This Letter presents the first search for a heavy particle decaying into an $e^{\pm }{\mu }^{\mp }$ final state in $\sqrt{s}=7\mathrm{TeV}$ $pp$ collisions at the LHC. The data were recorded by the ATLAS detector during 2010 and correspond to a total integrated luminosity of $35{\mathrm{pb}}^{-1}$. No excess above the standard model background expectation is observed. Exclusions at 95% confidence level are placed on two representative models. In an $R$-parity violating supersymmetric model, tau sneutrinos with a mass below 0.75 TeV are excluded, assuming all $R$-parity violating couplings are zero except ${\lambda }_{311}^{\prime }=0.11$ and ${\lambda }_{312}=0.07$. In a lepton flavor violating model, a $Z^{\prime }$-like vector boson with masses of 0.70–1.00 TeV and corresponding cross sections times branching ratios of 0.175–0.183 pb is excluded. These results extend to higher mass $R$-parity violating sneutrinos and lepton flavor violating $Z$’s than previous constraints from the Tevatron.

• Received 29 March 2011

DOI:https://doi.org/10.1103/PhysRevLett.106.251801

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© 2011 CERN, for the ATLAS Collaboration