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CMS-HIG-17-024 ; CERN-EP-2018-089
Search for an exotic decay of the Higgs boson to a pair of light pseudoscalars in the final state with two b quarks and two $\tau$ leptons in proton-proton collisions at $\sqrt{s} = $ 13 TeV
Phys. Lett. B 785 (2018) 462
Abstract: A search for an exotic decay of the Higgs boson to a pair of light pseudoscalar bosons is performed for the first time in the final state with two b quarks and two $\tau$ leptons. The search is motivated in the context of models of physics beyond the standard model (SM), such as two Higgs doublet models extended with a complex scalar singlet (2HDM+S), which include the next-to-minimal supersymmetric SM (NMSSM). The results are based on a data set of proton-proton collisions corresponding to an integrated luminosity of 35.9 fb$^{-1}$, accumulated by the CMS experiment at the LHC in 2016 at a center-of-mass energy of 13 TeV. Masses of the pseudoscalar boson between 15 and 60 GeV are probed, and no excess of events above the SM expectation is observed. Upper limits between 3 and 12% are set on the branching fraction ${\mathcal{B}({{{\mathrm{h}}\to{\mathrm{a}\mathrm{a}}\to2\tau2\mathrm{b}}}} )$ assuming the SM production of the Higgs boson. Upper limits are also set on the branching fraction of the Higgs boson to two light pseudoscalar bosons in different 2HDM+S scenarios. Assuming the SM production cross section for the Higgs boson, the upper limit on this quantity is as low as 20% for a mass of the pseudoscalar of 40 GeV in the NMSSM.
Figures & Tables Summary Additional Figures References CMS Publications
Figures

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Figure 1:
Predicted ${\mathcal {B}({{\mathrm {a}} {{\mathrm {a}} \to {{\mathrm {b}} {{\mathrm {b}} {\tau} {\tau}}}}})}$ for $ {m_{{{\mathrm {a}}}}} = $ 40 GeV in the different models of 2HDM+S, for various values of $\tan\beta $. The picture is essentially the same for all ${m_{{{\mathrm {a}}}}}$ hypotheses considered in this Letter. The branching fractions are computed following the formulas of Ref. [15].

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Figure 2:
Visible invariant mass of the leptons and the leading b jet, ${m_{{{\mathrm {b}} {\tau} {\tau}}^{\text {vis}}}}$, after the baseline selection, in the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ final state, for the signal with different mass hypotheses (left). Distribution of ${m_{{{\mathrm {b}} {\tau} {\tau}}^{\text {vis}}}}$ in the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ final state (right). The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}} $" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}}$, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to 10 times the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 100%.

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Figure 2-a:
Visible invariant mass of the leptons and the leading b jet, ${m_{{{\mathrm {b}} {\tau} {\tau}}^{\text {vis}}}}$, after the baseline selection, in the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ final state, for the signal with different mass hypotheses.

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Figure 2-b:
Distribution of ${m_{{{\mathrm {b}} {\tau} {\tau}}^{\text {vis}}}}$ in the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ final state. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}} $" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}}$, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to 10 times the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 100%.

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Figure 3:
Distributions of $ {m_{{\mathrm {T}}} ({{\mu}}, {{{\vec{p}}_{{\mathrm {T}}^{\text {miss}}}}}})$ (top left), $ {m_{{\mathrm {T}}} ({{\tau} _{\mathrm {h}}, {{{\vec{p}}_{{\mathrm {T}}^{\text {miss}}}}}})}$ (top right), and $D_\zeta $ (bottom) in the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ final state before the $ {m_{{{\mathrm {b}} {\tau} {\tau}}^{\text {vis}}}} $-based categorization. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}} $" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to 10 times the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 100%.

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Figure 3-a:
Distribution of $ {m_{{\mathrm {T}}} ({{\mu}}, {{{\vec{p}}_{{\mathrm {T}}^{\text {miss}}}}}})$ in the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ final state before the $ {m_{{{\mathrm {b}} {\tau} {\tau}}^{\text {vis}}}} $-based categorization. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}} $" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to 10 times the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 100%.

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Figure 3-b:
Distribution of $ {m_{{\mathrm {T}}} ({{\tau} _{\mathrm {h}}, {{{\vec{p}}_{{\mathrm {T}}^{\text {miss}}}}}})}$ in the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ final state before the $ {m_{{{\mathrm {b}} {\tau} {\tau}}^{\text {vis}}}} $-based categorization. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}} $" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to 10 times the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 100%.

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Figure 3-c:
Distribution of $D_\zeta $ in the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ final state before the $ {m_{{{\mathrm {b}} {\tau} {\tau}}^{\text {vis}}}} $-based categorization. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}} $" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to 10 times the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 100%.

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Figure 4:
Distributions of $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in the four categories of the $ {{\mathrm {e}} {{\mu}}}$ channel. The "Other" contribution includes events from single top quark, diboson, SM Higgs boson, and W+jets productions. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 4-a:
Distribution of $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in one of four categories of the $ {{\mathrm {e}} {{\mu}}}$ channel. The "Other" contribution includes events from single top quark, diboson, SM Higgs boson, and W+jets productions. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 4-b:
Distribution of $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in one of four categories of the $ {{\mathrm {e}} {{\mu}}}$ channel. The "Other" contribution includes events from single top quark, diboson, SM Higgs boson, and W+jets productions. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 4-c:
Distribution of $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in one of four categories of the $ {{\mathrm {e}} {{\mu}}}$ channel. The "Other" contribution includes events from single top quark, diboson, SM Higgs boson, and W+jets productions. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 4-d:
Distribution of $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in one of four categories of the $ {{\mathrm {e}} {{\mu}}}$ channel. The "Other" contribution includes events from single top quark, diboson, SM Higgs boson, and W+jets productions. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 5:
Distributions of $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in the four categories of the $ {{\mathrm {e}} {{\tau} _{\mathrm {h}}}} $ channel. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}} $" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 5-a:
Distributionsof $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in one of the four categories of the $ {{\mathrm {e}} {{\tau} _{\mathrm {h}}}} $ channel. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}} $" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 5-b:
Distributionsof $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in one of the four categories of the $ {{\mathrm {e}} {{\tau} _{\mathrm {h}}}} $ channel. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}} $" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 5-c:
Distributionsof $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in one of the four categories of the $ {{\mathrm {e}} {{\tau} _{\mathrm {h}}}} $ channel. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}} $" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 5-d:
Distributionsof $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in one of the four categories of the $ {{\mathrm {e}} {{\tau} _{\mathrm {h}}}} $ channel. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}} $" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 6:
Distributions of $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in the four categories of the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ channel. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}}$" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 6-a:
Distribution of $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in one of the four categories of the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ channel. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}}$" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 6-b:
Distribution of $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in one of the four categories of the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ channel. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}}$" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 6-c:
Distribution of $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in one of the four categories of the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ channel. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}}$" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 6-d:
Distribution of $ {m_{{\tau} {\tau}}^{\text {vis}}} $ in one of the four categories of the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ channel. The "$\textrm {jet}\to {{\tau} _{\mathrm {h}}}$" contribution includes all events with a jet misidentified as a $ {{\tau} _{\mathrm {h}}} $ candidate, whereas the rest of background contributions only include events where the reconstructed $ {{\tau} _{\mathrm {h}}} $ corresponds to a $ {{\tau} _{\mathrm {h}}} $, a muon, or an electron, at the generator level. The "Other" contribution includes events from single top quark, diboson, and SM Higgs boson processes. The signal histogram corresponds to the SM production cross section for $ {{\mathrm {g}} {{\mathrm {g}} {{\mathrm {h}}}}} $, VBF, and Vh processes, and assumes ${\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}= $ 10%. The normalization of the predicted background distributions corresponds to the result of the global fit.

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Figure 7:
Expected and observed 95% CL limits on $(\sigma ({{\mathrm {h}})/\sigma _\textrm {SM}) {\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}}$ in%. The $ {{\mathrm {e}} {{\mu}}}$ results are shown in the top left panel, $ {{\mathrm {e}} {{\tau} _{\mathrm {h}}}} $ in the top right, $ {{\mu}} {{\tau} _{\mathrm {h}}} $ in the bottom left, and the combination in the bottom right. The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis.

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Figure 7-a:
Expected and observed 95% CL limits on $(\sigma ({{\mathrm {h}})/\sigma _\textrm {SM}) {\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}}$ in% ($ {{\mathrm {e}} {{\mu}}}$). The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis.

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Figure 7-b:
Expected and observed 95% CL limits on $(\sigma ({{\mathrm {h}})/\sigma _\textrm {SM}) {\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}}$ in% ($ {{\mathrm {e}} {{\tau} _{\mathrm {h}}}} $). The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis.

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Figure 7-c:
Expected and observed 95% CL limits on $(\sigma ({{\mathrm {h}})/\sigma _\textrm {SM}) {\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}}$ in% ($ {{\mu}} {{\tau} _{\mathrm {h}}} $). The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis.

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Figure 7-d:
Expected and observed 95% CL limits on $(\sigma ({{\mathrm {h}})/\sigma _\textrm {SM}) {\mathcal {B}({{{{\mathrm {h}}}\to {{{\mathrm {a}} {{\mathrm {a}}}\to 2 {\tau}2{{{\mathrm {b}}}}}}}})}}$ in% (combination). The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis.

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Figure 8:
Observed 95% CL limits on $(\sigma (\mathrm {h})/\sigma _\textrm {SM}){\mathcal {B}}({{\mathrm {h}}} \to {{\mathrm {a}}} {{\mathrm {a}}})$ in 2HDM+S of type III (left), and type IV (right). The contours corresponding to a 95% CL exclusion of $(\sigma (\mathrm {h})/\sigma _\textrm {SM}){\mathcal {B}}({\mathrm {h}} \to {{\mathrm {a}}} {{\mathrm {a}}})= $ 1.00 and 0.34 are drawn with dashed lines. The number 34% corresponds to the limit on the branching fraction of the Higgs boson to beyond-the-SM particles at the 95% CL obtained with data collected at center-of-mass energies of 7 and 8 TeV by the ATLAS and CMS experiments [10].

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Figure 8-a:
Observed 95% CL limits on $(\sigma (\mathrm {h})/\sigma _\textrm {SM}){\mathcal {B}}({{\mathrm {h}}} \to {{\mathrm {a}}} {{\mathrm {a}}})$ in 2HDM+S of type III. The contours corresponding to a 95% CL exclusion of $(\sigma (\mathrm {h})/\sigma _\textrm {SM}){\mathcal {B}}({\mathrm {h}} \to {{\mathrm {a}}} {{\mathrm {a}}})= $ 1.00 and 0.34 are drawn with dashed lines. The number 34% corresponds to the limit on the branching fraction of the Higgs boson to beyond-the-SM particles at the 95% CL obtained with data collected at center-of-mass energies of 7 and 8 TeV by the ATLAS and CMS experiments [10].

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Figure 8-b:
Observed 95% CL limits on $(\sigma (\mathrm {h})/\sigma _\textrm {SM}){\mathcal {B}}({{\mathrm {h}}} \to {{\mathrm {a}}} {{\mathrm {a}}})$ in 2HDM+S of type IV. The contours corresponding to a 95% CL exclusion of $(\sigma (\mathrm {h})/\sigma _\textrm {SM}){\mathcal {B}}({\mathrm {h}} \to {{\mathrm {a}}} {{\mathrm {a}}})= $ 1.00 and 0.34 are drawn with dashed lines. The number 34% corresponds to the limit on the branching fraction of the Higgs boson to beyond-the-SM particles at the 95% CL obtained with data collected at center-of-mass energies of 7 and 8 TeV by the ATLAS and CMS experiments [10].

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Figure 9:
Observed 95% CL limits on $(\sigma ({{\mathrm {h}}})/\sigma _\textrm {SM}){\mathcal {B}}({{\mathrm {h}}} \to {{\mathrm {a}}} {{\mathrm {a}}})$ for various 2HDM+S types. The limit in type I 2HDM+S does not depend on $\tan\beta $.
Tables

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Table 1:
Baseline selection criteria for objects required in various final states. The numbers given for the ${p_{{\mathrm {T}}}}$ thresholds of the electron and muon in the $ \mathrm {e} {\mu}$ final state correspond to the leading and subleading particles. The ${p_{{\mathrm {T}}}}$ threshold for the $ \tau _{\mathrm {h}} $ candidates is the result of an optimization of the expected exclusion limits of the signal.

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Table 2:
Optimized selection and categorization in the various final states. The selection criterion $D_\zeta > -30 $ GeV in the $ {{\mathrm {e}} {{\mu}}}$ final state reduces the large $ {{{\mathrm {t}\overline {{\mathrm {t}}}}}} $ background. In the the other final states the $ {{{\mathrm {t}\overline {{\mathrm {t}}}}}} $ background is less important, and only events with $D_\zeta > $ 0 GeV are discarded in one of the categories of the $ {{\mu}} {{\tau} _{\mathrm {h}}} $ final state to reduce the $ {{\mathrm {Z}} \to {\tau} {\tau}}$ background. This selection criterion does not improve the sensitivity in the $ {{\mathrm {e}} {{\tau} _{\mathrm {h}}}} $ final state because of the lower expected signal and background yields, and is therefore not applied.
Summary
The first search for exotic decays of the Higgs boson to pairs of light bosons with two b quark jets and two $\tau$ leptons in the final state has been performed with 35.9 fb$^{-1}$ of data collected at 13 TeV center-of-mass energy in 2016. This decay channel has a large branching fraction in many models where the couplings to fermions are proportional to the fermion mass, and can be triggered in the dominant gluon fusion production mode because of the presence of light leptons from leptonic $\tau$ decays. No excess of events is found on top of the expected standard model background for masses of the light boson, ${m_{\mathrm{a}}} $, between 15 and 60 GeV. Upper limits between 3 and 12% are set on the branching fraction ${\mathcal{B}({{{\mathrm{h}}\to{\mathrm{a}\mathrm{a}}\to2\tau2\mathrm{b}}})}$ assuming the SM production of the Higgs boson. This translates to upper limits on ${\mathcal{B}({\mathrm{h}\to\mathrm{a}\mathrm{a}}}$ as low as 20% for ${m_{\mathrm{a}}} = $ 40 GeV in the NMSSM. These results improve by more than one order of magnitude the sensitivity to exotic Higgs boson decays to pairs of light pseudoscalars in the NMSSM from previous CMS results in other final states for 15 $ < {m_{\mathrm{a}}} < $ 25 GeV, and by a factor up to five for 25 $ < {m_{\mathrm{a}}} < $ 60 GeV [28,31].
Additional Figures

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Additional Figure 1:
Distributions of $m_{{\mathrm {b}} {\tau} {\tau}}^{\mathrm {vis}}$ in the $ {{\mu}} {\tau}_{\rm h}$ channel before the categorization. The selection criteria based on $m_{\mathrm{T}}({{\mu}},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, $m_{\mathrm{T}}({\tau}_{\rm h},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, and $D_\zeta $ are not applied. The "jet$\rightarrow {\tau}_{\rm h}$" contribution includes all events with a jet misidentified as a $ {\tau}_{\rm h}$ candidate, whereas the other contributions only include events where the reconstructed $ {\tau}_{\rm h}$ corresponds to a $ {\tau}_{\rm h}$, a muon, or an electron, at generated level. The "Others" contribution includes events from single top quark, diboson, and SM Higgs boson productions. The signal for $m_{\mathrm {a}}= $ 40 GeV is scaled to a cross section times branching fraction equal to 10 times the production cross section of the Higgs boson in the SM.

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Additional Figure 2:
Distributions of $m_{\mathrm{T}}({{\mu}},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$ in the $ {{\mu}} {\tau}_{\rm h}$ channel before the categorization. The selection criteria based on $m_{\mathrm{T}}({{\mu}},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, $m_{\mathrm{T}}({\tau}_{\rm h},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, and $D_\zeta $ are not applied. The "jet$\rightarrow {\tau}_{\rm h}$" contribution includes all events with a jet misidentified as a $ {\tau}_{\rm h}$ candidate, whereas the other contributions only include events where the reconstructed $ {\tau}_{\rm h}$ corresponds to a $ {\tau}_{\rm h}$, a muon, or an electron, at generated level. The "Others" contribution includes events from single top quark, diboson, and SM Higgs boson productions. The signal for $m_{\mathrm {a}}= $ 40 GeV is scaled to a cross section times branching fraction equal to 10 times the production cross section of the Higgs boson in the SM.

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Additional Figure 3:
Distributions of $m_{\mathrm{T}}({\tau}_{\rm h},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$ in the $ {{\mu}} {\tau}_{\rm h}$ channel before the categorization. The selection criteria based on $m_{\mathrm{T}}({{\mu}},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, $m_{\mathrm{T}}({\tau}_{\rm h},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, and $D_\zeta $ are not applied. The "jet$\rightarrow {\tau}_{\rm h}$" contribution includes all events with a jet misidentified as a $ {\tau}_{\rm h}$ candidate, whereas the other contributions only include events where the reconstructed $ {\tau}_{\rm h}$ corresponds to a $ {\tau}_{\rm h}$, a muon, or an electron, at generated level. The "Others" contribution includes events from single top quark, diboson, and SM Higgs boson productions. The signal for $m_{\mathrm {a}}= $ 40 GeV is scaled to a cross section times branching fraction equal to 10 times the production cross section of the Higgs boson in the SM.

png pdf
Additional Figure 4:
Distributions of $D_\zeta $ in the $ {{\mu}} {\tau}_{\rm h}$ channel before the categorization. The selection criteria based on $m_{\mathrm{T}}({{\mu}},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, $m_{\mathrm{T}}({\tau}_{\rm h},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, and $D_\zeta $ are not applied. The "jet$\rightarrow {\tau}_{\rm h}$" contribution includes all events with a jet misidentified as a $ {\tau}_{\rm h}$ candidate, whereas the other contributions only include events where the reconstructed $ {\tau}_{\rm h}$ corresponds to a $ {\tau}_{\rm h}$, a muon, or an electron, at generated level. The "Others" contribution includes events from single top quark, diboson, and SM Higgs boson productions. The signal for $m_{\mathrm {a}}= $ 40 GeV is scaled to a cross section times branching fraction equal to 10 times the production cross section of the Higgs boson in the SM.

png pdf
Additional Figure 5:
Distributions of $m_{{\mathrm {b}} {\tau} {\tau}}^{\mathrm {vis}}$ in the $ {\mathrm {e}} {\tau}_{\rm h}$ channel before the categorization. The selection criteria based on $m_{\mathrm{T}}({\mathrm {e}},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, $m_{\mathrm{T}}({\tau}_{\rm h},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, and $D_\zeta $ are not applied. The "jet$\rightarrow {\tau}_{\rm h}$" contribution includes all events with a jet misidentified as a $ {\tau}_{\rm h}$ candidate, whereas the other contributions only include events where the reconstructed $ {\tau}_{\rm h}$ corresponds to a $ {\tau}_{\rm h}$, a muon, or an electron, at generated level. The "Others" contribution includes events from single top quark, diboson, and SM Higgs boson productions. The signal for $m_{\mathrm {a}}= $ 40 GeV is scaled to a cross section times branching fraction equal to 10 times the production cross section of the Higgs boson in the SM.

png pdf
Additional Figure 6:
Distributions of $m_{\mathrm{T}}({\mathrm {e}},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$ in the $ {\mathrm {e}} {\tau}_{\rm h}$ channel before the categorization. The selection criteria based on $m_{\mathrm{T}}({\mathrm {e}},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, $m_{\mathrm{T}}({\tau}_{\rm h},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, and $D_\zeta $ are not applied. The "jet$\rightarrow {\tau}_{\rm h}$" contribution includes all events with a jet misidentified as a $ {\tau}_{\rm h}$ candidate, whereas the other contributions only include events where the reconstructed $ {\tau}_{\rm h}$ corresponds to a $ {\tau}_{\rm h}$, a muon, or an electron, at generated level. The "Others" contribution includes events from single top quark, diboson, and SM Higgs boson productions. The signal for $m_{\mathrm {a}}= $ 40 GeV is scaled to a cross section times branching fraction equal to 10 times the production cross section of the Higgs boson in the SM.

png pdf
Additional Figure 7:
Distributions of $m_{\mathrm{T}}({\tau}_{\rm h},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$ in the $ {\mathrm {e}} {\tau}_{\rm h}$ channel before the categorization. The selection criteria based on $m_{\mathrm{T}}({\mathrm {e}},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, $m_{\mathrm{T}}({\tau}_{\rm h},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, and $D_\zeta $ are not applied. The "jet$\rightarrow {\tau}_{\rm h}$" contribution includes all events with a jet misidentified as a $ {\tau}_{\rm h}$ candidate, whereas the other contributions only include events where the reconstructed $ {\tau}_{\rm h}$ corresponds to a $ {\tau}_{\rm h}$, a muon, or an electron, at generated level. The "Others" contribution includes events from single top quark, diboson, and SM Higgs boson productions. The signal for $m_{\mathrm {a}}= $ 40 GeV is scaled to a cross section times branching fraction equal to 10 times the production cross section of the Higgs boson in the SM.

png pdf
Additional Figure 8:
Distributions of $D_\zeta $ in the $ {\mathrm {e}} {\tau}_{\rm h}$ channel before the categorization. The selection criteria based on $m_{\mathrm{T}}({\mathrm {e}},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, $m_{\mathrm{T}}({\tau}_{\rm h},{\vec{p}}_{\mathrm {T}}^{\,\text {miss}})$, and $D_\zeta $ are not applied. The "jet$\rightarrow {\tau}_{\rm h}$" contribution includes all events with a jet misidentified as a $ {\tau}_{\rm h}$ candidate, whereas the other contributions only include events where the reconstructed $ {\tau}_{\rm h}$ corresponds to a $ {\tau}_{\rm h}$, a muon, or an electron, at generated level. The "Others" contribution includes events from single top quark, diboson, and SM Higgs boson productions. The signal for $m_{\mathrm {a}}= $ 40 GeV is scaled to a cross section times branching fraction equal to 10 times the production cross section of the Higgs boson in the SM.

png pdf
Additional Figure 9:
Expected and observed 95% CL limits on $({\sigma _h}/{\sigma _{\textrm {SM}}})\mathcal {B}(\textrm {h}\rightarrow \textrm {aa})$ 2HDM+S type II $\tan\beta =2$. Limits are shown as a function of the mass of the light boson, $\textrm {m}_\textrm {a}$. The branching fractions of the pseudoscalar boson to SM particles are computed following a model described in arXiv:1312.4992. Grey shaded regions correspond to regions where theoretical predictions for the branching fractions of the pseudoscalar boson to SM particles are not reliable. The limits for the bb$\tau \tau $ channel were obtained using an integrated luminosity of 35.9 fb$^{-1}$ collected at 13 TeV center-of-mass energy, while the other results were obtained using an integrated luminosity of 19.7 fb$^{-1}$ collected at 8 TeV center-of-mass energy [28] (arXiv:1701.02032).
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Compact Muon Solenoid
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