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| CMS-SUS-24-002 ; CERN-EP-2025-129 | ||
| Search for light pseudoscalar boson pairs produced from Higgs boson decays using the 4$ \tau $ and 2$ \mu2\tau $ final states in proton-proton collisions at $ \sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| 9 August 2025 | ||
| Submitted to JHEP | ||
| Abstract: A search for a pair of light pseudoscalar bosons ($ \mathrm{a}_{1} $) produced in the decay of the 125 GeV Higgs boson is presented. The analysis examines decay modes where one $ \mathrm{a}_{1} $ decays into a pair of tau leptons and the other decays into either another pair of tau leptons or a pair of muons. The $ \mathrm{a}_{1} $ boson mass probed in this study ranges from 4 to 15 GeV. The data sample was recorded by the CMS experiment in proton-proton collisions at a center-of-mass energy of 13 TeV and corresponds to an integrated luminosity of 138 fb$ ^{-1} $. No excess above standard model (SM) expectations is observed. The study combines the 4$ \tau $ and 2$ \mu2\tau $ channels to set upper limits at 95% confidence level (CL) on the product of the Higgs boson production cross section and the branching fraction to the 4$ \tau $ final state, relative to the Higgs boson production cross section predicted by the SM. In this interpretation, the $ \mathrm{a}_{1} $ boson is assumed to have Yukawa-like couplings to fermions, with coupling strengths proportional to the respective fermion masses. The observed (expected) upper limits range between 0.007 (0.011) and 0.079 (0.066) across the mass range considered. The results are also interpreted in the context of models with two Higgs doublets and an additional complex singlet field (2HD+S). The tightest constraints are obtained for the Type III 2HD+S model. In this case, assuming the Higgs boson production cross section equals the SM prediction, values of the branching ratio for the Higgs boson decay into a pair of $ \mathrm{a}_{1} $ bosons exceeding 16% are excluded at 95% CL for $ \mathrm{a}_{1} $ boson masses between 5 and 15 GeV and $ \tan\beta > $ 2, with the exception of scenarios in which the $ \mathrm{a}_{1} $ boson mixes with charm or bottom quark-antiquark bound states. | ||
| Links: e-print arXiv:2508.06947 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Illustration of the signal topology. The Higgs boson decays into two $ \mathrm{a}_{1} $ bosons, one of which further decays into a pair of tau leptons, and the other into a pair of muons or tau leptons. In the case of $ \mathrm{a}_{1} \to \tau \tau $ decays, one tau lepton is required to decay to a muon, and the other to a single charged particle---either an electron, a muon, or a charged hadron ($ h $). The targeted final state consists of one muon and one oppositely charged track arising from each $ \mathrm{a}_{1} $ boson decay. |
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Figure 2:
Binning of the 2D ($ m_1,m_2$) distribution. Each bin is labeled $ (i,j) $, where $ i $ is the bin index along $ m_1 $ ($ x $ axis) and $ j $ is the bin index along $ m_2$ ($ y $ axis). Bins $ (i,6) $ with $ i=1,..., $ 5 include all events with $ m_2 > $ 5.2 GeV, while bin $ (6,6) $ contains all events with $ m_1 $ and $ m_2 > $ 5.2 GeV. |
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Figure 3:
The observed invariant mass distribution, normalized to unity, of the first muon and the softest (left) or hardest (right) accompanying signal track for different isolation requirements imposed on the second muon: one isolation track ($ \textit{CR}\: N_{{\text{iso}},1} $; circles) or two to three isolation tracks ($ \textit{CR}\: N_{{\text{iso}},23} $; squares). Vertical bars represent statistical uncertainties, which are smaller than the marker size in most cases and thus imperceptible. The horizontal bars show the bin width. The lower panels show the ratio of the distribution in $ \textit{CR}\: N_{{\text{iso}},23} $ to that in $ \textit{CR}\: N_{{\text{iso}},1} $. The last bin in both distributions includes all entries with invariant mass of the muon+track system greater than 5.2 GeV. |
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Figure 3-a:
The observed invariant mass distribution, normalized to unity, of the first muon and the softest (left) or hardest (right) accompanying signal track for different isolation requirements imposed on the second muon: one isolation track ($ \textit{CR}\: N_{{\text{iso}},1} $; circles) or two to three isolation tracks ($ \textit{CR}\: N_{{\text{iso}},23} $; squares). Vertical bars represent statistical uncertainties, which are smaller than the marker size in most cases and thus imperceptible. The horizontal bars show the bin width. The lower panels show the ratio of the distribution in $ \textit{CR}\: N_{{\text{iso}},23} $ to that in $ \textit{CR}\: N_{{\text{iso}},1} $. The last bin in both distributions includes all entries with invariant mass of the muon+track system greater than 5.2 GeV. |
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Figure 3-b:
The observed invariant mass distribution, normalized to unity, of the first muon and the softest (left) or hardest (right) accompanying signal track for different isolation requirements imposed on the second muon: one isolation track ($ \textit{CR}\: N_{{\text{iso}},1} $; circles) or two to three isolation tracks ($ \textit{CR}\: N_{{\text{iso}},23} $; squares). Vertical bars represent statistical uncertainties, which are smaller than the marker size in most cases and thus imperceptible. The horizontal bars show the bin width. The lower panels show the ratio of the distribution in $ \textit{CR}\: N_{{\text{iso}},23} $ to that in $ \textit{CR}\: N_{{\text{iso}},1} $. The last bin in both distributions includes all entries with invariant mass of the muon+track system greater than 5.2 GeV. |
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Figure 4:
Invariant mass distribution of the muon+track pair, normalized to unity, for events passing the signal selection. Events in data are represented by black points with the vertical bars representing the statistical uncertainty and the horizontal bars the bin width. The expected background distribution derived from $ \textit{CR}\: N_{23} $ is shown by the solid blue histogram, with the grey band giving the uncertainty in the background prediction, including systematic and statistical components. Also shown are normalized distributions from signal simulations for four mass hypotheses, $ m_{\mathrm{a}_{1}} $= 5, 8, 12, and 15 GeV (dashed colored histograms). The lower panel displays the ratio of the data to the expected background. |
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Figure 5:
The correlation factors $ C(i,j)^{\textit{CR}}_{\text{data}} $ with their statistical uncertainties. |
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Figure 6:
The correlation factors $ C(i,j)^{\textit{CR}}_{\text{MC}} $ (top) and $ C(i,j)^{\textit{CR}}_{\text{MC}} $ (bottom) with their statistical uncertainties. |
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Figure 6-a:
The correlation factors $ C(i,j)^{\textit{CR}}_{\text{MC}} $ (top) and $ C(i,j)^{\textit{CR}}_{\text{MC}} $ (bottom) with their statistical uncertainties. |
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Figure 6-b:
The correlation factors $ C(i,j)^{\textit{CR}}_{\text{MC}} $ (top) and $ C(i,j)^{\textit{CR}}_{\text{MC}} $ (bottom) with their statistical uncertainties. |
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Figure 7:
The simulated signal $ (m_1, m_2) $ distribution, converted into a 1D array, for $ m_{\mathrm{a}_1} $ values of 5 (upper left), 8 (upper right), 12 (lower left), and 15 GeV (lower right). The contributions of the $ \mathrm{H}\to\mathrm{a}_{1}\mathrm{a}_{1}\to 4\tau $ (red histograms) and 2$ \mu2\tau $ (blue histograms) decays are shown. The distributions are normalized assuming SM Higgs production cross section and $ {\mathcal{B}} (\mathrm{H}\to\mathrm{a}_{1}\mathrm{a}_{1}){\mathcal{B}}^{2}(\mathrm{a}_{1}\to \tau\tau) $ = 0.05. The bin notation follows that of Fig. 2. |
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Figure 7-a:
The simulated signal $ (m_1, m_2) $ distribution, converted into a 1D array, for $ m_{\mathrm{a}_1} $ values of 5 (upper left), 8 (upper right), 12 (lower left), and 15 GeV (lower right). The contributions of the $ \mathrm{H}\to\mathrm{a}_{1}\mathrm{a}_{1}\to 4\tau $ (red histograms) and 2$ \mu2\tau $ (blue histograms) decays are shown. The distributions are normalized assuming SM Higgs production cross section and $ {\mathcal{B}} (\mathrm{H}\to\mathrm{a}_{1}\mathrm{a}_{1}){\mathcal{B}}^{2}(\mathrm{a}_{1}\to \tau\tau) $ = 0.05. The bin notation follows that of Fig. 2. |
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Figure 7-b:
The simulated signal $ (m_1, m_2) $ distribution, converted into a 1D array, for $ m_{\mathrm{a}_1} $ values of 5 (upper left), 8 (upper right), 12 (lower left), and 15 GeV (lower right). The contributions of the $ \mathrm{H}\to\mathrm{a}_{1}\mathrm{a}_{1}\to 4\tau $ (red histograms) and 2$ \mu2\tau $ (blue histograms) decays are shown. The distributions are normalized assuming SM Higgs production cross section and $ {\mathcal{B}} (\mathrm{H}\to\mathrm{a}_{1}\mathrm{a}_{1}){\mathcal{B}}^{2}(\mathrm{a}_{1}\to \tau\tau) $ = 0.05. The bin notation follows that of Fig. 2. |
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Figure 7-c:
The simulated signal $ (m_1, m_2) $ distribution, converted into a 1D array, for $ m_{\mathrm{a}_1} $ values of 5 (upper left), 8 (upper right), 12 (lower left), and 15 GeV (lower right). The contributions of the $ \mathrm{H}\to\mathrm{a}_{1}\mathrm{a}_{1}\to 4\tau $ (red histograms) and 2$ \mu2\tau $ (blue histograms) decays are shown. The distributions are normalized assuming SM Higgs production cross section and $ {\mathcal{B}} (\mathrm{H}\to\mathrm{a}_{1}\mathrm{a}_{1}){\mathcal{B}}^{2}(\mathrm{a}_{1}\to \tau\tau) $ = 0.05. The bin notation follows that of Fig. 2. |
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Figure 7-d:
The simulated signal $ (m_1, m_2) $ distribution, converted into a 1D array, for $ m_{\mathrm{a}_1} $ values of 5 (upper left), 8 (upper right), 12 (lower left), and 15 GeV (lower right). The contributions of the $ \mathrm{H}\to\mathrm{a}_{1}\mathrm{a}_{1}\to 4\tau $ (red histograms) and 2$ \mu2\tau $ (blue histograms) decays are shown. The distributions are normalized assuming SM Higgs production cross section and $ {\mathcal{B}} (\mathrm{H}\to\mathrm{a}_{1}\mathrm{a}_{1}){\mathcal{B}}^{2}(\mathrm{a}_{1}\to \tau\tau) $ = 0.05. The bin notation follows that of Fig. 2. |
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Figure 8:
The unrolled ($ m_1,m_2$) distribution used to extract the signal. The observed number of events in data is represented by the points, with the vertical bars giving the statistical uncertainty. The background is shown as the blue histogram, with its uncertainty depicted by the shaded grey band. The normalization for the background is obtained by fitting the observed data under the background-only hypothesis. Signal expectations for the 4$ \tau $ and 2$ \mu2\tau $ final states are shown as dashed histograms for the mass hypotheses $ m_{\mathrm{a}_{1}} $= 5, 8, 12, and 15 GeV. The relative normalization of the 4$ \tau $ and 2$ \mu2\tau $ final states is given by Eq. (1). The signal normalization is computed assuming that the Higgs boson is produced in pp collisions with a rate predicted by the SM and decays into the $ \mathrm{a}_{1} \mathrm{a}_{1} \to 4\tau $ final state with a branching fraction of 5%. The lower plot shows the ratio of the observed data events to the expected background yield in each bin of the ($ m_1,m_2$) distribution. |
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Figure 9:
The observed (points) and expected (red line) 95% CL upper limits on the product of the signal cross section and the branching fractions $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) {\mathcal{B}}^{2} (\mathrm{a}_{1} \to \tau \tau) $, relative to the inclusive Higgs boson production cross section $ \sigma_\text{SM} $ predicted in the SM. The green and yellow bands indicate the regions containing 68 and 95% of the expected limit ranges under the background-only hypothesis. |
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Figure 10:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ m_{\mathrm{a}_{1}} $ for different 2HD+S models at benchmark $ \tan\beta $ values: Type I ($ \tan\beta $ independent; upper left), Type II ($ \tan\beta = $ 5; upper right), Type III ($ \tan\beta = $ 2; lower left), and Type IV ($ \tan\beta = $ 0.5; lower right). |
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Figure 10-a:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ m_{\mathrm{a}_{1}} $ for different 2HD+S models at benchmark $ \tan\beta $ values: Type I ($ \tan\beta $ independent; upper left), Type II ($ \tan\beta = $ 5; upper right), Type III ($ \tan\beta = $ 2; lower left), and Type IV ($ \tan\beta = $ 0.5; lower right). |
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Figure 10-b:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ m_{\mathrm{a}_{1}} $ for different 2HD+S models at benchmark $ \tan\beta $ values: Type I ($ \tan\beta $ independent; upper left), Type II ($ \tan\beta = $ 5; upper right), Type III ($ \tan\beta = $ 2; lower left), and Type IV ($ \tan\beta = $ 0.5; lower right). |
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Figure 10-c:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ m_{\mathrm{a}_{1}} $ for different 2HD+S models at benchmark $ \tan\beta $ values: Type I ($ \tan\beta $ independent; upper left), Type II ($ \tan\beta = $ 5; upper right), Type III ($ \tan\beta = $ 2; lower left), and Type IV ($ \tan\beta = $ 0.5; lower right). |
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Figure 10-d:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ m_{\mathrm{a}_{1}} $ for different 2HD+S models at benchmark $ \tan\beta $ values: Type I ($ \tan\beta $ independent; upper left), Type II ($ \tan\beta = $ 5; upper right), Type III ($ \tan\beta = $ 2; lower left), and Type IV ($ \tan\beta = $ 0.5; lower right). |
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Figure 11:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ \tan\beta $ for the Type II 2HD+S model with $ m_{\mathrm{a}_{1}} $= 5 GeV (upper left), $ m_{\mathrm{a}_{1}} $= 8 GeV (upper right), $ m_{\mathrm{a}_{1}} $= 12 GeV (lower left), and $ m_{\mathrm{a}_{1}} $= 15 GeV (lower right). |
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Figure 11-a:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ \tan\beta $ for the Type II 2HD+S model with $ m_{\mathrm{a}_{1}} $= 5 GeV (upper left), $ m_{\mathrm{a}_{1}} $= 8 GeV (upper right), $ m_{\mathrm{a}_{1}} $= 12 GeV (lower left), and $ m_{\mathrm{a}_{1}} $= 15 GeV (lower right). |
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Figure 11-b:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ \tan\beta $ for the Type II 2HD+S model with $ m_{\mathrm{a}_{1}} $= 5 GeV (upper left), $ m_{\mathrm{a}_{1}} $= 8 GeV (upper right), $ m_{\mathrm{a}_{1}} $= 12 GeV (lower left), and $ m_{\mathrm{a}_{1}} $= 15 GeV (lower right). |
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Figure 11-c:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ \tan\beta $ for the Type II 2HD+S model with $ m_{\mathrm{a}_{1}} $= 5 GeV (upper left), $ m_{\mathrm{a}_{1}} $= 8 GeV (upper right), $ m_{\mathrm{a}_{1}} $= 12 GeV (lower left), and $ m_{\mathrm{a}_{1}} $= 15 GeV (lower right). |
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Figure 11-d:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ \tan\beta $ for the Type II 2HD+S model with $ m_{\mathrm{a}_{1}} $= 5 GeV (upper left), $ m_{\mathrm{a}_{1}} $= 8 GeV (upper right), $ m_{\mathrm{a}_{1}} $= 12 GeV (lower left), and $ m_{\mathrm{a}_{1}} $= 15 GeV (lower right). |
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Figure 12:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ \tan\beta $ for the Type III 2HD+S model with $ m_{\mathrm{a}_{1}} $= 5 GeV (upper left), $ m_{\mathrm{a}_{1}} $= 8 GeV (upper right), $ m_{\mathrm{a}_{1}} $= 12 GeV (lower left), and $ m_{\mathrm{a}_{1}} $= 15 GeV (lower right). |
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Figure 12-a:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ \tan\beta $ for the Type III 2HD+S model with $ m_{\mathrm{a}_{1}} $= 5 GeV (upper left), $ m_{\mathrm{a}_{1}} $= 8 GeV (upper right), $ m_{\mathrm{a}_{1}} $= 12 GeV (lower left), and $ m_{\mathrm{a}_{1}} $= 15 GeV (lower right). |
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Figure 12-b:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ \tan\beta $ for the Type III 2HD+S model with $ m_{\mathrm{a}_{1}} $= 5 GeV (upper left), $ m_{\mathrm{a}_{1}} $= 8 GeV (upper right), $ m_{\mathrm{a}_{1}} $= 12 GeV (lower left), and $ m_{\mathrm{a}_{1}} $= 15 GeV (lower right). |
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Figure 12-c:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ \tan\beta $ for the Type III 2HD+S model with $ m_{\mathrm{a}_{1}} $= 5 GeV (upper left), $ m_{\mathrm{a}_{1}} $= 8 GeV (upper right), $ m_{\mathrm{a}_{1}} $= 12 GeV (lower left), and $ m_{\mathrm{a}_{1}} $= 15 GeV (lower right). |
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Figure 12-d:
The observed (points) and expected (red line) 95% CL upper limits on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, as a function of $ \tan\beta $ for the Type III 2HD+S model with $ m_{\mathrm{a}_{1}} $= 5 GeV (upper left), $ m_{\mathrm{a}_{1}} $= 8 GeV (upper right), $ m_{\mathrm{a}_{1}} $= 12 GeV (lower left), and $ m_{\mathrm{a}_{1}} $= 15 GeV (lower right). |
| Tables | |
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Table 1:
The purpose and selection criteria for the two types of tracks used in the selection procedure. |
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Table 2:
Signal acceptance times selection efficiency $ {\mathcal{A}}\epsilon $, defined in the text, and the number of expected signal events after selection in the $ \textit{SR} $, computed using simulated signal samples for representative mass hypotheses. The Higgs boson cross sections from $ {\mathrm{g}\mathrm{g}}\text{F} $, VBF, and VH production mechanisms are set to the SM predictions [62]. The number of expected signal events is computed for a benchmark value of the branching fraction $ {\mathcal{B}}(\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) {\mathcal{B}}^{2} (\mathrm{a}_{1} \to \tau \tau)= $ 0.05. The quoted uncertainties in the predictions from simulation include only the statistical component. In data, 7803 events are selected in the $ \textit{SR} $. |
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Table 3:
Definition of the \CRs used to construct and validate the background model. The last row defines the $ \textit{SR} $. The symbols $ N_\text{iso} $ and $ N_\text{sig} $ denote the number of isolation and signal tracks, respectively, within a cone of $ \Delta R= $ 0.5 around the muon momentum direction. In cases where $ N_\text{sig} $ is not mentioned, there is no explicit requirement on the number of signal tracks. |
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Table 4:
Summary of systematic uncertainties affecting the estimation of signal and background. The terms ISR and FSR refer to initial- and final-state radiation, and the symbols $ \mu_\text{R} $ and $ \mu_\text{F} $ denote the renormalization and factorization scales, respectively. The impact of shape-altering and bin-by-bin uncertainties is quoted in terms of relative variations of yields across all bins of the modeled $ (m_1,m_2) $ distributions. For the normalization (norm.) uncertainties, the impact on the overall estimated yield is reported. The last column indicates how a given uncertainty is correlated across the data-taking years. Bin-by-bin statistical uncertainties for simulated signal samples are quoted for the most populated bins containing 80% of the total yield of selected signal events. |
| Summary |
| A search for a pair of light pseudoscalar bosons ($ \mathrm{a}_{1} $) produced in decays of the 125 GeV Higgs boson (H), $ \mathrm{H}\to\mathrm{a}_{1}\mathrm{a}_{1} $, in final states with two muons and two charged-particle tracks is presented. The search is performed using data from proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the CMS experiment at the LHC between 2016 and 2018, and corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The analysis exploits the gluon-gluon fusion, vector boson fusion, and Higgs-strahlung production modes, and targets the $ \mathrm{H}\to \mathrm{a}_{1} \mathrm{a}_{1} \to 4\tau $ and 2$ \mu2\tau $ decay channels. Masses of the $ \mathrm{a}_{1} $ boson ($ m_{\mathrm{a}_{1}} $) in the range 4--15 GeV are examined. No excess of data above the standard model (SM) background prediction is found. Upper limits on the product of the inclusive signal cross section and the branching fraction, $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) \mathcal{B}^2(\mathrm{a}_{1} \to \tau\tau) $, relative to the SM Higgs boson production cross section $ \sigma_\text{SM} $, are set at 95% confidence level (CL) by combining the 4$ \tau $ and 2$ \mu2\tau $ decay channels, assuming Yukawa-like couplings of $ \mathrm{a}_{1} $ to fermions. The observed limits range from 0.007 at $ m_{\mathrm{a}_{1}}= $ 11 GeV to 0.079 at $ m_{\mathrm{a}_{1}}= $ 4 GeV. The expected limits in the absence of signal span from 0.011 at $ m_{\mathrm{a}_{1}}= $ 11 GeV to 0.066 at $ m_{\mathrm{a}_{1}}= $ 4 GeV. The results are a significant improvement over the previous CMS analysis at 13 TeV [29], exceeding the anticipated improvement from the larger data sample alone. The sensitivity is enhanced by a factor of 2 to 4, depending on $ m_{\mathrm{a}_1} $, which can be attributed to the introduction of a veto for b-tagged jets and further optimization of the selection criteria targeting the $ \mathrm{a}_{1}\to\tau\tau $ and $ \mu\mu $ decays. The analysis also exceeds the sensitivity of a similar search performed by the ATLAS Collaboration in the same channel using a comparable amount of integrated luminosity [33]. The results of the search are also interpreted in the context of various models with two Higgs doublets and an additional complex singlet field (2HD+S). For the Type II 2HD+S scenario, realized in the next-to-minimal supersymmetric SM, 95% CL upper limits between 0.013 and 0.092 are set on $ \sigma (\mathrm{p}\mathrm{p} \to \mathrm{H}+\mathrm{X}) {\mathcal{B}} (\mathrm{H} \to \mathrm{a}_{1} \mathrm{a}_{1}) $, relative to $ \sigma_\text{SM} $, for 4$ < m_{\mathrm{a}_{1}} < $ 9 GeV and $ \tan\beta > $ 5. The analysis sets the most stringent constraints to date for the Type III 2HD+S scenario. Upper limits in the range 0.010--0.057 are obtained for probed mass hypotheses in the ranges 5 $ < m_{\mathrm{a}_{1}} < $ 9 GeV and 12$ < m_{\mathrm{a}_{1}} < $ 14 GeV for $ \tan\beta > $ 2. |
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