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CMS-PAS-HIG-18-012
Search for 2HDM neutral Higgs bosons through the $\mathrm{H} \to \mathrm{Z}\mathrm{A} \to \ell^{+}\ell^{-}\mathrm{b}\overline{\mathrm{b}}$ process in proton-proton collisions at $\sqrt{s} = $ 13 TeV
Abstract: This note reports on a search for an extended scalar sector of the standard model, where a new CP-even (odd) scalar decays to a Z boson and a lighter CP-odd (even) scalar, which further decays to $\mathrm{b}\overline{\mathrm{b}}$. The Z boson is reconstructed via its decays to leptons. The analysed data were recorded in proton-proton collisions at $\sqrt{s} = $ 13 TeV, collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Data and predictions from the standard model are in agreement within uncertainties. Upper limits at 95% confidence level are set on the production cross section times branching ratio, with masses of the resonances ranging up to 1000 GeV. The results are interpreted in the context of the Two-Higgs Doublet Model.
Figures & Tables Summary Additional Figures References CMS Publications
Figures

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Figure 1:
2HDM mass hierarchies considered in the study: MSSM-like, where a heavy A is degenerate in mass with the charged scalars, and im2HDM [11], where the hierarchy is inverted making H the heaviest neutral scalar and degenerate in mass with charged scalars.

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Figure 2:
H and A branching fractions as a function of $\cos(\beta - \alpha)$ for the following set of parameters: $\tan\beta = $ 1.5, $m_{\mathrm{H}} = $ 300 GeV, $m_{A} = $ 200 GeV (left). H and A branching fractions as a function of $\tan\beta $ for the following set of parameters: $\cos(\beta - \alpha) = $ 0.01, $m_{\mathrm{H}} = $ 300 GeV and $m_{A} = $ 200 GeV (right).

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Figure 2-a:
H and A branching fractions as a function of $\cos(\beta - \alpha)$ for the following set of parameters: $\tan\beta = $ 1.5, $m_{\mathrm{H}} = $ 300 GeV, $m_{A} = $ 200 GeV.

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Figure 2-b:
H and A branching fractions as a function of $\tan\beta $ for the following set of parameters: $\cos(\beta - \alpha) = $ 0.01, $m_{\mathrm{H}} = $ 300 GeV and $m_{A} = $ 200 GeV.

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Figure 3:
The $ {\mathrm {m}_{\ell\ell{\mathrm {j}} {\mathrm {j}}}} $-$ {\mathrm {m}_{{\mathrm {j}} {\mathrm {j}}}} $ plane for signal samples under three different mass hypotheses, on which the parametrised ellipse is shown (left). A signal hypothesis with $m_{\mathrm{H}}=$ 500 GeV and $m_{{\text {A}}}=$ 300 GeV is shown in the $ {\mathrm {m}_{\ell\ell{\mathrm {j}} {\mathrm {j}}}} $-$ {\mathrm {m}_{{\mathrm {j}} {\mathrm {j}}}} $ plane (right). The different ellipses show the variation of the $\rho $ parameter in steps of 0.5, from 0 to 3.

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Figure 3-a:
The $ {\mathrm {m}_{\ell\ell{\mathrm {j}} {\mathrm {j}}}} $-$ {\mathrm {m}_{{\mathrm {j}} {\mathrm {j}}}} $ plane for signal samples under three different mass hypotheses, on which the parametrised ellipse is shown. The different ellipses show the variation of the $\rho $ parameter in steps of 0.5, from 0 to 3.

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Figure 3-b:
A signal hypothesis with $m_{\mathrm{H}}=$ 500 GeV and $m_{{\text {A}}}=$ 300 GeV is shown in the $ {\mathrm {m}_{\ell\ell{\mathrm {j}} {\mathrm {j}}}} $-$ {\mathrm {m}_{{\mathrm {j}} {\mathrm {j}}}} $ plane. The different ellipses show the variation of the $\rho $ parameter in steps of 0.5, from 0 to 3.

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Figure 4:
The $ {\mathrm {m}_{{\mathrm {j}} {\mathrm {j}}}} $ and $ {\mathrm {m}_{\ell\ell{\mathrm {j}} {\mathrm {j}}}} $ distributions in data and simulated background events after requiring all the analysis cuts, for $\mathrm{e^{+}} \mathrm{e^{-}} $ (left), and $\mu^{+} \mu^{-} $ (right) events. The various signal hypotheses displayed have been scaled to a cross section of 1 pb for display purposes. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties prior to the fit.

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Figure 4-a:
The $ {\mathrm {m}_{{\mathrm {j}} {\mathrm {j}}}} $ distribution in data and simulated background events after requiring all the analysis cuts, for $\mathrm{e^{+}} \mathrm{e^{-}} $ events. The various signal hypotheses displayed have been scaled to a cross section of 1 pb for display purposes. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties prior to the fit.

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Figure 4-b:
The $ {\mathrm {m}_{{\mathrm {j}} {\mathrm {j}}}} $ distribution in data and simulated background events after requiring all the analysis cuts, for $\mu^{+} \mu^{-} $ events. The various signal hypotheses displayed have been scaled to a cross section of 1 pb for display purposes. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties prior to the fit.

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Figure 4-c:
The $ {\mathrm {m}_{\ell\ell{\mathrm {j}} {\mathrm {j}}}} $ distribution in data and simulated background events after requiring all the analysis cuts, for $\mathrm{e^{+}} \mathrm{e^{-}} $ events. The various signal hypotheses displayed have been scaled to a cross section of 1 pb for display purposes. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties prior to the fit.

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Figure 4-d:
The $ {\mathrm {m}_{\ell\ell{\mathrm {j}} {\mathrm {j}}}} $ distribution in data and simulated background events after requiring all the analysis cuts, for $\mu^{+} \mu^{-} $ events. The various signal hypotheses displayed have been scaled to a cross section of 1 pb for display purposes. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties prior to the fit.

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Figure 5:
$\rho $ distributions for $\mu^{+} \mu^{-}$ (top left), $\mathrm{e^{+}} \mathrm{e^{-}}$ (top right), and $\mathrm{e^{\pm}} {\mu ^\mp}$ (bottom) events corresponding to a particle signal hypothesis with $m_{\mathrm{H}}=$ 609 GeV and $m_{{\text {A}}}=$ 505 GeV. The signal is normalised to 20 nb. The mixed-flavour lepton category is a control region used in the template fit to further constrain the ${\mathrm{t} \mathrm{\bar{t}}}$ background. The bins are defined by steps of 0.5 in $\rho $. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties.

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Figure 5-a:
$\rho $ distribution for $\mu^{+} \mu^{-}$ events corresponding to a particle signal hypothesis with $m_{\mathrm{H}}=$ 609 GeV and $m_{{\text {A}}}=$ 505 GeV. The signal is normalised to 20 nb. The mixed-flavour lepton category is a control region used in the template fit to further constrain the ${\mathrm{t} \mathrm{\bar{t}}}$ background. The bins are defined by steps of 0.5 in $\rho $. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties.

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Figure 5-b:
$\rho $ distribution for $\mathrm{e^{+}} \mathrm{e^{-}}$ events corresponding to a particle signal hypothesis with $m_{\mathrm{H}}=$ 609 GeV and $m_{{\text {A}}}=$ 505 GeV. The signal is normalised to 20 nb. The mixed-flavour lepton category is a control region used in the template fit to further constrain the ${\mathrm{t} \mathrm{\bar{t}}}$ background. The bins are defined by steps of 0.5 in $\rho $. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties.

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Figure 5-c:
$\rho $ distribution for $\mathrm{e^{\pm}} {\mu ^\mp}$ events corresponding to a particle signal hypothesis with $m_{\mathrm{H}}=$ 609 GeV and $m_{{\text {A}}}=$ 505 GeV. The signal is normalised to 20 nb. The mixed-flavour lepton category is a control region used in the template fit to further constrain the ${\mathrm{t} \mathrm{\bar{t}}}$ background. The bins are defined by steps of 0.5 in $\rho $. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties.

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Figure 6:
$\rho $ distributions for $\mu^{+} \mu^{-}$ (top left), $\mathrm{e^{+}} \mathrm{e^{-}}$ (top right), and $\mathrm{e^{\pm}} {\mu ^\mp}$ (bottom) events corresponding to a particle signal hypothesis with $m_{\mathrm{H}}=$ 261 GeV and $m_{{\text {A}}}=$ 150 GeV. The signal is normalised to 20 nb. The mixed-flavour lepton category is a control region used in the template fit to further constrain the ${\mathrm{t} \mathrm{\bar{t}}}$ background. The bins are defined by steps of 0.5 in $\rho $. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties.

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Figure 6-a:
$\rho $ distribution for $\mu^{+} \mu^{-}$ events corresponding to a particle signal hypothesis with $m_{\mathrm{H}}=$ 261 GeV and $m_{{\text {A}}}=$ 150 GeV. The signal is normalised to 20 nb. The mixed-flavour lepton category is a control region used in the template fit to further constrain the ${\mathrm{t} \mathrm{\bar{t}}}$ background. The bins are defined by steps of 0.5 in $\rho $. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties.

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Figure 6-b:
$\rho $ distribution for $\mathrm{e^{+}} \mathrm{e^{-}}$ events corresponding to a particle signal hypothesis with $m_{\mathrm{H}}=$ 261 GeV and $m_{{\text {A}}}=$ 150 GeV. The signal is normalised to 20 nb. The mixed-flavour lepton category is a control region used in the template fit to further constrain the ${\mathrm{t} \mathrm{\bar{t}}}$ background. The bins are defined by steps of 0.5 in $\rho $. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties.

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Figure 6-c:
$\rho $ distribution for $\mathrm{e^{\pm}} {\mu ^\mp}$ events corresponding to a particle signal hypothesis with $m_{\mathrm{H}}=$ 261 GeV and $m_{{\text {A}}}=$ 150 GeV. The signal is normalised to 20 nb. The mixed-flavour lepton category is a control region used in the template fit to further constrain the ${\mathrm{t} \mathrm{\bar{t}}}$ background. The bins are defined by steps of 0.5 in $\rho $. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties.

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Figure 7:
Expected (with $ \pm$1, $ \pm$2 standard deviation bands) and observed 95% CL exclusion limits for $\tan\beta =$ 1.5 and $\cos(\beta - \alpha)=$ 0.01 as a function of $m_{{\text {A}}}$ and $m_{\mathrm{H}}$ in the Type-II 2HDM benchmark scenario. The limits are computed using the asymptotic CLs method, combining the $\mathrm{e^{+}} \mathrm{e^{-}}$ and $\mu^{+} \mu^{-}$ channels.

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Figure 8:
Expected (with $ \pm$1, $ \pm$2 standard deviation bands) and observed 95% CL exclusion limits for $m_{\mathrm{H}}=$ 379 GeV and $m_{{\text {A}}}= $ 172 GeV as a function of $\tan\beta $ and $\cos(\beta - \alpha)$ in the Type-II 2HDM benchmark scenario. The limits are computed using the asymptotic CLs method, combining the $\mathrm{e^{+}} \mathrm{e^{-}}$ and $\mu^{+} \mu^{-}$ channels.
Tables

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Table 1:
Expected and observed event yields prior to the fit in the signal region with $m_{\mathrm{H}}=$ 500 GeV and $m_{{\text {A}}}=$ 200 GeV for each elliptical bin. The $\mathrm{e^{+}} \mathrm{e^{-}}$ and $\mu^{+} \mu^{-}$ categories are summed together.

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Table 2:
Summary of the systematic uncertainties prior to the fit and their impact in percentages on the total event yields for background and for a particular signal hypothesis of $m_{\mathrm{H}} = $ 379 GeV and $m_{{\text {A}}} = $ 172 GeV.
Summary
This note reports on a search for a new CP-even neutral Higgs boson, H, decaying into a Z boson and a lighter CP-odd Higgs boson, A, where the Z decays into $\ell^{+}\ell^{-}$, and the A into $\mathrm{b\bar{b}}$. The search is based on LHC proton-proton collision data at $\sqrt{s}=$ 13 TeV collected by the CMS experiment, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. We consider decays such that $\mathrm{H} \to \mathrm{Z}{\text{A}} \to \ell\ell\mathrm{b\bar{b}}$, where H and A are additional neutral Higgs bosons with masses ranging from ${\mathrm{m}}_{\mathrm{Z}} $ to 1000 GeV for H and 30 to 1000 GeV for A. Deviations from the SM expectations are observed with a global significance of 1.3$\sigma$ and so upper limits on the product of cross section and branching ratio are set. Limits are also set on the parameters of the type-II 2HDM model. Within this theoretical framework, H and A are the CP-even and CP-odd scalar bosons, respectively. The specific benchmark scenario corresponding to $\tan\beta=$ 1.5 and $\cos (\beta - \alpha)=$ 0.01 is excluded for $m_{\mathrm{H}}$ in the range 150-700 GeV and $m_{{\text{A}} }$ in the range 30-295 GeV with $m_{\mathrm{H}} > m_{{\text{A}} }$, or alternatively, for $m_{\mathrm{H}}$ in the range 125-280 GeV and $m_{{\text{A}} }$ in the range 200-700 with $m_{\mathrm{H}} < m_{{\text{A}} }$. Results are also interpreted in the benchmark scenario where $m_{\mathrm{H}}=$ 379 GeV and $m_{{\text pbA}} = $ 172 GeV. In this context, the region with $\cos (\beta - \alpha)$ in the range -0.9-0.3 and $\tan\beta$ in the range 0.5-7 is excluded. A larger region of the Type-II 2HDM parameter space is excluded with respect to previous searches.
Additional Figures

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Additional Figure 1:
95% CL upper limit on the observed cross section across the mass parameter space.

Text files for expected and observed 95% CL upper limits on the cross section [fb] for given values of $m_{{\text {A}}}$ and $m_{\mathrm{H}}$ [GeV].

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Additional Figure 2:
Theoretical cross section across the mass parameter space computed under the Type-II 2HDM benchmark scenario with $\tan\beta = $ 1.5 and $\cos(\beta - \alpha) = $ 0.01.

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Additional Figure 3:
$\rho $ distributions in the signal region corresponding to $m_{{\mathrm {H}}}=$ 627 GeV and $m_{{\text {A}}}=$ 162 GeV, where the local significance amounts to 3.9$\sigma $. The ${\mathrm {e}^+} {\mathrm {e}^-}$ (left) and ${\mu ^+} {\mu ^-}$ (right) categories are shown. The signal is normalised to 0.3 pb. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties.

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Additional Figure 3-a:
$\rho $ distribution in the signal region corresponding to $m_{{\mathrm {H}}}=$ 627 GeV and $m_{{\text {A}}}=$ 162 GeV, where the local significance amounts to 3.9$\sigma $, in the ${\mathrm {e}^+} {\mathrm {e}^-}$ category. The signal is normalised to 0.3 pb. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties.

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Additional Figure 3-b:
$\rho $ distribution in the signal region corresponding to $m_{{\mathrm {H}}}=$ 627 GeV and $m_{{\text {A}}}=$ 162 GeV, where the local significance amounts to 3.9$\sigma $, in the ${\mu ^+} {\mu ^-}$ category. The signal is normalised to 0.3 pb. Error bars indicate statistical uncertainties, while shaded bands show systematic uncertainties.
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