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| CMS-PAS-EXO-18-003 | ||
| Search for long-lived particles decaying to displaced leptons in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| July 2021 | ||
| Abstract: A search for new long-lived particles decaying to leptons is presented using proton-proton collisions produced by the CERN LHC at a center-of-mass energy of 13 TeV. Events are selected with two leptons (an electron [e] and a muon [$\mu$], two electrons, or two muons) that both have transverse impact parameter values between 0.01 cm and 10 cm. Data used for the analysis were collected by the CMS detector in 2016, 2017, and 2018, and correspond to an integrated luminosity of 113 fb$^{-1}$ for the e$\mu$ and $\mu\mu$ channels and 118 fb$^{-1}$ for the ee. The search is designed to be sensitive to a wide range of models with nonprompt e$\mu$, ee, and $\mu\mu$ final states. The results are interpreted with models involving top squarks that decay to displaced leptons via R-parity-violating interactions, a gauge-mediated supersymmetry breaking model with lepton superpartners that decay to gravitinos and displaced leptons, and a model involving exotic Higgs bosons that decay to long-lived scalars, which in turn decay to displaced leptons. This is the first search at CMS for displaced leptons at a center-of-mass energy of 13 TeV, and the first search at CMS for displaced leptons in the ee and $\mu\mu$ channels that does not require the leptons to come from a common displaced vertex. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, EPJC 82 (2022) 153. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Feynman diagrams for $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{b} \,\ell \mathrm{\bar{b}} $ (upper left), $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{d} \,\ell \mathrm{\bar{d}} $ (upper right), $\tilde{\ell} \overline{\tilde{\ell}} \to \bar{\ell} \tilde{\mathrm{G}} \,\ell \tilde{\mathrm{G}} $ (lower left), and $\mathrm{H} \to \mathrm{S} \mathrm{S} \to \ell \bar{\ell} \,\ell \bar{\ell} $ (lower right). |
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Figure 1-a:
Feynman diagrams for $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{b} \,\ell \mathrm{\bar{b}} $ (upper left), $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{d} \,\ell \mathrm{\bar{d}} $ (upper right), $\tilde{\ell} \overline{\tilde{\ell}} \to \bar{\ell} \tilde{\mathrm{G}} \,\ell \tilde{\mathrm{G}} $ (lower left), and $\mathrm{H} \to \mathrm{S} \mathrm{S} \to \ell \bar{\ell} \,\ell \bar{\ell} $ (lower right). |
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Figure 1-b:
Feynman diagrams for $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{b} \,\ell \mathrm{\bar{b}} $ (upper left), $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{d} \,\ell \mathrm{\bar{d}} $ (upper right), $\tilde{\ell} \overline{\tilde{\ell}} \to \bar{\ell} \tilde{\mathrm{G}} \,\ell \tilde{\mathrm{G}} $ (lower left), and $\mathrm{H} \to \mathrm{S} \mathrm{S} \to \ell \bar{\ell} \,\ell \bar{\ell} $ (lower right). |
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Figure 1-c:
Feynman diagrams for $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{b} \,\ell \mathrm{\bar{b}} $ (upper left), $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{d} \,\ell \mathrm{\bar{d}} $ (upper right), $\tilde{\ell} \overline{\tilde{\ell}} \to \bar{\ell} \tilde{\mathrm{G}} \,\ell \tilde{\mathrm{G}} $ (lower left), and $\mathrm{H} \to \mathrm{S} \mathrm{S} \to \ell \bar{\ell} \,\ell \bar{\ell} $ (lower right). |
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Figure 1-d:
Feynman diagrams for $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{b} \,\ell \mathrm{\bar{b}} $ (upper left), $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{d} \,\ell \mathrm{\bar{d}} $ (upper right), $\tilde{\ell} \overline{\tilde{\ell}} \to \bar{\ell} \tilde{\mathrm{G}} \,\ell \tilde{\mathrm{G}} $ (lower left), and $\mathrm{H} \to \mathrm{S} \mathrm{S} \to \ell \bar{\ell} \,\ell \bar{\ell} $ (lower right). |
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Figure 2:
A diagram of the ABCD method, shown for illustration on background simulation passing the e$ \mu $ preselection with 2018 conditions. If a $ | d_0 | $ value is less than unity, it is set to unity for plotting. A, B, and C are control regions, and D is the inclusive SR. SRs I-IV are described in the text below. |
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Figure 3:
The number of estimated background and observed events in each channel and SR, with a representative signal overlaid. For each background estimate and signal yield, the total uncertainty (statistical plus systematic) is given. The distributions shown are those obtained before the final maximum likelihood fit to the data. |
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Figure 4:
The observed 95% $ 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}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{b} \,\ell \mathrm{\bar{b}} $ (left) and $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{d} \,\ell \mathrm{\bar{d}} $ (right) processes are shown. 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% $ CL $s. |
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Figure 4-a:
The observed 95% $ 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}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{b} \,\ell \mathrm{\bar{b}} $ (left) and $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{d} \,\ell \mathrm{\bar{d}} $ (right) processes are shown. 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% $ CL $s. |
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Figure 4-b:
The observed 95% $ 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}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{b} \,\ell \mathrm{\bar{b}} $ (left) and $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{d} \,\ell \mathrm{\bar{d}} $ (right) processes are shown. 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% $ CL $s. |
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Figure 5:
The 95% $ CL $ upper limits on the long-lived slepton production cross section, in the $c\tau _0$ - mass plane. The $\tilde{\tau}$ and co-NLSP limits are shown for the three channels combined, while the $\tilde{\mathrm{e}}$ and $\tilde{\mu}$ NLSP limits are shown for the ee and $\mu \mu $ channels, respectively. The area to the left of the solid curves represents the observed exclusion region, and the dashed lines indicate the expected limits. |
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Figure 6:
The 95% $ CL $ upper limits on the $\mathrm{H} \to \mathrm{S} \mathrm{S} \to \ell \bar{\ell} \,\ell \bar{\ell} $ branching fraction as a function of $c\tau _0$, for a Higgs boson with a mass of 125 GeV and a long-lived scalar with a mass of 30 GeV or 50 GeV, for the three channels combined. The area above the solid (dashed) curve represents the observed (expected) exclusion region. |
| Tables | |
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Table 1:
The cumulative efficiency for $\tilde{\mathrm{t}} \overline{\tilde{\mathrm{t}}} \to \bar{\ell} \mathrm{b} \,\ell \mathrm{\bar{b}} $ signal events to pass the 2018 inclusive signal region selection, for several choices of $\tilde{\mathrm{t}}$ mass and $c\tau _0$. The corrections described in Section 6 are applied. |
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Table 2:
Closure test results in data, background simulation, and background simulation with the DY$\to \tau \tau $ events removed in the 100-500 m region. The extrapolated ratios of the actual yield to the estimated yield (averaged over the two one-prompt/one-displaced sidebands) and their statistical uncertainties are given. |
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Table 3:
Closure test results in data, background simulation, and background simulation with the DY$\to \tau \tau $ events removed in the 500 m -10 cm region. The ratios of the actual yield to the estimated yield (averaged over the two one-prompt/one-displaced sidebands) and their statistical uncertainties are given. |
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Table 4:
Closure test results in background simulation in the SRs, with the correction applied. The estimated number of events, the actual number of events, and their total uncertainties (statistical plus systematic) are given. In cases where the actual number of events is zero, the uncertainty is given by the product of the average background simulation event weight and the upper bound of the 68% Poisson interval given by a single observation of zero events. |
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Table 5:
Systematic uncertainties in the signal efficiency, for all three years and the three channels. For many sources of uncertainty, a range indicating the 68% $ CL $ of the spread is given, followed by the mean. Uncertainties in the same row are treated as correlated among the years of data taking, except for the displaced tracking and muon pixel hit efficiencies, where the 2016 uncertainty is treated as uncorrelated with the 2017 and 2018 uncertainties. |
| Summary |
| A search has been presented for long-lived particles decaying to displaced leptons in proton-proton collisions at a center-of-mass energy of 13 TeV at the CERN LHC. With collision data recorded in 2016, 2017, and 2018, and corresponding to an integrated luminosity of 113-118 fb$^{-1}$, no excess above the estimated background has been observed. Upper limits have been set at 95% confidence level. Top squarks with masses between 100 and at least 460 GeV have been excluded for proper decay lengths between 0.01 and 1000 cm, with a maximum exclusion of 1500 GeV occurring at a proper decay length of 2 cm, assuming 100% of the top squarks decay to a lepton and a b or d quark, where the lepton has an equal probability of being an electron, muon, or tau lepton. The following exclusions assume that the superpartners of the left- and right-handed leptons are mass degenerate. Electron superpartners with masses of at least 50 GeV have been excluded for proper decay lengths between 0.007 and 70 cm, with a maximum exclusion of 610 GeV occurring at a proper decay length of 0.7 cm. Muon superpartners with masses of at least 50 GeV have been excluded for proper decay lengths between 0.005 and 265 cm, with a maximum exclusion of 610 GeV occurring at a proper decay length of 3 cm. Tau lepton superpartners with masses of at least 50 GeV have been excluded for proper decay lengths between 0.015 and 20 cm, with a maximum exclusion of 405 GeV occurring at a proper decay length of 2 cm. In the case that electron, muon, and tau lepton superpartners are mass degenerate, lepton superpartners with masses between 50 and at least 270 GeV have been excluded for proper decay length between 0.005 and 265 cm, with a maximum exclusion of 680 GeV occurring at a proper decay length of 2 cm. For proper decay lengths between 0.10 and 12 cm, Higgs bosons with a mass of 125 GeV and with branching ratios to two long-lived scalars greater than 0.03% have been excluded, assuming each scalar has a mass of 30 GeV and decays with equal probability to electrons or muons. This is the first search at CMS for displaced leptons at a center-of-mass energy of 13 TeV, and the first search at CMS for displaced leptons in the electron-electron and muon-muon channels that does not require the leptons to come from a common displaced vertex. As a result of the larger center-of-mass energy and integrated luminosity, as well as the addition of the same-flavor channels, the mass exclusion limits for this search improve upon previous CMS results by approximately a factor of 2. |
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Compact Muon Solenoid LHC, CERN |
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