CMSHIG22007 ; CERNEP2023284  
Search for exotic decays of the Higgs boson to a pair of pseudoscalars in the $ \mu\mu\mathrm{b}\mathrm{b} $ and $ \tau\tau\mathrm{b}\mathrm{b} $ final states  
CMS Collaboration  
21 February 2024  
Eur. Phys. J. C 84 (2024) 493  
Abstract: A search for exotic decays of the Higgs boson (H) with a mass of 125 GeV to a pair of light pseudoscalars $ a_{1} $ is performed in final states where one pseudoscalar decays to two b quarks and the other to a pair of muons or $ \tau $ leptons. A data sample of protonproton collisions at $ \sqrt{s}= $ 13 TeV corresponding to an integrated luminosity of 138 fb$ ^{1} $ recorded with the CMS detector is analyzed. No statistically significant excess is observed over the standard model backgrounds. Upper limits are set at 95% confidence level (CL) on the Higgs boson branching fraction to $ \mu\mu\mathrm{b}\mathrm{b} $ and to $ \tau\tau\mathrm{b}\mathrm{b} $, via a pair of $ a_{1} $s. The limits depend on the pseudoscalar mass $ m_{a_{1}} $ and are observed to be in the range (0.173.3) $ \times $ 10$^{4} $ and (1.77.7) $ \times $ 10$^{2} $ in the $ \mu\mu\mathrm{b}\mathrm{b} $ and $ \tau\tau\mathrm{b}\mathrm{b} $ final states, respectively. In the framework of models with two Higgs doublets and a complex scalar singlet (2HDM+S), the results of the two final states are combined to determine modelindependent upper limits on the branching fraction $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to \ell\ell\mathrm{b}\mathrm{b}) $ at 95% CL, with $ \ell $ being a muon or a $ \tau $ lepton. For different types of 2HDM+S, upper bounds on the branching fraction $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} ) $ are extracted from the combination of the two channels. In most of the Type II 2HDM+S parameter space, $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} ) $ values above 0.23 are excluded at 95% CL for $ m_{a_{1}} $ values between 15 and 60 GeV.  
Links: eprint arXiv:2402.13358 [hepex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; 
Figures  
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Figure 1:
The distributions of leading and subleading (upper) muon $ p_{\mathrm{T}} $ and (lower) b jet $ p_{\mathrm{T}} $ in the selected events. The uncertainty band in the lower panel represents the limited size of the simulated samples together with a 30% uncertainty in the lowmass DY cross section. Simulated samples are normalized using the corresponding theoretical cross sections. To evaluate the normalization of the signal, SM Higgs boson cross sections are multiplied by the $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ value that is calculated in the Type III model with $ \tan\beta = $ 2. 
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Figure 1a:
The distributions of leading and subleading (upper) muon $ p_{\mathrm{T}} $ and (lower) b jet $ p_{\mathrm{T}} $ in the selected events. The uncertainty band in the lower panel represents the limited size of the simulated samples together with a 30% uncertainty in the lowmass DY cross section. Simulated samples are normalized using the corresponding theoretical cross sections. To evaluate the normalization of the signal, SM Higgs boson cross sections are multiplied by the $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ value that is calculated in the Type III model with $ \tan\beta = $ 2. 
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Figure 1b:
The distributions of leading and subleading (upper) muon $ p_{\mathrm{T}} $ and (lower) b jet $ p_{\mathrm{T}} $ in the selected events. The uncertainty band in the lower panel represents the limited size of the simulated samples together with a 30% uncertainty in the lowmass DY cross section. Simulated samples are normalized using the corresponding theoretical cross sections. To evaluate the normalization of the signal, SM Higgs boson cross sections are multiplied by the $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ value that is calculated in the Type III model with $ \tan\beta = $ 2. 
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Figure 1c:
The distributions of leading and subleading (upper) muon $ p_{\mathrm{T}} $ and (lower) b jet $ p_{\mathrm{T}} $ in the selected events. The uncertainty band in the lower panel represents the limited size of the simulated samples together with a 30% uncertainty in the lowmass DY cross section. Simulated samples are normalized using the corresponding theoretical cross sections. To evaluate the normalization of the signal, SM Higgs boson cross sections are multiplied by the $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ value that is calculated in the Type III model with $ \tan\beta = $ 2. 
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Figure 1d:
The distributions of leading and subleading (upper) muon $ p_{\mathrm{T}} $ and (lower) b jet $ p_{\mathrm{T}} $ in the selected events. The uncertainty band in the lower panel represents the limited size of the simulated samples together with a 30% uncertainty in the lowmass DY cross section. Simulated samples are normalized using the corresponding theoretical cross sections. To evaluate the normalization of the signal, SM Higgs boson cross sections are multiplied by the $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ value that is calculated in the Type III model with $ \tan\beta = $ 2. 
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Figure 2:
The $ p_{\mathrm{T}} $ distributions of the (left) dimuon systems and (right) dibjet system. The uncertainty band in the lower panel represents the limited size of the simulated samples together with a 30% uncertainty in the lowmass DY cross section. Simulated samples are normalized to using the corresponding theoretical cross sections. To evaluate the normalization of the signal, SM Higgs boson cross sections are multiplied by the $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ value that is calculated in the Type III model with $ \tan\beta = $ 2. 
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Figure 2a:
The $ p_{\mathrm{T}} $ distributions of the (left) dimuon systems and (right) dibjet system. The uncertainty band in the lower panel represents the limited size of the simulated samples together with a 30% uncertainty in the lowmass DY cross section. Simulated samples are normalized to using the corresponding theoretical cross sections. To evaluate the normalization of the signal, SM Higgs boson cross sections are multiplied by the $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ value that is calculated in the Type III model with $ \tan\beta = $ 2. 
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Figure 2b:
The $ p_{\mathrm{T}} $ distributions of the (left) dimuon systems and (right) dibjet system. The uncertainty band in the lower panel represents the limited size of the simulated samples together with a 30% uncertainty in the lowmass DY cross section. Simulated samples are normalized to using the corresponding theoretical cross sections. To evaluate the normalization of the signal, SM Higgs boson cross sections are multiplied by the $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ value that is calculated in the Type III model with $ \tan\beta = $ 2. 
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Figure 3:
The distribution of $ \chi_{\mathrm{b}\mathrm{b}} $ versus $ \chi_{\mathrm{H}} $ as defined in Eq. (1) for (left) simulated background processes and (right) the signal process with $ m_{a_{1}} = $ 40 GeV. The contours indicate lines of constant $ \chi_\text{tot}^2 $. The gray scale represents the expected yields in data. To evaluate the yield of the signal, SM Higgs boson cross sections are multiplied by the $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ value that is calculated in the Type III model with $ \tan\beta= $ 2. 
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Figure 3a:
The distribution of $ \chi_{\mathrm{b}\mathrm{b}} $ versus $ \chi_{\mathrm{H}} $ as defined in Eq. (1) for (left) simulated background processes and (right) the signal process with $ m_{a_{1}} = $ 40 GeV. The contours indicate lines of constant $ \chi_\text{tot}^2 $. The gray scale represents the expected yields in data. To evaluate the yield of the signal, SM Higgs boson cross sections are multiplied by the $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ value that is calculated in the Type III model with $ \tan\beta= $ 2. 
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Figure 3b:
The distribution of $ \chi_{\mathrm{b}\mathrm{b}} $ versus $ \chi_{\mathrm{H}} $ as defined in Eq. (1) for (left) simulated background processes and (right) the signal process with $ m_{a_{1}} = $ 40 GeV. The contours indicate lines of constant $ \chi_\text{tot}^2 $. The gray scale represents the expected yields in data. To evaluate the yield of the signal, SM Higgs boson cross sections are multiplied by the $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ value that is calculated in the Type III model with $ \tan\beta= $ 2. 
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Figure 4:
Signal ($ m_{a_{1}}= $ 40 GeV) versus background efficiency for different thresholds on $ \chi_\text{tot}^2 $ (gray) and $ \chi_\mathrm{d}^2 $ (red) variables. The black star indicates signal efficiency versus that of background for the optimized $ \chi_\mathrm{d}^2 $ requirement. 
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Figure 5:
Prefit distributions of the DNN score for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel divided into events with one (left) or at least two (right) b jets. The shape of the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is indicated assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. The lower panel shows the ratio of the observed data to the expected yields. The gray band represents the unconstrained statistical and systematic uncertainties. 
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Figure 5a:
Prefit distributions of the DNN score for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel divided into events with one (left) or at least two (right) b jets. The shape of the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is indicated assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. The lower panel shows the ratio of the observed data to the expected yields. The gray band represents the unconstrained statistical and systematic uncertainties. 
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Figure 5b:
Prefit distributions of the DNN score for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel divided into events with one (left) or at least two (right) b jets. The shape of the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is indicated assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. The lower panel shows the ratio of the observed data to the expected yields. The gray band represents the unconstrained statistical and systematic uncertainties. 
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Figure 6:
The best fit background models for the $ \mu\mu\mathrm{b}\mathrm{b} $ channel together with a 68% CL uncertainty band from the fit to the data under the backgroundonly hypothesis for the (upper left) Low $ p_{\mathrm{T}} $ category, (middle left) VBF category, (middle right) TL category, (lower left) TM category, and (lower right) TT category. For comparison, the signalplusbackground is shown for the (upper right) Low $ p_{\mathrm{T}} $ category for a signal with $ m_{a_{1}} = $ 40 GeV. The expected signal yield is evaluated assuming the SM production of the Higgs boson and a $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ as predicted in the Type III 2HDM+S with $ \tan\beta= $ 2. The bin widths depend on statistics, irrelevant for the final fit. 
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Figure 6a:
The best fit background models for the $ \mu\mu\mathrm{b}\mathrm{b} $ channel together with a 68% CL uncertainty band from the fit to the data under the backgroundonly hypothesis for the (upper left) Low $ p_{\mathrm{T}} $ category, (middle left) VBF category, (middle right) TL category, (lower left) TM category, and (lower right) TT category. For comparison, the signalplusbackground is shown for the (upper right) Low $ p_{\mathrm{T}} $ category for a signal with $ m_{a_{1}} = $ 40 GeV. The expected signal yield is evaluated assuming the SM production of the Higgs boson and a $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ as predicted in the Type III 2HDM+S with $ \tan\beta= $ 2. The bin widths depend on statistics, irrelevant for the final fit. 
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Figure 6b:
The best fit background models for the $ \mu\mu\mathrm{b}\mathrm{b} $ channel together with a 68% CL uncertainty band from the fit to the data under the backgroundonly hypothesis for the (upper left) Low $ p_{\mathrm{T}} $ category, (middle left) VBF category, (middle right) TL category, (lower left) TM category, and (lower right) TT category. For comparison, the signalplusbackground is shown for the (upper right) Low $ p_{\mathrm{T}} $ category for a signal with $ m_{a_{1}} = $ 40 GeV. The expected signal yield is evaluated assuming the SM production of the Higgs boson and a $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ as predicted in the Type III 2HDM+S with $ \tan\beta= $ 2. The bin widths depend on statistics, irrelevant for the final fit. 
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Figure 6c:
The best fit background models for the $ \mu\mu\mathrm{b}\mathrm{b} $ channel together with a 68% CL uncertainty band from the fit to the data under the backgroundonly hypothesis for the (upper left) Low $ p_{\mathrm{T}} $ category, (middle left) VBF category, (middle right) TL category, (lower left) TM category, and (lower right) TT category. For comparison, the signalplusbackground is shown for the (upper right) Low $ p_{\mathrm{T}} $ category for a signal with $ m_{a_{1}} = $ 40 GeV. The expected signal yield is evaluated assuming the SM production of the Higgs boson and a $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ as predicted in the Type III 2HDM+S with $ \tan\beta= $ 2. The bin widths depend on statistics, irrelevant for the final fit. 
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Figure 6d:
The best fit background models for the $ \mu\mu\mathrm{b}\mathrm{b} $ channel together with a 68% CL uncertainty band from the fit to the data under the backgroundonly hypothesis for the (upper left) Low $ p_{\mathrm{T}} $ category, (middle left) VBF category, (middle right) TL category, (lower left) TM category, and (lower right) TT category. For comparison, the signalplusbackground is shown for the (upper right) Low $ p_{\mathrm{T}} $ category for a signal with $ m_{a_{1}} = $ 40 GeV. The expected signal yield is evaluated assuming the SM production of the Higgs boson and a $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ as predicted in the Type III 2HDM+S with $ \tan\beta= $ 2. The bin widths depend on statistics, irrelevant for the final fit. 
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Figure 6e:
The best fit background models for the $ \mu\mu\mathrm{b}\mathrm{b} $ channel together with a 68% CL uncertainty band from the fit to the data under the backgroundonly hypothesis for the (upper left) Low $ p_{\mathrm{T}} $ category, (middle left) VBF category, (middle right) TL category, (lower left) TM category, and (lower right) TT category. For comparison, the signalplusbackground is shown for the (upper right) Low $ p_{\mathrm{T}} $ category for a signal with $ m_{a_{1}} = $ 40 GeV. The expected signal yield is evaluated assuming the SM production of the Higgs boson and a $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ as predicted in the Type III 2HDM+S with $ \tan\beta= $ 2. The bin widths depend on statistics, irrelevant for the final fit. 
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Figure 6f:
The best fit background models for the $ \mu\mu\mathrm{b}\mathrm{b} $ channel together with a 68% CL uncertainty band from the fit to the data under the backgroundonly hypothesis for the (upper left) Low $ p_{\mathrm{T}} $ category, (middle left) VBF category, (middle right) TL category, (lower left) TM category, and (lower right) TT category. For comparison, the signalplusbackground is shown for the (upper right) Low $ p_{\mathrm{T}} $ category for a signal with $ m_{a_{1}} = $ 40 GeV. The expected signal yield is evaluated assuming the SM production of the Higgs boson and a $ \mathcal{B}(a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ as predicted in the Type III 2HDM+S with $ \tan\beta= $ 2. The bin widths depend on statistics, irrelevant for the final fit. 
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Figure 7:
Postfit distributions of $ m_{\tau\tau} $ for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel signal regions in events with exactly one b tagged jet: SR1 (upper left ), SR2 (upper right), and SR3 (lower). The shape of the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is indicated assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. 
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Figure 7a:
Postfit distributions of $ m_{\tau\tau} $ for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel signal regions in events with exactly one b tagged jet: SR1 (upper left ), SR2 (upper right), and SR3 (lower). The shape of the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is indicated assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. 
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Figure 7b:
Postfit distributions of $ m_{\tau\tau} $ for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel signal regions in events with exactly one b tagged jet: SR1 (upper left ), SR2 (upper right), and SR3 (lower). The shape of the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is indicated assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. 
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Figure 7c:
Postfit distributions of $ m_{\tau\tau} $ for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel signal regions in events with exactly one b tagged jet: SR1 (upper left ), SR2 (upper right), and SR3 (lower). The shape of the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is indicated assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. 
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Figure 8:
Postfit distributions of the $ m_{\tau\tau} $ for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel signal regions in events with at least two b tagged jets: SR1 (left) and SR2 (right). The shape of the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is indicated assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. 
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Figure 8a:
Postfit distributions of the $ m_{\tau\tau} $ for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel signal regions in events with at least two b tagged jets: SR1 (left) and SR2 (right). The shape of the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is indicated assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. 
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Figure 8b:
Postfit distributions of the $ m_{\tau\tau} $ for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel signal regions in events with at least two b tagged jets: SR1 (left) and SR2 (right). The shape of the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is indicated assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. 
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Figure 9:
Postfit distributions of the $ m_{\tau\tau} $ for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel control regions in events with exactly one b tagged jet (left) and at least two b tagged jets (right). The contamination from the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is barely visible assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. 
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Figure 9a:
Postfit distributions of the $ m_{\tau\tau} $ for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel control regions in events with exactly one b tagged jet (left) and at least two b tagged jets (right). The contamination from the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is barely visible assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. 
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Figure 9b:
Postfit distributions of the $ m_{\tau\tau} $ for the $ \mu\hspace{.04em}\tau_\mathrm{h} $ channel control regions in events with exactly one b tagged jet (left) and at least two b tagged jets (right). The contamination from the $ \mathrm{H}\to a_{1} a_{1} $ signal, where $ m_{a_{1}} = $ 35 GeV, is barely visible assuming $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ to be 10%. 
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Figure 10:
Observed and expected upper limits at 95% CL on $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to \mu\mu\mathrm{b}\mathrm{b}) $ as functions of $ m_{a_{1}} $. The inner and outer bands indicate the regions containing the distribution of limits located within 68 and 95% confidence intervals, respectively, of the expectation under the backgroundonly hypothesis. 
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Figure 11:
Observed and expected 95% CL exclusion limits on $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ in percent, for the (upper left) $ \mu\hspace{.04em}\tau_\mathrm{h} $, (upper right) $ \mathrm{e}\hspace{.04em}\tau_\mathrm{h} $, (lower left) $ \mathrm{e}\hspace{.04em}\mu $ channels, and (lower right) for the combination of all the channels. 
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Figure 11a:
Observed and expected 95% CL exclusion limits on $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ in percent, for the (upper left) $ \mu\hspace{.04em}\tau_\mathrm{h} $, (upper right) $ \mathrm{e}\hspace{.04em}\tau_\mathrm{h} $, (lower left) $ \mathrm{e}\hspace{.04em}\mu $ channels, and (lower right) for the combination of all the channels. 
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Figure 11b:
Observed and expected 95% CL exclusion limits on $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ in percent, for the (upper left) $ \mu\hspace{.04em}\tau_\mathrm{h} $, (upper right) $ \mathrm{e}\hspace{.04em}\tau_\mathrm{h} $, (lower left) $ \mathrm{e}\hspace{.04em}\mu $ channels, and (lower right) for the combination of all the channels. 
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Figure 11c:
Observed and expected 95% CL exclusion limits on $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ in percent, for the (upper left) $ \mu\hspace{.04em}\tau_\mathrm{h} $, (upper right) $ \mathrm{e}\hspace{.04em}\tau_\mathrm{h} $, (lower left) $ \mathrm{e}\hspace{.04em}\mu $ channels, and (lower right) for the combination of all the channels. 
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Figure 11d:
Observed and expected 95% CL exclusion limits on $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $ in percent, for the (upper left) $ \mu\hspace{.04em}\tau_\mathrm{h} $, (upper right) $ \mathrm{e}\hspace{.04em}\tau_\mathrm{h} $, (lower left) $ \mathrm{e}\hspace{.04em}\mu $ channels, and (lower right) for the combination of all the channels. 
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Figure 12:
Observed and expected 95% CL upper limits on $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to \ell\ell\mathrm{b}\mathrm{b}) $ in %, where $ \ell $ stands for muons or $ \tau $ leptons, obtained from the combination of the $ \mu\mu\mathrm{b}\mathrm{b} $ and $ \tau\tau{\mathrm{b}}{\mathrm{b}} $ channels. The results are obtained as functions of $ m_{a_{1}} $ for 2HDM+S models, independent of the type and $ \tan\beta $ parameter. 
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Figure 13:
Observed and expected 95% CL upper limits on $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} ) $ in %, obtained from the combination of the $ \mu\mu\mathrm{b}\mathrm{b} $ and $ \tau\tau{\mathrm{b}}{\mathrm{b}} $ channels. The results are obtained as functions of $ m_{a_{1}} $ for 2HDM+S Type I (independent of $ \tan\beta $), Type II ($ \tan\beta= $ 2.0), Type III ($ \tan\beta= $ 2.0), and Type IV ($ \tan\beta= $ 0.6), respectively. 
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Figure 14:
Observed 95% CL upper limits on $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} ) $ in %, for the combination of the $ \mu\mu\mathrm{b}\mathrm{b} $ and $ \tau\tau{\mathrm{b}}{\mathrm{b}} $ channels for Type III (left) and Type IV (right) 2HDM+S in the $ \tan\beta \mbox{\textsl{vs.}} m_{a_{1}} $ parameter space. The limits are calculated in a grid of 5 GeV in $ m_{a_{1}} $ and 0.10.5 in $ \tan\beta $, interpolating the points in between. The contours corresponding to branching fractions of 100 and 16% are drawn using dashed lines, where 16% refers to the combined upper limit on Higgs boson to undetected particle decays from previous Run 2 results [15]. All points inside the contour are allowed within that upper limit. 
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Figure 14a:
Observed 95% CL upper limits on $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} ) $ in %, for the combination of the $ \mu\mu\mathrm{b}\mathrm{b} $ and $ \tau\tau{\mathrm{b}}{\mathrm{b}} $ channels for Type III (left) and Type IV (right) 2HDM+S in the $ \tan\beta \mbox{\textsl{vs.}} m_{a_{1}} $ parameter space. The limits are calculated in a grid of 5 GeV in $ m_{a_{1}} $ and 0.10.5 in $ \tan\beta $, interpolating the points in between. The contours corresponding to branching fractions of 100 and 16% are drawn using dashed lines, where 16% refers to the combined upper limit on Higgs boson to undetected particle decays from previous Run 2 results [15]. All points inside the contour are allowed within that upper limit. 
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Figure 14b:
Observed 95% CL upper limits on $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} ) $ in %, for the combination of the $ \mu\mu\mathrm{b}\mathrm{b} $ and $ \tau\tau{\mathrm{b}}{\mathrm{b}} $ channels for Type III (left) and Type IV (right) 2HDM+S in the $ \tan\beta \mbox{\textsl{vs.}} m_{a_{1}} $ parameter space. The limits are calculated in a grid of 5 GeV in $ m_{a_{1}} $ and 0.10.5 in $ \tan\beta $, interpolating the points in between. The contours corresponding to branching fractions of 100 and 16% are drawn using dashed lines, where 16% refers to the combined upper limit on Higgs boson to undetected particle decays from previous Run 2 results [15]. All points inside the contour are allowed within that upper limit. 
Tables  
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Table 1:
The electron, muon, and $ \tau_\mathrm{h} p_{\mathrm{T}} $ thresholds in GeV at trigger level for the $ \mu\mu\mathrm{b}\mathrm{b} $ and $ \tau\tau{\mathrm{b}}{\mathrm{b}} $ channels. 
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Table 3:
Summary of the categorization requirements in the $ \mu\mu\mathrm{b}\mathrm{b} $ channel. Events in these categories contain two muons and two b jets. As stated in the text, L, M, and T stand for the loose, medium, and tight b tagging criteria, respectively. 
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Table 4:
The expected yields for backgrounds and different signal hypotheses in each category of the $ \mu\mu\mathrm{b}\mathrm{b} $ channel. 
Summary 
A search for an exotic decay of the 125 GeV Higgs boson (H) to a pair of light pseudoscalar bosons (a_{1} ) in the final state with two b quarks and two muons or two $ \tau $ leptons has been presented. The results are based on a data sample of protonproton collisions corresponding to an integrated luminosity of 138 fb$^{1}$, accumulated by the CMS experiment at the LHC during Run 2 at a centerofmass energy of 13 TeV. Final states with at least one leptonic $ \tau $ decay are studied in the $ \tau\tau{\mathrm{b}}{\mathrm{b}} $ channel, excluding those with two muons or two electrons. The results show significant improvement, with respect to the earlier CMS analyses at 13 TeV, beyond what is merely expected from the increase in the size of the data sample. A more thorough analysis of the signal properties using a single discriminating variable improves the $ \mu\mu\mathrm{b}\mathrm{b} $ analysis, while the $ \tau\tau{\mathrm{b}}{\mathrm{b}} $ analysis gains from a deep neural network based signal categorization. No significant excess in the data over the standard model backgrounds is observed. Upper limits are set, at 95% confidence level, on branching fractions $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\mu\mu\mathrm{b}\mathrm{b}) $ and $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to\tau\tau{\mathrm{b}}{\mathrm{b}}) $, in the $ \mu\mu\mathrm{b}\mathrm{b} $ and $ \tau\tau{\mathrm{b}}{\mathrm{b}} $ analyses, respectively. Both analyses provide the most stringent expected limits to date. In the $ \mu\mu\mathrm{b}\mathrm{b} $ channel, the observed limits are in the range (0.173.3) $ \times $ 10$^{4} $ for a pseudoscalar mass, $ m_{a_{1}} $, between 15 and 62.5 GeV. Combining all final states in the $ \tau\tau{\mathrm{b}}{\mathrm{b}} $ channel, limits are observed to be in the range 1.77.7% for $ m_{a_{1}} $ between 12 and 60 GeV. By combining the $ \mu\mu\mathrm{b}\mathrm{b} $ and $ \tau\tau{\mathrm{b}}{\mathrm{b}} $ channels, modelindependent limits are set on the branching fraction $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} \to \ell\ell\mathrm{b}\mathrm{b}) $, where $ \ell $ stands for muons or $ \tau $ leptons. The observed upper limits range between 0.6 and 7.7% depending on the $ m_{a_{1}} $. The results can also be interpreted in different types of 2HDM+S models. For $ m_{a_{1}} $ values between 15 and 60 GeV, $ \mathcal{B}(\mathrm{H}\to a_{1} a_{1} ) $ values above 23% are excluded, at 95% confidence level, in most of the Type II scenarios. In Types III and IV, observed upper limits as low as 1 and 3% are obtained, respectively, for $ \tan\beta= $ 2.0 and 0.5. 
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