CMSEXO21008 ; CERNEP2024008  
Search for longlived particles decaying in the CMS muon detectors in protonproton collisions at $ \sqrt{s}= $ 13 TeV  
CMS Collaboration  
3 February 2024  
Accepted for publication in Phys. Rev. D  
Abstract: A search for longlived particles (LLPs) decaying in the CMS muon detectors is presented. A data sample of protonproton collisions at $ \sqrt{s}= $ 13 TeV corresponding to an integrated luminosity of 138 fb$ ^{1} $, recorded at the LHC in 20162018, is used. The decays of LLPs are reconstructed as high multiplicity clusters of hits in the muon detectors. In the context of twin Higgs models, the search is sensitive to LLP masses from 0.4 to 55 GeV and a broad range of LLP decay modes, including decays to hadrons, $ \tau $ leptons, electrons, or photons. No excess of events above the standard model background is observed. The most stringent limits to date from LHC data are set on the branching fraction of the Higgs boson decay to a pair of LLPs with masses below 10 GeV. This search also provides the best limits for various intervals of LLP proper decay length and mass. Finally, this search sets the first limits at the LHC on a dark quantum chromodynamic sector whose particles couple to the Higgs boson through gluon, Higgs boson, photon, vector, and darkphoton portals, and is sensitive to branching fractions of the Higgs boson to dark quarks as low as 2 $ \times $ 10$^{3} $.  
Links: eprint arXiv:2402.01898 [hepex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ; 
Figures  
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Figure 1:
Diagram representing the twin Higgs and darkshower models. The SM Higgs boson (H) decays to a pair of neutral longlived scalars (S) in the twin Higgs model or to a pair of darksector quarks ($\Psi$) in the dark shower model. 
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Figure 2:
The cluster reconstruction efficiency, including both DT and CSC clusters, as a function of the simulated $ r $ and $ z $ decay positions of the particle S decaying to $ \mathrm{d}\overline{\mathrm{d}} $ in events with $ p_{\mathrm{T}}^\text{miss} > $ 200 GeV, for a mass of 40 GeV and a range of $ c\tau $ values uniformly distributed between 1 and 10 m. The cluster reconstruction efficiency appears to be nonzero beyond MB4 because the MB4 chambers are staggered so that the outer radius of the CMS detector ranges from 738 to 800 cm. The barrel and endcap muon stations are drawn as black boxes and labeled by their station names. The region between labeled sections are mostly steel return yoke. 
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Figure 3:
The DT (left) and CSC (right) cluster reconstruction efficiency as a function of the simulated $ r $ or $ z $ decay positions of S decaying to $ \mathrm{d}\overline{\mathrm{d}} $ in events with $ p_{\mathrm{T}}^\text{miss} > $ 200 GeV, for a mass of 40 GeV and a range of $ c\tau $ values between 1 and 10 m. The DT cluster reconstruction efficiency is shown for events where the LLP decay occurs at $ z < $ 700 cm. The DT cluster reconstruction efficiency appears to be nonzero beyond MB4 because the MB4 chambers are staggered so that the outer radius of the CMS detector ranges from 738 to 800 cm. The CSC cluster reconstruction efficiency is shown for events where the LLP decay occurs at $ r < $ 700 cm and $ \eta < $ 2.6. The clusters are selected from signal events satisfying the $ p_{\mathrm{T}}^\text{miss} > $ 200 GeV requirement. Regions occupied by steel shielding are shaded in gray. 
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Figure 3a:
The DT cluster reconstruction efficiency as a function of the simulated $ r $ or $ z $ decay positions of S decaying to $ \mathrm{d}\overline{\mathrm{d}} $ in events with $ p_{\mathrm{T}}^\text{miss} > $ 200 GeV, for a mass of 40 GeV and a range of $ c\tau $ values between 1 and 10 m. The DT cluster reconstruction efficiency is shown for events where the LLP decay occurs at $ z < $ 700 cm. The DT cluster reconstruction efficiency appears to be nonzero beyond MB4 because the MB4 chambers are staggered so that the outer radius of the CMS detector ranges from 738 to 800 cm. Regions occupied by steel shielding are shaded in gray. 
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Figure 3b:
The CSC cluster reconstruction efficiency as a function of the simulated $ r $ or $ z $ decay positions of S decaying to $ \mathrm{d}\overline{\mathrm{d}} $ in events with $ p_{\mathrm{T}}^\text{miss} > $ 200 GeV, for a mass of 40 GeV and a range of $ c\tau $ values between 1 and 10 m. The CSC cluster reconstruction efficiency is shown for events where the LLP decay occurs at $ r < $ 700 cm and $ \eta < $ 2.6. The clusters are selected from signal events satisfying the $ p_{\mathrm{T}}^\text{miss} > $ 200 GeV requirement. Regions occupied by steel shielding are shaded in gray. 
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Figure 4:
The geometric acceptance multiplied by the efficiency of the $ p_{\mathrm{T}}^\text{miss} > $ 200 GeV selection, as a function of the proper decay length $ c\tau $ for a scalar particle S with a mass of 40 GeV. 
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Figure 5:
Distributions of the cluster time ($ t_\text{cluster} $) for signal, where S decays to $ \mathrm{d}\overline{\mathrm{d}} $ with a proper decay length $ c\tau $ of 1 m and mass of 40 GeV, and for a backgroundenriched sample in data selected by inverting the $ N_\text{hits} $ requirement. 
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Figure 6:
The distributions of $ N_\text{hits} $ (left) and $ \Delta\phi{\mathrm{(}{\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}}\mathrm{, cluster)}} $ (right) for single CSC clusters are shown for S decaying to $ \mathrm{d}\overline{\mathrm{d}} $ for a proper decay length of 1 m and various masses compared to the OOT background ($ t_\text{cluster} <  $12.5 ns). The OOT background is representative of the overall background shape, because the background passing all the selections described above is dominated by pileup and underlying events. The shaded bands show the statistical uncertainty in the background. 
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Figure 6a:
The distribution of $ N_\text{hits} $ for single CSC clusters are shown for S decaying to $ \mathrm{d}\overline{\mathrm{d}} $ for a proper decay length of 1 m and various masses compared to the OOT background ($ t_\text{cluster} <  $12.5 ns). The OOT background is representative of the overall background shape, because the background passing all the selections described above is dominated by pileup and underlying events. The shaded bands show the statistical uncertainty in the background. 
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Figure 6b:
The distribution of $ \Delta\phi{\mathrm{(}{\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}}\mathrm{, cluster)}} $ for single CSC clusters are shown for S decaying to $ \mathrm{d}\overline{\mathrm{d}} $ for a proper decay length of 1 m and various masses compared to the OOT background ($ t_\text{cluster} <  $12.5 ns). The OOT background is representative of the overall background shape, because the background passing all the selections described above is dominated by pileup and underlying events. The shaded bands show the statistical uncertainty in the background. 
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Figure 7:
The distributions of $ N_\text{hits} $ (left) and $ \Delta\phi{\mathrm{(}{\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}}\mathrm{, cluster)}} $ (right) for DT clusters are shown for S decaying to $ \mathrm{d}\overline{\mathrm{d}} $ for a proper decay length of 1 m and various masses compared to the shape of background in a selection in which the cluster is not matched to any RPC hit and passes all other selections. The background is dominated by clusters from noise and low$ p_{\mathrm{T}} $ particles. The shaded bands show the statistical uncertainty in the background. 
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Figure 7a:
The distribution of $ N_\text{hits} $ for DT clusters are shown for S decaying to $ \mathrm{d}\overline{\mathrm{d}} $ for a proper decay length of 1 m and various masses compared to the shape of background in a selection in which the cluster is not matched to any RPC hit and passes all other selections. The background is dominated by clusters from noise and low$ p_{\mathrm{T}} $ particles. The shaded bands show the statistical uncertainty in the background. 
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Figure 7b:
The distribution of $ \Delta\phi{\mathrm{(}{\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}}\mathrm{, cluster)}} $ for DT clusters are shown for S decaying to $ \mathrm{d}\overline{\mathrm{d}} $ for a proper decay length of 1 m and various masses compared to the shape of background in a selection in which the cluster is not matched to any RPC hit and passes all other selections. The background is dominated by clusters from noise and low$ p_{\mathrm{T}} $ particles. The shaded bands show the statistical uncertainty in the background. 
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Figure 8:
Diagrams illustrating the ABCD plane for the DTCSC category (left), and for the DTDT and CSCCSC categories (right). The variable $ c_1 $ is the passfail ratio of the $ N_\text{hits} $ selection for the background clusters. Bin A is the signal region (SR) for all categories. The size of the blue boxes on the left represents the approximate size of the expected background yield in each bin. 
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Figure 8a:
Diagram illustrating the ABCD plane for the DTCSC category. Bin A is the signal region (SR). The size of the blue boxes represents the approximate size of the expected background yield in each bin. 
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Figure 8b:
Diagram illustrating the ABCD plane for the DTDT and CSCCSC categories. The variable $ c_1 $ is the passfail ratio of the $ N_\text{hits} $ selection for the background clusters. Bin A is the signal region (SR) for all categories. 
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Figure 9:
Diagram illustrating the ABCD plane for the singleCSCcluster category, where bin A is the signal region (SR). 
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Figure 10:
The signal (assuming $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) = $ 1%, $ \mathrm{S} \to \mathrm{d}\overline{\mathrm{d}} $, and $ c\tau= $ 1 m), background, and data distributions of $ N_\text{clusters} $ passing the $ N_\text{hits} $ selection in the search region for CSCCSC (upper left), DTDT (upper right), and DTCSC (lower) categories. The background prediction is obtained from the fit to the observed data assuming no signal contribution, and is shown in blue with the shaded region showing the fitted uncertainty. 
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Figure 10a:
The signal (assuming $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) = $ 1%, $ \mathrm{S} \to \mathrm{d}\overline{\mathrm{d}} $, and $ c\tau= $ 1 m), background, and data distributions of $ N_\text{clusters} $ passing the $ N_\text{hits} $ selection in the search region for the CSCCSC category. The background prediction is obtained from the fit to the observed data assuming no signal contribution, and is shown in blue with the shaded region showing the fitted uncertainty. 
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Figure 10b:
The signal (assuming $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) = $ 1%, $ \mathrm{S} \to \mathrm{d}\overline{\mathrm{d}} $, and $ c\tau= $ 1 m), background, and data distributions of $ N_\text{clusters} $ passing the $ N_\text{hits} $ selection in the search region for the DTDT category. The background prediction is obtained from the fit to the observed data assuming no signal contribution, and is shown in blue with the shaded region showing the fitted uncertainty. 
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Figure 10c:
The signal (assuming $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) = $ 1%, $ \mathrm{S} \to \mathrm{d}\overline{\mathrm{d}} $, and $ c\tau= $ 1 m), background, and data distributions of $ N_\text{clusters} $ passing the $ N_\text{hits} $ selection in the search region for the DTCSC category. The background prediction is obtained from the fit to the observed data assuming no signal contribution, and is shown in blue with the shaded region showing the fitted uncertainty. 
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Figure 11:
Distributions of $ N_\text{hits} $ (left) and $ \Delta\phi{\mathrm{(}{\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}}\mathrm{, cluster)}} $ (right) in the search region of the singleCSCcluster category. The background prediction is obtained from the fit to the observed data assuming no signal contribution, and is shown in blue with the shaded region showing the fitted uncertainty. The expected signal with $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) = $ 1%, $ \mathrm{S}\to\mathrm{d}\overline{\mathrm{d}} $, and $ c\tau= $ 1 m is shown for $ m_\mathrm{S} $ of 3, 7, 15, 40, and 55 GeV in various colors and dotted lines. The $ N_\text{hits} $ distribution includes only events in bins A and D, while the $ \Delta\phi{\mathrm{(}{\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}}\mathrm{, cluster)}} $ distribution includes only events in bins A and B. The rightmost bin in the $ N_\text{hits} $ distribution includes overflow events. 
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Figure 11a:
Distribution of $ N_\text{hits} $ in the search region of the singleCSCcluster category. The background prediction is obtained from the fit to the observed data assuming no signal contribution, and is shown in blue with the shaded region showing the fitted uncertainty. The expected signal with $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) = $ 1%, $ \mathrm{S}\to\mathrm{d}\overline{\mathrm{d}} $, and $ c\tau= $ 1 m is shown for $ m_\mathrm{S} $ of 3, 7, 15, 40, and 55 GeV in various colors and dotted lines. The distribution includes only events in bins A and D. The rightmost bin includes overflow events. 
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Figure 11b:
Distribution of $ \Delta\phi{\mathrm{(}{\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}}\mathrm{, cluster)}} $ in the search region of the singleCSCcluster category. The background prediction is obtained from the fit to the observed data assuming no signal contribution, and is shown in blue with the shaded region showing the fitted uncertainty. The expected signal with $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) = $ 1%, $ \mathrm{S}\to\mathrm{d}\overline{\mathrm{d}} $, and $ c\tau= $ 1 m is shown for $ m_\mathrm{S} $ of 3, 7, 15, 40, and 55 GeV in various colors and dotted lines. The distribution includes only events in bins A and B. 
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Figure 12:
Distributions of $ N_\text{hits} $ (left) and $ \Delta\phi{\mathrm{(}{\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}}\mathrm{, cluster)}} $ (right) in the search region of the singleDTcluster category. The background prediction is obtained from the fit to the observed data assuming no signal contribution, and is shown in blue with the shaded region showing the fitted uncertainty. The expected signal with $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) = $ 1%, $ \mathrm{S}\to\mathrm{d}\overline{\mathrm{d}} $, and $ c\tau= $ 1 m is shown for $ m_\mathrm{S} $ of 3, 7, 15, 40, and 55 GeV in various colors and dotted lines. The $ N_\text{hits} $ distribution includes only events in bins A and D, while the $ \Delta\phi{\mathrm{(}{\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}}\mathrm{, cluster)}} $ one includes only events in bins A and B. The rightmost bin in the $ N_\text{hits} $ distribution includes overflow events. 
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Figure 12a:
Distribution of $ N_\text{hits} $ $ \Delta\phi{\mathrm{(}{\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}}\mathrm{, cluster)}} $ in the search region of the singleDTcluster category. The background prediction is obtained from the fit to the observed data assuming no signal contribution, and is shown in blue with the shaded region showing the fitted uncertainty. The expected signal with $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) = $ 1%, $ \mathrm{S}\to\mathrm{d}\overline{\mathrm{d}} $, and $ c\tau= $ 1 m is shown for $ m_\mathrm{S} $ of 3, 7, 15, 40, and 55 GeV in various colors and dotted lines. 
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Figure 12b:
Distributions of $ N_\text{hits} $ (left) and $ \Delta\phi{\mathrm{(}{\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}}\mathrm{, cluster)}} $ (right) in the search region of the singleDTcluster category. The background prediction is obtained from the fit to the observed data assuming no signal contribution, and is shown in blue with the shaded region showing the fitted uncertainty. The expected signal with $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) = $ 1%, $ \mathrm{S}\to\mathrm{d}\overline{\mathrm{d}} $, and $ c\tau= $ 1 m is shown for $ m_\mathrm{S} $ of 3, 7, 15, 40, and 55 GeV in various colors and dotted lines. The $ N_\text{hits} $ distribution includes only events in bins A and D, while the $ \Delta\phi{\mathrm{(}{\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}}\mathrm{, cluster)}} $ one includes only events in bins A and B. The rightmost bin in the $ N_\text{hits} $ distribution includes overflow events. 
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Figure 13:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of $ c\tau $ for the $ \mathrm{S}\to\mathrm{d}\overline{\mathrm{d}} $ (upper left), $ \mathrm{S}\to\pi^{0}\pi^{0} $ (upper right), and $ \mathrm{S}\to\tau^{+}\tau^{} $ (lower) decay modes. The exclusion limits are shown for different mass hypotheses. 
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Figure 13a:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of $ c\tau $ for the $ \mathrm{S}\to\mathrm{d}\overline{\mathrm{d}} $ decay mode. The exclusion limits are shown for different mass hypotheses. 
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Figure 13b:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of $ c\tau $ for the $ \mathrm{S}\to\pi^{0}\pi^{0} $ decay mode. The exclusion limits are shown for different mass hypotheses. 
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Figure 13c:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of $ c\tau $ for the $ \mathrm{S}\to\tau^{+}\tau^{} $ decay mode. The exclusion limits are shown for different mass hypotheses. 
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Figure 14:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of mass and $ c\tau $ for the $ \mathrm{S}\to\mathrm{d}\overline{\mathrm{d}} $ (upper left), $ \mathrm{S}\to\pi^{0}\pi^{0} $ (upper right), and $ \mathrm{S}\to\tau^{+}\tau^{} $ (lower) decay modes. The area inside the solid contours represents the region for which the limit is smaller than 0.01. 
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Figure 14a:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of mass and $ c\tau $ for the $ \mathrm{S}\to\mathrm{d}\overline{\mathrm{d}} $ decay mode. The area inside the solid contours represents the region for which the limit is smaller than 0.01. 
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Figure 14b:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of mass and $ c\tau $ for the $ \mathrm{S}\to\pi^{0}\pi^{0} $ decay mode. The area inside the solid contours represents the region for which the limit is smaller than 0.01. 
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Figure 14c:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of mass and $ c\tau $ for the $ \mathrm{S}\to\tau^{+}\tau^{} $ decay mode. The area inside the solid contours represents the region for which the limit is smaller than 0.01. 
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Figure 15:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of $ c\tau $ for the $ \mathrm{S}\to\mathrm{b}\overline{\mathrm{b}} $ (upper left), $ \mathrm{S}\to\pi^{+}\pi^{} $ (upper right), $ \mathrm{S}\to \mathrm{K^+}\mathrm{K^} $ (middle left), $ \mathrm{S}\to \mathrm{K^0}\mathrm{K^0} $ (middle right), $ \mathrm{S}\to \gamma\gamma $ (lower left), and $ \mathrm{S}\to \mathrm{e}^+\mathrm{e}^ $ (lower right) decay modes. The exclusion limits are shown for different mass hypotheses. 
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Figure 15a:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of $ c\tau $ for the $ \mathrm{S}\to\mathrm{b}\overline{\mathrm{b}} $ decay mode. The exclusion limits are shown for different mass hypotheses. 
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Figure 15b:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of $ c\tau $ for the $ \mathrm{S}\to\pi^{+}\pi^{} $ decay mode. The exclusion limits are shown for different mass hypotheses. 
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Figure 15c:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of $ c\tau $ for the $ \mathrm{S}\to \mathrm{K^+}\mathrm{K^} $ decay mode. The exclusion limits are shown for different mass hypotheses. 
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Figure 15d:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of $ c\tau $ for the $ \mathrm{S}\to \mathrm{K^0}\mathrm{K^0} $ decay mode. The exclusion limits are shown for different mass hypotheses. 
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Figure 15e:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of $ c\tau $ for the $ \mathrm{S}\to \gamma\gamma $ decay mode. The exclusion limits are shown for different mass hypotheses. 
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Figure 15f:
The 95% CL expected (dotted curves) and observed (solid curves) upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of $ c\tau $ for the $ \mathrm{S}\to \mathrm{e}^+\mathrm{e}^ $ decay mode. The exclusion limits are shown for different mass hypotheses. 
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Figure 16:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of mass and $ c\tau $ for the $ \mathrm{S}\to\mathrm{b}\overline{\mathrm{b}} $ (upper left), $ \mathrm{S}\to\pi^{+}\pi^{} $ (upper right), $ \mathrm{S}\to \mathrm{K^+}\mathrm{K^} $ (middle left), $ \mathrm{S}\to \mathrm{K^0}\mathrm{K^0} $ (middle right), $ \mathrm{S}\to\gamma\gamma $ (lower left), $ \mathrm{S}\to \mathrm{e}^+\mathrm{e}^ $ (lower right) decay modes. The area inside the solid contours represents the region for which the limit is smaller than 0.01. 
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Figure 16a:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of mass and $ c\tau $ for the $ \mathrm{S}\to\mathrm{b}\overline{\mathrm{b}} $ decay modes. The area inside the solid contours represents the region for which the limit is smaller than 0.01. 
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Figure 16b:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of mass and $ c\tau $ for the $ \mathrm{S}\to\pi^{+}\pi^{} $ decay modes. The area inside the solid contours represents the region for which the limit is smaller than 0.01. 
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Figure 16c:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of mass and $ c\tau $ for the $ \mathrm{S}\to \mathrm{K^+}\mathrm{K^} $ decay modes. The area inside the solid contours represents the region for which the limit is smaller than 0.01. 
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Figure 16d:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of mass and $ c\tau $ for the $ \mathrm{S}\to \mathrm{K^0}\mathrm{K^0} $ decay modes. The area inside the solid contours represents the region for which the limit is smaller than 0.01. 
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Figure 16e:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of mass and $ c\tau $ for the $ \mathrm{S}\to\gamma\gamma $ decay modes. The area inside the solid contours represents the region for which the limit is smaller than 0.01. 
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Figure 16f:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as functions of mass and $ c\tau $ for the $ \mathrm{S}\to \mathrm{e}^+\mathrm{e}^ $ decay modes. The area inside the solid contours represents the region for which the limit is smaller than 0.01. 
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Figure 17:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) $ as a function of mass and $ c\tau $, assuming the branching fractions for \mathrm{S} are identical to those of a Higgs boson evaluated at $ m_\mathrm{S} $ [66]. The area inside the solid contours represents the region for which the limit is smaller than 0.01. 
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Figure 18:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower vector portal assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (1, 1) $. The exclusion limits are shown for different LLP mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
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Figure 19:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower gluon portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (1, 1) $ (upper left), $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 1.0) $ (upper right), and $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 2.5) $ (lower). The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
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Figure 19a:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower gluon portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (1, 1) $. The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
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Figure 19b:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower gluon portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 1.0) $. The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
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Figure 19c:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower gluon portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 2.5) $. The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
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Figure 20:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower photon portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (1, 1) $ (upper left), $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 1.0) $ (upper right), and $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 2.5) $ (lower). The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
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Figure 20a:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower photon portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (1, 1) $. The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
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Figure 20b:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower photon portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 1.0) $. The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
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Figure 20c:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower photon portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 2.5) $. The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
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Figure 21:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower Higgs boson portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (1, 1) $ (upper left), $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 1.0) $ (upper right), and $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 2.5) $ (lower). The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
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Figure 21a:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower Higgs boson portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (1, 1) $. The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
png pdf 
Figure 21b:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower Higgs boson portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 1.0) $. The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
png pdf 
Figure 21c:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower Higgs boson portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 2.5) $. The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
png pdf 
Figure 22:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower darkphoton portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (1, 1) $ (upper left), $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 1.0) $ (upper right), and $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 2.5) $ (lower). The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
png pdf 
Figure 22a:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower darkphoton portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (1, 1) $. The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
png pdf 
Figure 22b:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower darkphoton portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 1.0) $. The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
png pdf 
Figure 22c:
The 95% CL observed upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\Psi\Psi) $ as functions of $ c\tau $ for the dark shower darkphoton portal, assuming $ (\xi_{\omega} $, $ \xi_{\Lambda}) = (2.5, 2.5) $. The exclusion limits are shown for different mass hypotheses. The limits are calculated only at the proper decay lengths indicated by the markers and the lines connecting the markers are linear interpolations. 
Tables  
png pdf 
Table 1:
Validation of the ABCD method for the doublecluster category in both VRs. The uncertainty in the prediction is the statistical uncertainty propagated from bins B, C, and D or bins BD and C. The expected background event rate in bin A ($ \lambda_\mathrm{A} $) and the background event rate in bins B, C, D, BD, and A ($ N_\mathrm{B} $, $ N_\mathrm{C} $, $ N_\mathrm{D} $, $ N_\mathrm{BD} $, $ N_\mathrm{A} $) are shown. 
png pdf 
Table 2:
Validation of the ABCD method for the singleCSCcluster category in both VRs. The uncertainty in the prediction is the statistical uncertainty propagated from bins B, C, and D. The expected background event rate in bin A ($ \lambda_\mathrm{A} $) and the background event rate in bins B, C, D, and A ($ N_\mathrm{B} $, $ N_\mathrm{C} $, $ N_\mathrm{D} $, $ N_\mathrm{A} $) are shown. 
png pdf 
Table 3:
Validation of the ABCD method for the singleDTcluster category in a pileupenriched region. The uncertainty in the prediction is the statistical uncertainty propagated from bins B, C, and D. Bins C and D for the MB3 and MB4 categories are combined to reduce the statistical uncertainty in the two regions. The final ABCD fit in the SR will also be performed with those bins combined. 
png pdf 
Table 4:
Validation of the punchthrough jet background prediction method for the singleDTcluster category. The uncertainty in the prediction is the statistical uncertainty propagated from the extrapolated MB1/MB2 hit veto efficiency. 
png pdf 
Table 5:
Number of predicted background and observed events in the doublecluster category. The background prediction is obtained from a fit to the observed data assuming no signal contribution. 
png pdf 
Table 6:
Number of predicted background and observed events in the singleCSCcluster category. The background prediction is obtained from a fit to the observed data assuming no signal contribution. 
png pdf 
Table 7:
Number of predicted background and observed events in the singleDTcluster category. The background prediction is obtained from a fit to the observed data assuming no signal contribution. 
png pdf 
Table 8:
Expected number of signal events in bin A for each category, for a few benchmark signal models assuming $ \mathcal{B}(\mathrm{H}\to\mathrm{S}\mathrm{S}) = $ 1%. 
Summary 
Data from protonproton collisions at $ \sqrt{s} = $ 13 TeV recorded by the CMS experiment in 20162018, corresponding to an integrated luminosity of 138 fb$ ^{1} $, have been used to conduct the first search that uses both the barrel and endcap CMS muon detectors to detect hadronic and electromagnetic showers from longlived particle (LLP) decays. Based on this unique detector signature, the search is largely modelindependent, with sensitivity to a broad range of LLP decay modes and masses extending below the GeVns scale. With the excellent shielding provided by the inner CMS detector, the CMS magnet and its steel fluxreturn yoke, the background is suppressed to a low level and a search for both single and pairs of LLPs decays is possible. No significant deviation from the standard model background is observed. The most stringent LHC constraints to date are set on the branching fraction of the Higgs boson to LLPs with masses below 10 GeV and decaying to particles other than muons. For higher LLP masses the search provides the most stringent branching fraction limits for specific proper decay lengths: 0.040.40 m and above 5 m for 15 GeV LLP; 0.30.9 m and above 3 m for 40 GeV LLP; and above 0.9 m for 55 GeV LLP. Finally, the first LHC limits on models of dark showers produced via H decay are set, and constrain branching fractions of the H decay to dark quarks as low as 2 $ \times $ 10$^{3} $ at 95% confidence level. 
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