Search for long-lived particles decaying to leptons with large impact parameter in proton-proton collisions at s = 13 Te

Abstract

A search for new long-lived particles decaying to leptons using proton-proton collision data produced by the CERN LHC at $\sqrt{s}=13\text{Te}$ is presented. Events are selected with two leptons (an electron and a muon, two electrons, or two muons) that both have transverse impact parameter values between 0.01 and 10 $\text{cm}$ and are not required to form a common vertex. Data used for the analysis were collected with the CMS detector in 2016, 2017, and 2018, and correspond to an integrated luminosity of 118 (113) ${\text{fb}}^{-1}$ in the $e_{}^{}{e}_{}^{}$ channel ( $e_{}^{}\mu$ and $\mu \mu$ channels). The search is designed to be sensitive to a wide range of models with displaced $e_{}^{}\mu$ , $e_{}^{}{e}_{}^{}$ , and $\mu \mu$ final states. The results constrain several well-motivated models involving new long-lived particles that decay to displaced leptons. For some areas of the available phase space, these are the most stringent constraints to date.

Fig. 1

Feynman diagrams for $\tilde{\mathrm{t}}\to \mathrm{b}\ell$ (upper left), $\tilde{\mathrm{t}}\to \mathrm{d}\ell$ (upper right), $\tilde{\ell }\to \ell \tilde{\mathrm{G}}$ (lower left), and ${\mathrm{H}}_{}^{}\to {\mathrm{S}}_{}^{}{\mathrm{S}}_{}^{}$, ${\mathrm{S}}_{}^{}\to {\ell }^{+}{\ell }^{-}$ (lower right)

Fig. 2

A diagram of the ABCD method, shown for illustration on simulated background events passing the ${\mathrm{e}}_{}^{}\mathrm{\mu }$ preselection with 2018 conditions. In each $|d_0^{\text{a}}|$-$|d_0^{\text{b}}|$ bin, the number of events divided by the bin area is plotted. A, B, and C are CRs. SRs I–IV are described in Sect. 7.2

Fig. 3

The number of observed and estimated background events in each channel and SR, with a representative signal overlaid. The lower panel shows the fractional difference between the data and the background. For each background estimate and signal yield, the total uncertainty (statistical plus systematic) is given. For each observed yield, the 68% Poisson confidence interval is given. The distributions shown are those obtained before the final maximum likelihood fit to the data

Fig. 4

The observed 95% $\text{CL}$ upper limits on the long-lived top squark production cross section, in the $c{\tau }_0$-mass plane, for the three channels combined. The $\tilde{\mathrm{t}}\to \mathrm{b}\ell$ (upper) and $\tilde{\mathrm{t}}\to \mathrm{d}\ell$ (lower) processes are shown. These limits assume either $\mathcal{B}(\tilde{\mathrm{t}}\to \mathrm{d}\ell )$ or $\mathcal{B}(\tilde{\mathrm{t}}\to \mathrm{b}\ell )$ is 100%, and each lepton has an equal probability of being an electron, a muon, or a tau lepton. The area to the left of the black curve represents the observed exclusion region, and the dashed red lines indicate the expected limits and their 68% confidence intervals

Fig. 5

The 95% $\text{CL}$ constraints on the long-lived slepton $c{\tau }_0$ and mass. The $\tilde{\mathrm{\tau }}$ and co-NLSP limits are shown for the three channels combined, while the $\tilde{\mathrm{e}}$ and $\tilde{\mathrm{\mu }}$ NLSP limits are shown for the ${\mathrm{e}}_{}^{}{\mathrm{e}}_{}^{}$ and $\mathrm{\mu }\mathrm{\mu }$ channels, respectively. These limits assume that the superpartners of the left- and right-handed leptons are degenerate in mass and $\mathcal{B}(\tilde{\ell }\to \ell \tilde{\mathrm{G}})$ is 100%. The area to the left of the solid curves represents the observed exclusion region, and the dashed lines indicate the expected limits

Fig. 6

The 95% $\text{CL}$ upper limits on the ${\mathrm{H}}_{}^{}\to {\mathrm{S}}_{}^{}{\mathrm{S}}_{}^{}$, ${\mathrm{S}}_{}^{}\to {\ell }^{+}{\ell }^{-}$ branching fraction as a function of $c{\tau }_0$, for a Higgs boson with a mass of 125$\text{Ge}$ and a long-lived scalar with a mass of 30$\text{Ge}$ or 50$\text{Ge}$, for the three channels combined. These limits assume that $\mathcal{B}({\mathrm{H}}_{}^{}\to {\mathrm{S}}_{}^{}{\mathrm{S}}_{}^{})=100\%$ and each ${\mathrm{S}}_{}^{}$ has a 50% probability of decaying to two electrons or two muons. The area above the solid (dashed) curve represents the observed (expected) exclusion region