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CMS-PAS-EXO-23-013
Search for low-mass long-lived particles decaying to displaced jets in proton-proton collisions at $ \sqrt{s} = $ 13.6 TeV
Abstract: A search for low-mass long-lived particles decaying to displaced jets is presented, using a data sample corresponding to an integrated luminosity of 34.7 fb$ ^{-1} $, collected with the CMS detector at the CERN LHC in 2022. Novel trigger, reconstruction, and machine learning techniques were developed for and employed in this search. Limits are presented on the branching fraction of the Higgs boson to long-lived particles that subsequently decay to quark pairs or tau lepton pairs. Up to a factor of 10 improvement is achieved over previous limits for models with proper decay lengths smaller than 1 m.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
The Feynman diagram for the benchmark signal model, in which the SM-like Higgs boson with a mass of 125 GeV decays to two long-lived neutral scalars S, and each of them decays to a pair of SM fermions.

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Figure 2:
The predicted background yields and the number of observed events for the data with $ g_{\text{prompt}} > $ 0.985, shown for different bins of the displaced-dijet GNN score $ g_{\text{displaced}} $. Expected signal yields for the $ \mathrm{H}\to\text{S}\text{S} $, $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $ signature are also shown for models with $ m_{\text{S}}= $ 40 GeV and $ c\tau_{0}= $ 1, 10, or 100 mm, assuming a branching fraction of 1% for the $ \mathrm{H}\to\text{S}\text{S} $ decay.

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Figure 3:
The 95% CL upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\text{S}\text{S}) $ for $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $ (upper left), $ \text{S}\to\mathrm{d}\overline{\mathrm{d}} $ (upper right), and $ \text{S}\to\tau\tau $ (lower), for different LLP masses $ m_{\text{S}} $ and proper decay lengths $ c\tau_{0} $.

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Figure 3-a:
The 95% CL upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\text{S}\text{S}) $ for $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $ (upper left), $ \text{S}\to\mathrm{d}\overline{\mathrm{d}} $ (upper right), and $ \text{S}\to\tau\tau $ (lower), for different LLP masses $ m_{\text{S}} $ and proper decay lengths $ c\tau_{0} $.

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Figure 3-b:
The 95% CL upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\text{S}\text{S}) $ for $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $ (upper left), $ \text{S}\to\mathrm{d}\overline{\mathrm{d}} $ (upper right), and $ \text{S}\to\tau\tau $ (lower), for different LLP masses $ m_{\text{S}} $ and proper decay lengths $ c\tau_{0} $.

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Figure 3-c:
The 95% CL upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to\text{S}\text{S}) $ for $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $ (upper left), $ \text{S}\to\mathrm{d}\overline{\mathrm{d}} $ (upper right), and $ \text{S}\to\tau\tau $ (lower), for different LLP masses $ m_{\text{S}} $ and proper decay lengths $ c\tau_{0} $.

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Figure 4:
Comparisons of the observed limits from this search and other results, for $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $, $ m_{\text{S}}= $ 40 GeV (upper left); $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $, $ m_{\text{S}}= $ 15 GeV (upper right); and $ \text{S}\to\mathrm{d}\overline{\mathrm{d}} $, $ m_{\text{S}}= $ 15 GeV (lower). The other results include the previous CMS displaced-jets search [40] (red dashed lines) and the CMS Z + displaced jets search [41] (green dashed lines).

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Figure 4-a:
Comparisons of the observed limits from this search and other results, for $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $, $ m_{\text{S}}= $ 40 GeV (upper left); $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $, $ m_{\text{S}}= $ 15 GeV (upper right); and $ \text{S}\to\mathrm{d}\overline{\mathrm{d}} $, $ m_{\text{S}}= $ 15 GeV (lower). The other results include the previous CMS displaced-jets search [40] (red dashed lines) and the CMS Z + displaced jets search [41] (green dashed lines).

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Figure 4-b:
Comparisons of the observed limits from this search and other results, for $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $, $ m_{\text{S}}= $ 40 GeV (upper left); $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $, $ m_{\text{S}}= $ 15 GeV (upper right); and $ \text{S}\to\mathrm{d}\overline{\mathrm{d}} $, $ m_{\text{S}}= $ 15 GeV (lower). The other results include the previous CMS displaced-jets search [40] (red dashed lines) and the CMS Z + displaced jets search [41] (green dashed lines).

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Figure 4-c:
Comparisons of the observed limits from this search and other results, for $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $, $ m_{\text{S}}= $ 40 GeV (upper left); $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $, $ m_{\text{S}}= $ 15 GeV (upper right); and $ \text{S}\to\mathrm{d}\overline{\mathrm{d}} $, $ m_{\text{S}}= $ 15 GeV (lower). The other results include the previous CMS displaced-jets search [40] (red dashed lines) and the CMS Z + displaced jets search [41] (green dashed lines).

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Figure 5:
The 95% CL limits on the LLP mass $ m_{\text{S}} $ for different proper decay lengths $ c\tau_{0} $ assuming a branching fraction of 1% for the $ \mathrm{H}\to\text{S}\text{S} $ decay, and with subsequent $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $ (left) or $ \text{S}\to\mathrm{d}\overline{\mathrm{d}} $ (right) decays.

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Figure 5-a:
The 95% CL limits on the LLP mass $ m_{\text{S}} $ for different proper decay lengths $ c\tau_{0} $ assuming a branching fraction of 1% for the $ \mathrm{H}\to\text{S}\text{S} $ decay, and with subsequent $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $ (left) or $ \text{S}\to\mathrm{d}\overline{\mathrm{d}} $ (right) decays.

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Figure 5-b:
The 95% CL limits on the LLP mass $ m_{\text{S}} $ for different proper decay lengths $ c\tau_{0} $ assuming a branching fraction of 1% for the $ \mathrm{H}\to\text{S}\text{S} $ decay, and with subsequent $ \text{S}\to\mathrm{b}\overline{\mathrm{b}} $ (left) or $ \text{S}\to\mathrm{d}\overline{\mathrm{d}} $ (right) decays.

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Figure 6:
The 95% CL observed limits on the hidden-sector top partner mass $ m_{{\mathrm{T}} } $ for different hidden glueball masses $ m_{0} $, in the fraternal Twin Higgs model [29] (left) and the folded SUSY model [44] (right).

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Figure 6-a:
The 95% CL observed limits on the hidden-sector top partner mass $ m_{{\mathrm{T}} } $ for different hidden glueball masses $ m_{0} $, in the fraternal Twin Higgs model [29] (left) and the folded SUSY model [44] (right).

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Figure 6-b:
The 95% CL observed limits on the hidden-sector top partner mass $ m_{{\mathrm{T}} } $ for different hidden glueball masses $ m_{0} $, in the fraternal Twin Higgs model [29] (left) and the folded SUSY model [44] (right).
Tables

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Table 1:
Summary of the systematic uncertainties in the signal yields.
Summary
A search for low-mass long-lived particles (LLPs) decaying into jets has been performed using proton-proton collision data corresponding to an integrated luminosity of 34.7 fb$ ^{-1} $, collected with the CMS experiment at a center-of-mass energy of 13.6 TeV in 2022. Novel techniques in trigger, reconstruction, and machine learning were developed for and employed in this search, leading to significant improvements over existing results, despite utilizing a much smaller data set compared to other existing searches. The observed yields are consistent with the background predictions. The best limits to date are set for LLPs with masses between 15 and 55 GeV and with proper decay lengths smaller than $ \approx $1 m. The search provides the first exclusions of hardonically decaying tau leptons with decay lengths smaller than $ \approx $1 m. For the signature where the Higgs boson decays to two LLPs that further decay to bottom (down) quark pairs, branching fractions for the exotic Higgs boson decay larger than 1% are excluded for an LLP mass larger than 40 GeV and mean proper decay lengths between 2 (1) and 370 mm (380 mm), and the branching fraction limits outperform previous results by a factor of up to 10 (8).
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Compact Muon Solenoid
LHC, CERN