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| CMS-PAS-EXO-24-008 | ||
| Search for low-mass hidden valley dark showers with displaced dimuons in proton-proton collisions at $ \sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| 15 April 2025 | ||
| Abstract: A search for signatures of a dark analog to quantum chromodynamics is performed. The analysis targets long-lived dark mesons that decay into standard model particles with a high branching fraction to muons. A unique dataset with $ 10^{10} $ B meson events is used. It was collected by the CMS experiment at the CERN LHC in 2018 using displaced muon triggers, which have a high trigger efficiency for the signal models. Resonant dimuon signatures are searched for, with both pointing and non-pointing topologies. No significant excess is observed beyond the standard model expectation. Upper limits on the branching ratio of Higgs boson decays to dark partons are determined to be less than $ 10^{-3} $, at 95% confidence level, surpassing and extending existing limits for the mean proper lifetime of less than approximately 0.1 $ \mathrm{m} $ for a mass as low as 2 GeV. First limits are set for extended dark shower models, probing the low-mass region down to 0.33 $ \textrm{GeV} $. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Feynman diagram for the vector portal model. Dark partons $ \psi\bar{\psi} $ are first produced from the decay of the SM Higgs boson, which then hadronise to form dark vector mesons $ \tilde{\omega} $ and dark scalar mesons $ \tilde{\eta} $. The $ \tilde{\omega} $ then undergo displaced decay into SM fermions. |
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Figure 2:
Feynman diagram for the Scenario A model (left) and the Scenario B1 model (right). In these extended models the dark hadronization produces a spectrum of dark mesons, including the dark pions $ \pi_{1}, \pi_{2} $ and $ \pi_{3} $. The $ \pi_{3} $ then decays into SM fermions through the dark photon $ A' $. The $ A' $ is a long-lived particle in Scenario A, while the $ \pi_{3} $ is a long-lived particle in Scenario B1, as indicated by the green lines. |
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Figure 2-a:
Feynman diagram for the Scenario A model (left) and the Scenario B1 model (right). In these extended models the dark hadronization produces a spectrum of dark mesons, including the dark pions $ \pi_{1}, \pi_{2} $ and $ \pi_{3} $. The $ \pi_{3} $ then decays into SM fermions through the dark photon $ A' $. The $ A' $ is a long-lived particle in Scenario A, while the $ \pi_{3} $ is a long-lived particle in Scenario B1, as indicated by the green lines. |
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Figure 2-b:
Feynman diagram for the Scenario A model (left) and the Scenario B1 model (right). In these extended models the dark hadronization produces a spectrum of dark mesons, including the dark pions $ \pi_{1}, \pi_{2} $ and $ \pi_{3} $. The $ \pi_{3} $ then decays into SM fermions through the dark photon $ A' $. The $ A' $ is a long-lived particle in Scenario A, while the $ \pi_{3} $ is a long-lived particle in Scenario B1, as indicated by the green lines. |
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Figure 3:
Distributions of the number of muons (left) and the muon transverse impact parameter (right), which are examples of variables that are used in the BDT training for the QCD background and benchmark signal models. |
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Figure 3-a:
Distributions of the number of muons (left) and the muon transverse impact parameter (right), which are examples of variables that are used in the BDT training for the QCD background and benchmark signal models. |
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Figure 3-b:
Distributions of the number of muons (left) and the muon transverse impact parameter (right), which are examples of variables that are used in the BDT training for the QCD background and benchmark signal models. |
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Figure 4:
The multiplicity of dark vector mesons $ \tilde{\omega} $ for representative vector portal models (left), and the multiplicity of dark mesons $ \pi_{3} $ for representative Scenario A and B1 models (right). The fraction of generated events is shown against the multiplicity of dark mesons. |
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Figure 4-a:
The multiplicity of dark vector mesons $ \tilde{\omega} $ for representative vector portal models (left), and the multiplicity of dark mesons $ \pi_{3} $ for representative Scenario A and B1 models (right). The fraction of generated events is shown against the multiplicity of dark mesons. |
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Figure 4-b:
The multiplicity of dark vector mesons $ \tilde{\omega} $ for representative vector portal models (left), and the multiplicity of dark mesons $ \pi_{3} $ for representative Scenario A and B1 models (right). The fraction of generated events is shown against the multiplicity of dark mesons. |
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Figure 5:
Dimuon mass distributions in the single vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 5-a:
Dimuon mass distributions in the single vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 5-b:
Dimuon mass distributions in the single vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 5-c:
Dimuon mass distributions in the single vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 5-d:
Dimuon mass distributions in the single vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 5-e:
Dimuon mass distributions in the single vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 5-f:
Dimuon mass distributions in the single vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 6:
Dimuon mass distributions in the multi vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 6-a:
Dimuon mass distributions in the multi vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 6-b:
Dimuon mass distributions in the multi vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 6-c:
Dimuon mass distributions in the multi vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 6-d:
Dimuon mass distributions in the multi vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
png pdf |
Figure 6-e:
Dimuon mass distributions in the multi vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 6-f:
Dimuon mass distributions in the multi vertex categories for data and two benchmark signal models in the vector portal. The shaded regions indicate mass regions of known SM resonances, which are masked in the search. |
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Figure 7:
Dimuon mass distributions in a mass window around 0.67 GeV (left) and a mass window arond 1.33 GeV (right), both in the single vertex category with 1 $ \,\text{cm} < l_{xy} < 10\,\text{cm} $ and $ \textrm{pointing angle} < $ 0.2. The background fit is shown together with the signal expected for a representative Scenario A model (left) and a representative Scenario B1 model (right). |
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Figure 7-a:
Dimuon mass distributions in a mass window around 0.67 GeV (left) and a mass window arond 1.33 GeV (right), both in the single vertex category with 1 $ \,\text{cm} < l_{xy} < 10\,\text{cm} $ and $ \textrm{pointing angle} < $ 0.2. The background fit is shown together with the signal expected for a representative Scenario A model (left) and a representative Scenario B1 model (right). |
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Figure 7-b:
Dimuon mass distributions in a mass window around 0.67 GeV (left) and a mass window arond 1.33 GeV (right), both in the single vertex category with 1 $ \,\text{cm} < l_{xy} < 10\,\text{cm} $ and $ \textrm{pointing angle} < $ 0.2. The background fit is shown together with the signal expected for a representative Scenario A model (left) and a representative Scenario B1 model (right). |
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Figure 8:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the vector portal model. It is assumed that $ m_{\tilde{\omega}}=\tilde{\Lambda}=m_{\tilde{\eta}} $, where $ m_{\tilde{\omega}} $, $ \tilde{\Lambda} $ and $ m_{\tilde{\eta}} $ are the mass of the dark sector spin-one meson, the dark sector confinement scale, and the mass of the dark sector spin-zero meson respectively. |
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Figure 8-a:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the vector portal model. It is assumed that $ m_{\tilde{\omega}}=\tilde{\Lambda}=m_{\tilde{\eta}} $, where $ m_{\tilde{\omega}} $, $ \tilde{\Lambda} $ and $ m_{\tilde{\eta}} $ are the mass of the dark sector spin-one meson, the dark sector confinement scale, and the mass of the dark sector spin-zero meson respectively. |
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Figure 8-b:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the vector portal model. It is assumed that $ m_{\tilde{\omega}}=\tilde{\Lambda}=m_{\tilde{\eta}} $, where $ m_{\tilde{\omega}} $, $ \tilde{\Lambda} $ and $ m_{\tilde{\eta}} $ are the mass of the dark sector spin-one meson, the dark sector confinement scale, and the mass of the dark sector spin-zero meson respectively. |
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Figure 8-c:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the vector portal model. It is assumed that $ m_{\tilde{\omega}}=\tilde{\Lambda}=m_{\tilde{\eta}} $, where $ m_{\tilde{\omega}} $, $ \tilde{\Lambda} $ and $ m_{\tilde{\eta}} $ are the mass of the dark sector spin-one meson, the dark sector confinement scale, and the mass of the dark sector spin-zero meson respectively. |
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Figure 8-d:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the vector portal model. It is assumed that $ m_{\tilde{\omega}}=\tilde{\Lambda}=m_{\tilde{\eta}} $, where $ m_{\tilde{\omega}} $, $ \tilde{\Lambda} $ and $ m_{\tilde{\eta}} $ are the mass of the dark sector spin-one meson, the dark sector confinement scale, and the mass of the dark sector spin-zero meson respectively. |
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Figure 9:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the Scenario A model. It is assumed that $ m_{\eta}=\tilde{\Lambda}=4m_{\pi_{2}} $ and $ \textrm{sin}\:\theta= $ 0.1, where $ m_{\eta} $ is the mass of the dark sector pseudoscalar and $ \theta $ is the mixing angle parametrizing the isospin violation. The branching ratio $ \mathcal{B}(\pi_{3}\rightarrow A'A') $ is assumed to be one. |
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Figure 9-a:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the Scenario A model. It is assumed that $ m_{\eta}=\tilde{\Lambda}=4m_{\pi_{2}} $ and $ \textrm{sin}\:\theta= $ 0.1, where $ m_{\eta} $ is the mass of the dark sector pseudoscalar and $ \theta $ is the mixing angle parametrizing the isospin violation. The branching ratio $ \mathcal{B}(\pi_{3}\rightarrow A'A') $ is assumed to be one. |
png pdf |
Figure 9-b:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the Scenario A model. It is assumed that $ m_{\eta}=\tilde{\Lambda}=4m_{\pi_{2}} $ and $ \textrm{sin}\:\theta= $ 0.1, where $ m_{\eta} $ is the mass of the dark sector pseudoscalar and $ \theta $ is the mixing angle parametrizing the isospin violation. The branching ratio $ \mathcal{B}(\pi_{3}\rightarrow A'A') $ is assumed to be one. |
png pdf |
Figure 9-c:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the Scenario A model. It is assumed that $ m_{\eta}=\tilde{\Lambda}=4m_{\pi_{2}} $ and $ \textrm{sin}\:\theta= $ 0.1, where $ m_{\eta} $ is the mass of the dark sector pseudoscalar and $ \theta $ is the mixing angle parametrizing the isospin violation. The branching ratio $ \mathcal{B}(\pi_{3}\rightarrow A'A') $ is assumed to be one. |
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Figure 9-d:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the Scenario A model. It is assumed that $ m_{\eta}=\tilde{\Lambda}=4m_{\pi_{2}} $ and $ \textrm{sin}\:\theta= $ 0.1, where $ m_{\eta} $ is the mass of the dark sector pseudoscalar and $ \theta $ is the mixing angle parametrizing the isospin violation. The branching ratio $ \mathcal{B}(\pi_{3}\rightarrow A'A') $ is assumed to be one. |
png pdf |
Figure 10:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the Scenario B1 model. It is assumed that $ m_{\eta}=\tilde{\Lambda}=4m_{\pi_{2}} $ and $ \textrm{sin}\:\theta= $ 0.1, where $ m_{\eta} $ is the mass of the dark sector pseudoscalar and $ \theta $ is the mixing angle parametrizing the isospin violation. The branching ratio $ \mathcal{B}(\pi_{3}\rightarrow A'A') $ is assumed to be one. |
png pdf |
Figure 10-a:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the Scenario B1 model. It is assumed that $ m_{\eta}=\tilde{\Lambda}=4m_{\pi_{2}} $ and $ \textrm{sin}\:\theta= $ 0.1, where $ m_{\eta} $ is the mass of the dark sector pseudoscalar and $ \theta $ is the mixing angle parametrizing the isospin violation. The branching ratio $ \mathcal{B}(\pi_{3}\rightarrow A'A') $ is assumed to be one. |
png pdf |
Figure 10-b:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the Scenario B1 model. It is assumed that $ m_{\eta}=\tilde{\Lambda}=4m_{\pi_{2}} $ and $ \textrm{sin}\:\theta= $ 0.1, where $ m_{\eta} $ is the mass of the dark sector pseudoscalar and $ \theta $ is the mixing angle parametrizing the isospin violation. The branching ratio $ \mathcal{B}(\pi_{3}\rightarrow A'A') $ is assumed to be one. |
png pdf |
Figure 10-c:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the Scenario B1 model. It is assumed that $ m_{\eta}=\tilde{\Lambda}=4m_{\pi_{2}} $ and $ \textrm{sin}\:\theta= $ 0.1, where $ m_{\eta} $ is the mass of the dark sector pseudoscalar and $ \theta $ is the mixing angle parametrizing the isospin violation. The branching ratio $ \mathcal{B}(\pi_{3}\rightarrow A'A') $ is assumed to be one. |
png pdf |
Figure 10-d:
Exclusion limits at 95% CL on the branching ratio $ \mathcal{B}(H\rightarrow\psi\psi) $ for representative mass hypotheses for the Scenario B1 model. It is assumed that $ m_{\eta}=\tilde{\Lambda}=4m_{\pi_{2}} $ and $ \textrm{sin}\:\theta= $ 0.1, where $ m_{\eta} $ is the mass of the dark sector pseudoscalar and $ \theta $ is the mixing angle parametrizing the isospin violation. The branching ratio $ \mathcal{B}(\pi_{3}\rightarrow A'A') $ is assumed to be one. |
| Tables | |
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Table 1:
Model parameters of the different classes of signal models interpreted by the analysis. |
| Summary |
| A search for dark showers has been performed with displaced dimuons, using proton-proton collisions at a centre-of-mass energy of 13 TeV collected by the CMS experiment in 2018. The dataset uses a novel trigger and data processing strategy, resulting in a sample of about $ 10^{10} $ recorded events and accessibility to low-mass phase space down to the sub-GeV scale. No significant excess beyond the standard model expectation is observed. Upper limits on the branching ratio of Higgs boson decays into dark partons are set at $ < 10^{-3} $ at 95% confidence level, which give the most stringent limits to date on the vector portal model for mean proper lifetime below approximately 0.1 m and mass as low as 2 GeV. First limits have also been set for extended dark shower models (Scenario A and Scenario B1) with mass down to 0.33 GeV. |
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