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CMS-HIG-17-009 ; CERN-EP-2018-127
Search for resonant pair production of Higgs bosons decaying to bottom quark-antiquark pairs in proton-proton collisions at 13 TeV
JHEP 08 (2018) 152
Abstract: A search for a narrow-width resonance decaying into two Higgs bosons, each decaying into a bottom quark-antiquark pair, is presented. The search is performed using proton-proton collision data corresponding to an integrated luminosity of 35.9 fb$^{-1}$ at $\sqrt{s} = $ 13 TeV recorded by the CMS detector at the LHC. No evidence for such a signal is observed. Upper limits are set on the product of the production cross section for the resonance and the branching fraction for the selected decay mode in the resonance mass range from 260 to 1200 GeV.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
The selection efficiency for simulated ${\mathrm {X}\to {\mathrm {H}} ({{\mathrm {b}} {\overline {\mathrm {b}}}}) {\mathrm {H}} ({{\mathrm {b}} {\overline {\mathrm {b}}}})}$ events for a spin-0 radion (left) and a spin-2 bulk KK-graviton (right), at different stages of the event selection for each mass hypothesis, for the low-mass region (solid) and the medium-mass region (dashed). The vertical line at 580 GeV corresponds to the transition between the two selections.

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Figure 1-a:
The selection efficiency for simulated ${\mathrm {X}\to {\mathrm {H}} ({{\mathrm {b}} {\overline {\mathrm {b}}}}) {\mathrm {H}} ({{\mathrm {b}} {\overline {\mathrm {b}}}})}$ events for a spin-0 radion, at different stages of the event selection for each mass hypothesis, for the low-mass region (solid) and the medium-mass region (dashed). The vertical line at 580 GeV corresponds to the transition between the two selections.

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Figure 1-b:
The selection efficiency for simulated ${\mathrm {X}\to {\mathrm {H}} ({{\mathrm {b}} {\overline {\mathrm {b}}}}) {\mathrm {H}} ({{\mathrm {b}} {\overline {\mathrm {b}}}})}$ events for a spin-2 bulk KK-graviton, at different stages of the event selection for each mass hypothesis, for the low-mass region (solid) and the medium-mass region (dashed). The vertical line at 580 GeV corresponds to the transition between the two selections.

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Figure 2:
The $m_{\mathrm {X}}$ distribution for simulated signal events (spin-2 bulk KK-graviton) after the event selection criteria for the 450, 750, and 1000 GeV mass hypotheses, with and without the correction obtained by constraining $m_{{\mathrm {H}}}$ (kinematic constraint) and the specific b jet energy corrections (regression). The distributions are normalized so that the area under the curve for each mass is the same

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Figure 3:
Definition of the SR and the SB in the ($m_{{\mathrm {H}} _1}$, $m_{{\mathrm {H}} _2}$) plane used to motivate and validate the parametric model for the multijet background. The quantities $m_{{\mathrm {H}} _1}$ and $m_{{\mathrm {H}} _2}$ are the two reconstructed Higgs boson masses after applying the multivariate regression described in Section 4. Data corresponds to a selection in the MMR.

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Figure 4:
The predicted $m_{\mathrm {X}}$ distribution in data for the LMR (square) and the actual distribution in the SR (circle) defined centered at a Higgs boson mass of 150 GeV.

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Figure 5:
The $m_{\mathrm {X}}$ distribution in the SB of the MMR and the fit to the background multijet shape are shown. The shaded regions correspond to variations of $\pm $1 and $\pm $2 standard deviation (s.d.) in this parametrized form. Here $n$ is the number of degrees of freedom in each fit. The lower panel shows the difference between the data and the fits, divided by the uncertainty in the number of data events.

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Figure 6:
The $m_{\mathrm {X}}$ distributions for LMR1 (left) and LMR2 (right) in the SR. These distributions are fitted in the two ranges to the reference model. A fit to the background-only hypothesis, multijets, is shown. The shaded regions correspond to variations of $\pm $1 and $\pm $2 standard deviation (s.d.) in the parametrized form. Here $n$ is the number of degrees of freedom in each fit. The lower panels show the difference between the data and the fits, divided by the uncertainty in the number of data events.

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Figure 6-a:
The $m_{\mathrm {X}}$ distribution for LMR1 in the SR. These distributions are fitted in the two ranges to the reference model. A fit to the background-only hypothesis, multijets, is shown. The shaded regions correspond to variations of $\pm $1 and $\pm $2 standard deviation (s.d.) in the parametrized form. Here $n$ is the number of degrees of freedom in each fit. The lower panel shows the difference between the data and the fits, divided by the uncertainty in the number of data events.

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Figure 6-b:
The $m_{\mathrm {X}}$ distribution for LMR2 in the SR. These distributions are fitted in the two ranges to the reference model. A fit to the background-only hypothesis, multijets, is shown. The shaded regions correspond to variations of $\pm $1 and $\pm $2 standard deviation (s.d.) in the parametrized form. Here $n$ is the number of degrees of freedom in each fit. The lower panel shows the difference between the data and the fits, divided by the uncertainty in the number of data events.

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Figure 7:
The $m_{\mathrm {X}}$ distribution for the multijet background in the SR in data for the MMR. A fit to the background-only hypothesis, which consists of the multijet shape, is shown. The shaded regions correspond to variations of $\pm $1 and $\pm $2 standard deviation (s.d.) in this parametrized form. Here $n$ is the number of degrees of freedom in each fit. The lower panel shows the difference between the data and the fits, divided by the uncertainty in the number of data events.

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Figure 8:
The observed and expected upper limits on the cross section for a spin-2 resonance ${\mathrm {X}\to {\mathrm {H}} ({{\mathrm {b}} {\overline {\mathrm {b}}}}) {\mathrm {H}} ({{\mathrm {b}} {\overline {\mathrm {b}}}})}$ at 95% CL, using the asymptotic CLs method. The theoretical cross section for the bulk KK-graviton, with $\kappa / {{M_\mathrm {Pl}}} = $ 0.5, $\kappa l = $ 35, decaying to four b jets via Higgs bosons is overlaid. The transition between the LMR and the MMR is based on the expected sensitivity, resulting in the observed discontinuity.

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Figure 9:
The observed and expected upper limits on the cross section for a spin-0 resonance ${\mathrm {X}\to {\mathrm {H}} ({{\mathrm {b}} {\overline {\mathrm {b}}}}) {\mathrm {H}} ({{\mathrm {b}} {\overline {\mathrm {b}}}})}$ at 95% CL, using the asymptotic CLs method. The theoretical cross section for the production of a radion, with $\Lambda = $ 3 TeV, $\kappa l= $ 35, and no radion-Higgs boson mixing, decaying to four b jets via Higgs bosons is overlaid. The transition between the LMR and the MMR is based on the expected sensitivity, resulting in the observed discontinuity.
Tables

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Table 1:
Definitions of the control regions we used to test the functional form as described in the text.

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Table 2:
Impact of systematic uncertainties on the signal efficiencies in the LMR and the MMR.
Summary
A search for a narrow-width resonance decaying into two Higgs bosons, each decaying into a bottom quark-antiquark pair, is presented. The search is performed using proton-proton collision data corresponding to an integrated luminosity of 35.9 fb$^{-1}$ at $\sqrt{s} = $ 13 TeV recorded by the CMS detector at the LHC. No evidence for a signal is observed and upper limits at 95% confidence level on the production cross section for such spin-0 and spin-2 resonances, in the mass range from 260 to 1200 GeV, are set. In particular, a bulk KK-graviton (with $\kappa l$ = 35, $\kappa = 0.5\,{{M_\mathrm{Pl}}}$) in the mass range 320-450 GeV and 480-720 GeV, and a radion (with decay constant $\Lambda = $ 3 TeV) in the mass range 260-280 GeV, 300-450 GeV and 480-1120 GeV are excluded at a 95% confidence level. This analysis outperforms a similar search by CMS using 17.9 fb$^{-1}$ collected at 8 TeV [15] and extends the sensitivity to the gluon fusion production of a radion with decay constant $\Lambda = $ 3 TeV and to bulk graviton with $\kappa$ set to $0.5\, {{M_\mathrm{Pl}}}$.
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