at $\sqrt{s} = 7$ TeV at LHC with the ATLAS detector

In this thesis, a search for $e\mu$ resonance of $\tau$ sneutrino ($\tilde{\mu}_{\tau}$) decay using $pp$ collision data of $\sqrt{s}=7$ TeV collected by the ATLAS experiment at LHC is described. The Standard Model (SM) processes having electron and muon in the final state are estimated with Monte Carlo simulations. A matrix method was used to estimate the jet fake background. We first analysed the 35 pb$^{-1}$ data taken in 2010 from March to December and later updated the results with 1.07 fb$^{-1}$ data taken in 2011 from March 2011 to June 2011. Seeing no significance deviation of data from the SM prediction, we set the limit on the $\tau$ sneutrino mass and also the limit of coupling parameter as function of $\tilde{\mu}_{\tau}$ mass. $\tau$ sneutrino of $m<1.32$ TeV is excludes assuming the coupling parameters $\lambda_{312}=0.05$ and $\lambda'_{311}=0.10$. The limit observed surpasses the results reported by the Tevatron experiments. The limits on the coupling parameters are the best results to date for $m_{\tilde{\mu}_{\tau}}>275\GeV$. The limits on the production cross section of $\tau$ sneutrino can also be interpreted to other models predicting a heavy particle decaying to electron and muon pair.