A search for long-lived particles using final states including a displaced vertex with low-momentum tracks, large missing transverse momentum, and a jet from initial-state radiation is presented. This search uses proton-proton collision data at a center-of-mass energy of 13 TeV collected by the CMS experiment at the CERN LHC in 2017 and 2018, with a total integrated luminosity of 100 fb$^{-1}$. This analysis adopts specific supersymmetric (SUSY) coannihilation scenarios as benchmark signal models, characterized by a next-to-lightest SUSY particle (NLSP) with a mass difference of less than 25GeV relative to the lightest SUSY particle, assumed to be a bino-like neutralino. In the top squark ($\tilde{\mathrm{t}}$) NLSP model, the NLSP is a long-lived $\tilde{\mathrm{t}}$, while in the bino-wino NLSP scenario, the mass-degenerate NLSPs are a wino-like long-lived neutralino and a short-lived chargino. The search excludes top squarks with masses less than 400$-$1100 GeV and wino-like neutralinos with masses less than 220$-$550 GeV, depending on the signal parameters, including the mass difference, mass, and lifetime of the long-lived particle. It sets the most stringent limits to date for the $\tilde{\mathrm{t}}$ and bino-wino NLSP models.
The number of observed and predicted background events after the fit to the regions of the search planes. In addition, two representative signals are shown. The predicted background is shown with its associated uncertainties. The observed data are displayed with the 68% confidence level Poisson confidence intervals. The lower panel shows the fractional difference between the observed data and the predicted background.
Observed 95% CL upper limits on the $\tilde{t}$ production cross section, as functions of $m_{\tilde{t}}$ and $\Delta m$, for $\mathcal{B}(\tilde{t} \to bf\overline{f}'\tilde{\chi}^{0}_{1})$ of 10%. The observed (solid black) and expected (dashed red) exclusion curves are overlaid on the plots. The search excludes the region to the left of the exclusion curves.
Observed 95% CL upper limits on the $\tilde{t}$ production cross section, as functions of $m_{\tilde{t}}$ and $\Delta m$, for $\mathcal{B}(\tilde{t} \to bf\overline{f}'\tilde{\chi}^{0}_{1})$ of 10%. The observed (solid black) and expected (dashed red) exclusion curves are overlaid on the plots. The search excludes the region to the left of the exclusion curves.
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