Search for Displaced Supersymmetry in Events with an Electron and a Muon with Large Impact Parameters

A search for new long-lived particles decaying to leptons is presented using proton-proton collisions produced by the LHC at $\sqrt{s}=8\mathrm{TeV}$. Data used for the analysis were collected by the CMS detector and correspond to an integrated luminosity of $19.7{\mathrm{fb}}^{-1}$. Events are selected with an electron and muon with opposite charges that both have transverse impact parameter values between 0.02 and 2 cm. The search has been designed to be sensitive to a wide range of models with nonprompt $e\text{−}\mu$ final states. Limits are set on the “displaced supersymmetry” model, with pair production of top squarks decaying into an $e\text{−}\mu$ final state via $R$-parity-violating interactions. The results are the most restrictive to date on this model, with the most stringent limit being obtained for a top squark lifetime corresponding to $c\tau =2\mathrm{cm}$, excluding masses below 790 GeV at 95% confidence level.

Figure 1

Lepton transverse impact parameter distributions for data and expected background processes after the preselection requirements have been applied, for electrons (top) and muons (bottom). The signal expected from 500 GeV top squark pair production is overlaid for $c\tau =0.1$, 1, and 10 cm. The event yields per bin have been rescaled to account for the varying bin sizes. The rightmost bin of each plot contains the overflow entries.

Figure 2

Expected and observed 95% C.L. cross section exclusion contours for top squark pair production in the plane of top squark lifetime ($c\tau$) and top squark mass. These limits assume a branching fraction of 100% through the RPV vertex $\tilde{t}\to bl$, where the branching fraction to any lepton flavor is equal to $1/3$. As indicated in the plot, the region to the left of the contours is excluded by this search.