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CMS-B2G-16-026 ; CERN-EP-2017-238
Search for a massive resonance decaying to a pair of Higgs bosons in the four b quark final state in proton-proton collisions at $\sqrt{s}=$ 13 TeV
Phys. Lett. B 781 (2018) 244
Abstract: A search for a massive resonance decaying into a pair of standard model Higgs bosons, in a final state consisting of two b quark-antiquark pairs, is performed. A data sample of proton-proton collisions at a centre-of-mass energy of 13 TeV is used, collected by the CMS experiment at the CERN LHC in 2016, and corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The Higgs bosons are highly Lorentz-boosted and are each reconstructed as a single large-area jet. The signal is characterized by a peak in the dijet invariant mass distribution, above a background from the standard model multijet production. The observations are consistent with the background expectations, and are interpreted as upper limits on the products of the $s$-channel production cross sections and branching fractions of narrow bulk gravitons and radions in warped extra-dimensional models. The limits range from 126 to 1.4 fb at 95% confidence level for resonances with masses between 750 and 3000 GeV, and are the most stringent to date, over the explored mass range.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
The soft-drop mass (upper left), the N-subjettiness $ {\tau _{21}} $ (upper right), and the double-b tagger discriminator (lower) distributions of the selected AK8 jets. The multijet background components for the different jet flavours are shown, along with simulated bulk graviton and radion signals of masses 1400 and 2500 GeV. The number of signal and background events correspond to an integrated luminosity of 35.9 fb$^{-1}$. A signal cross section of 20 pb is assumed for all the mass hypotheses. The events are required to have passed the trigger selection, lepton rejection, the AK8 jet kinematic selections $ {p_{\mathrm {T}}} > $ 300 GeV and $ < \eta > < $ 2.4, and $ < {\Delta \eta (\text {j}_{1}, \text {j}_{2})} > < $ 1.3. The reduced dijet invariant mass $ {m_{\text {jj,red}}} $ is required to be greater than 750 GeV. The N-subjettiness requirement of $ {\tau _{21}} < $ 0.55 is applied to the upper left and lower figures. The soft-drop masses of the two jets are between 105-135 GeV for the upper right and lower figures.

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Figure 1-a:
The soft-drop mass distribution of the selected AK8 jets. The multijet background components for the different jet flavours are shown, along with simulated bulk graviton and radion signals of masses 1400 and 2500 GeV. The number of signal and background events correspond to an integrated luminosity of 35.9 fb$^{-1}$. A signal cross section of 20 pb is assumed for all the mass hypotheses. The events are required to have passed the trigger selection, lepton rejection, the AK8 jet kinematic selections $ {p_{\mathrm {T}}} > $ 300 GeV and $ < \eta > < $ 2.4, and $ < {\Delta \eta (\text {j}_{1}, \text {j}_{2})} > < $ 1.3. The reduced dijet invariant mass $ {m_{\text {jj,red}}} $ is required to be greater than 750 GeV. The N-subjettiness requirement of $ {\tau _{21}} < $ 0.55 is applied to the upper left and lower figures. The soft-drop masses of the two jets are between 105-135 GeV for the upper right and lower figures.

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Figure 1-b:
The N-subjettiness $ {\tau _{21}} $ distribution of the selected AK8 jets. The multijet background components for the different jet flavours are shown, along with simulated bulk graviton and radion signals of masses 1400 and 2500 GeV. The number of signal and background events correspond to an integrated luminosity of 35.9 fb$^{-1}$. A signal cross section of 20 pb is assumed for all the mass hypotheses. The events are required to have passed the trigger selection, lepton rejection, the AK8 jet kinematic selections $ {p_{\mathrm {T}}} > $ 300 GeV and $ < \eta > < $ 2.4, and $ < {\Delta \eta (\text {j}_{1}, \text {j}_{2})} > < $ 1.3. The reduced dijet invariant mass $ {m_{\text {jj,red}}} $ is required to be greater than 750 GeV. The N-subjettiness requirement of $ {\tau _{21}} < $ 0.55 is applied to the upper left and lower figures. The soft-drop masses of the two jets are between 105-135 GeV for the upper right and lower figures.

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Figure 1-c:
The double-b tagger discriminator distribution of the selected AK8 jets. The multijet background components for the different jet flavours are shown, along with simulated bulk graviton and radion signals of masses 1400 and 2500 GeV. The number of signal and background events correspond to an integrated luminosity of 35.9 fb$^{-1}$. A signal cross section of 20 pb is assumed for all the mass hypotheses. The events are required to have passed the trigger selection, lepton rejection, the AK8 jet kinematic selections $ {p_{\mathrm {T}}} > $ 300 GeV and $ < \eta > < $ 2.4, and $ < {\Delta \eta (\text {j}_{1}, \text {j}_{2})} > < $ 1.3. The reduced dijet invariant mass $ {m_{\text {jj,red}}} $ is required to be greater than 750 GeV. The N-subjettiness requirement of $ {\tau _{21}} < $ 0.55 is applied to the upper left and lower figures. The soft-drop masses of the two jets are between 105-135 GeV for the upper right and lower figures.

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Figure 2:
The jet separation $ < {\Delta \eta (\text {j}_{1}, \text {j}_{2})} >$ (left) and the reduced dijet invariant mass $ {m_{\text {jj,red}}} $ (right) distributions. The multijet background components for the different jet flavours are shown, along with simulated bulk graviton and radion signals of masses 1400 and 2500 GeV. The numbers of signal and background events correspond to an integrated luminosity of 35.9 fb$^{-1}$. The signal cross section is assumed to be 20 {\text { pb}} for all the mass hypotheses. The events are required to have passed the online selection, lepton rejection, the AK8 jet kinematic selections $ {p_{\mathrm {T}}} > $ 300 GeV, $ < \eta > < $ 2.4. The soft-drop masses of the two jets are between 105 and 135 GeV, and the N-subjettiness requirement of $ {\tau _{21}} < $ 0.55 is applied. The $ < {\Delta \eta (\text {j}_{1}, \text {j}_{2})} >$ distributions (left) require $ {m_{\text {jj,red}}} < $ 750 GeV; the $ {m_{\text {jj,red}}} $ distributions (right) require $ < {\Delta \eta (\text {j}_{1}, \text {j}_{2})} > < $ 1.3.

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Figure 2-a:
The jet separation $ < {\Delta \eta (\text {j}_{1}, \text {j}_{2})} >$ distribution. The multijet background components for the different jet flavours are shown, along with simulated bulk graviton and radion signals of masses 1400 and 2500 GeV. The numbers of signal and background events correspond to an integrated luminosity of 35.9 fb$^{-1}$. The signal cross section is assumed to be 20 {\text { pb}} for all the mass hypotheses. The events are required to have passed the online selection, lepton rejection, the AK8 jet kinematic selections $ {p_{\mathrm {T}}} > $ 300 GeV, $ < \eta > < $ 2.4. The soft-drop masses of the two jets are between 105 and 135 GeV, and the N-subjettiness requirement of $ {\tau _{21}} < $ 0.55 is applied. The $ < {\Delta \eta (\text {j}_{1}, \text {j}_{2})} >$ distributions (left) require $ {m_{\text {jj,red}}} < $ 750 GeV; the $ {m_{\text {jj,red}}} $ distributions (right) require $ < {\Delta \eta (\text {j}_{1}, \text {j}_{2})} > < $ 1.3.

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Figure 2-b:
The reduced dijet invariant mass $ {m_{\text {jj,red}}} $ distribution. The multijet background components for the different jet flavours are shown, along with simulated bulk graviton and radion signals of masses 1400 and 2500 GeV. The numbers of signal and background events correspond to an integrated luminosity of 35.9 fb$^{-1}$. The signal cross section is assumed to be 20 {\text { pb}} for all the mass hypotheses. The events are required to have passed the online selection, lepton rejection, the AK8 jet kinematic selections $ {p_{\mathrm {T}}} > $ 300 GeV, $ < \eta > < $ 2.4. The soft-drop masses of the two jets are between 105 and 135 GeV, and the N-subjettiness requirement of $ {\tau _{21}} < $ 0.55 is applied. The $ < {\Delta \eta (\text {j}_{1}, \text {j}_{2})} >$ distributions (left) require $ {m_{\text {jj,red}}} < $ 750 GeV; the $ {m_{\text {jj,red}}} $ distributions (right) require $ < {\Delta \eta (\text {j}_{1}, \text {j}_{2})} > < $ 1.3.

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Figure 3:
The signal selection efficiencies for the bulk graviton and radion models for different mass hypotheses of the resonances, shown for the LL and the TT signal event categories. Owing to the large sample sizes of the simulated events, the statistical uncertainties are negligible.

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Figure 4:
The bulk graviton signal $ {m_{\text {jj,red}}} $ distribution for the LL category, modelled using the sum of Gaussian and Crystal Ball functions. This modelling is performed for signals in the range 1100 $ < {m_{\text {jj,red}}} < $ 3000 GeV, where the background distribution falls smoothly. No events are observed above this value of ${m_{\text {jj,red}}}$.

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Figure 5:
The pass-fail ratio $R_\text {p/f}$ of the leading $ {p_{\mathrm {T}}} $ jet for the LL (left) and TT (right) signal region categories as a function of the difference between the soft-drop mass of the leading jet and the Higgs boson mass, ${{m_{\text {j}_{1}}} - {m_{\mathrm{H}}}}$. The measured ratio in different bins of $ {m_{\text {j}_{1}}} - {m_{\mathrm{H}}} $ is used in the fit (red solid line), except in the region around $ {m_{\text {j}_{1}}} - {m_{\mathrm{H}}} = 0$, which corresponds to the signal region (blue triangular markers). The fitted function is interpolated to obtain $R_\text {p/f}$ in the signal region. The horizontal bars on the data points indicate the bin widths.

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Figure 5-a:
The pass-fail ratio $R_\text {p/f}$ of the leading $ {p_{\mathrm {T}}} $ jet for the LL signal region category as a function of the difference between the soft-drop mass of the leading jet and the Higgs boson mass, ${{m_{\text {j}_{1}}} - {m_{\mathrm{H}}}}$. The measured ratio in different bins of $ {m_{\text {j}_{1}}} - {m_{\mathrm{H}}} $ is used in the fit (red solid line), except in the region around $ {m_{\text {j}_{1}}} - {m_{\mathrm{H}}} = 0$, which corresponds to the signal region (blue triangular markers). The fitted function is interpolated to obtain $R_\text {p/f}$ in the signal region. The horizontal bars on the data points indicate the bin widths.

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Figure 5-b:
The pass-fail ratio $R_\text {p/f}$ of the leading $ {p_{\mathrm {T}}} $ jet for the TT signal region category as a function of the difference between the soft-drop mass of the leading jet and the Higgs boson mass, ${{m_{\text {j}_{1}}} - {m_{\mathrm{H}}}}$. The measured ratio in different bins of $ {m_{\text {j}_{1}}} - {m_{\mathrm{H}}} $ is used in the fit (red solid line), except in the region around $ {m_{\text {j}_{1}}} - {m_{\mathrm{H}}} = 0$, which corresponds to the signal region (blue triangular markers). The fitted function is interpolated to obtain $R_\text {p/f}$ in the signal region. The horizontal bars on the data points indicate the bin widths.

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Figure 6:
The reduced mass distributions ${m_{\text {jj,red}}}$ for the LL (left) and TT (right) signal region categories. The points with bars show the data, the histogram with shaded band shows the estimated background and associated uncertainty. The ${m_{\text {jj,red}}}$ spectrum for the background is obtained by weighting the ${m_{\text {jj,red}}}$ spectrum in the antitag region by the ratio $R_\text {p/f}$ of Fig. 5. The signal predictions for a bulk graviton of mass 1000 GeV, are overlaid for comparison, assuming a production cross section of 10 fb. The last bins of the distributions contain all events with $ {m_{\text {jj,red}}} > $ 3000 GeV. The differences between the data and the predicted background, divided by the data statistical uncertainty (data unc.) as given by the Garwood interval [66], are shown in the lower panels.

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Figure 6-a:
The reduced mass distributions ${m_{\text {jj,red}}}$ for the LL signal region category. The points with bars show the data, the histogram with shaded band shows the estimated background and associated uncertainty. The ${m_{\text {jj,red}}}$ spectrum for the background is obtained by weighting the ${m_{\text {jj,red}}}$ spectrum in the antitag region by the ratio $R_\text {p/f}$ of Fig. 5. The signal predictions for a bulk graviton of mass 1000 GeV, are overlaid for comparison, assuming a production cross section of 10 fb. The last bins of the distributions contain all events with $ {m_{\text {jj,red}}} > $ 3000 GeV. The differences between the data and the predicted background, divided by the data statistical uncertainty (data unc.) as given by the Garwood interval [66], are shown in the lower panels.

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Figure 6-b:
The reduced mass distributions ${m_{\text {jj,red}}}$ for the TT signal region category. The points with bars show the data, the histogram with shaded band shows the estimated background and associated uncertainty. The ${m_{\text {jj,red}}}$ spectrum for the background is obtained by weighting the ${m_{\text {jj,red}}}$ spectrum in the antitag region by the ratio $R_\text {p/f}$ of Fig. 5. The signal predictions for a bulk graviton of mass 1000 GeV, are overlaid for comparison, assuming a production cross section of 10 fb. The last bins of the distributions contain all events with $ {m_{\text {jj,red}}} > $ 3000 GeV. The differences between the data and the predicted background, divided by the data statistical uncertainty (data unc.) as given by the Garwood interval [66], are shown in the lower panels.

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Figure 7:
The reduced mass ${m_{\text {jj,red}}}$ distributions in the antitag region for the LL (left) and TT (right) categories. The black markers are the data while the curves show the pre-fit and post-fit background shapes. The differences between the data and the predicted background, divided by the statistical uncertainty in the data (data unc.) as given by the Garwood interval [66], are shown in the lower panels.

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Figure 7-a:
The reduced mass ${m_{\text {jj,red}}}$ distributions in the antitag region for the LL category. The black markers are the data while the curves show the pre-fit and post-fit background shapes. The differences between the data and the predicted background, divided by the statistical uncertainty in the data (data unc.) as given by the Garwood interval [66], are shown in the lower panels.

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Figure 7-b:
The reduced mass ${m_{\text {jj,red}}}$ distributions in the antitag region for the TT category. The black markers are the data while the curves show the pre-fit and post-fit background shapes. The differences between the data and the predicted background, divided by the statistical uncertainty in the data (data unc.) as given by the Garwood interval [66], are shown in the lower panels.

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Figure 8:
The reduced mass ${m_{\text {jj,red}}}$ distributions in the signal region for the LL (left) and the TT (right) categories. The black markers are the data while the curves show the pre-fit and post-fit background shapes. The contribution of bulk gravitons of masses 1600 and 2500 GeV in the signal region are shown assuming a production cross section of 10 fb. The differences between the data and the predicted background, divided by the statistical uncertainty in the data (data unc.) as given by the Garwood interval [66], are shown in the lower panels.

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Figure 8-a:
The reduced mass ${m_{\text {jj,red}}}$ distributions in the signal region for the LL category. The black markers are the data while the curves show the pre-fit and post-fit background shapes. The contribution of bulk gravitons of masses 1600 and 2500 GeV in the signal region are shown assuming a production cross section of 10 fb. The differences between the data and the predicted background, divided by the statistical uncertainty in the data (data unc.) as given by the Garwood interval [66], are shown in the lower panels.

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Figure 8-b:
The reduced mass ${m_{\text {jj,red}}}$ distributions in the signal region for the TT category. The black markers are the data while the curves show the pre-fit and post-fit background shapes. The contribution of bulk gravitons of masses 1600 and 2500 GeV in the signal region are shown assuming a production cross section of 10 fb. The differences between the data and the predicted background, divided by the statistical uncertainty in the data (data unc.) as given by the Garwood interval [66], are shown in the lower panels.

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Figure 9:
The limits for the spin-0 radion (left) and the spin-2 bulk graviton (right) models. The result for $ {m_{\mathrm{X}}} < $ 1200 GeV uses the background predicted using the control regions, while for $ {m_{\mathrm{X}}} \ge $ 1200 GeV the background is derived from a combined signal and background fit to the data in the control and the signal regions. The predicted theoretical cross sections for a narrow radion or a bulk graviton are also shown.

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Figure 9-a:
The limits for the spin-0 radion model. The result for $ {m_{\mathrm{X}}} < $ 1200 GeV uses the background predicted using the control regions, while for $ {m_{\mathrm{X}}} \ge $ 1200 GeV the background is derived from a combined signal and background fit to the data in the control and the signal regions. The predicted theoretical cross sections for a narrow radion or a bulk graviton are also shown.

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Figure 9-b:
The limits for the spin-2 bulk graviton model. The result for $ {m_{\mathrm{X}}} < $ 1200 GeV uses the background predicted using the control regions, while for $ {m_{\mathrm{X}}} \ge $ 1200 GeV the background is derived from a combined signal and background fit to the data in the control and the signal regions. The predicted theoretical cross sections for a narrow radion or a bulk graviton are also shown.
Tables

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Table 1:
Definition of the signal, the antitag, and the sideband regions used for the background estimation. The regions are defined in terms of the soft-drop masses of the leading $ {p_{\mathrm {T}}} $ H jet (j$_{1}$) and the subleading $ {p_{\mathrm {T}}} $ H jet (j$_{2}$), and their double-b tagger discriminator values.

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Table 2:
Summary of systematic uncertainties in the signal and background yields.
Summary
A search for a narrow massive resonance decaying to two standard model Higgs bosons is performed using the LHC proton-proton collision data collected at a centre-of-mass energy of 13 TeV by the CMS detector, and corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The final state consists of events with both Higgs bosons decaying to b quark-antiquark pairs, which were identified using jet substructure and b-tagging techniques applied to large-area jets. The data are found to be consistent with the standard model expectations, dominated by multijet events. Upper limits are set on the products of the resonant production cross sections of a Kaluza-Klein bulk graviton and a Randall-Sundrum radion, and their branching fraction to $\mathrm{H}\mathrm{H} \to \mathrm{b\bar{b}}\mathrm{b\bar{b}}$. The limits range from 126 to 1.4 fb at 95% confidence level for bulk gravitons and radions in the mass range 750-3000 GeV. For the mass scale $\lambda_{\mathrm{R}} = $ 3 TeV, a radion of mass between 970 and 1400 GeV (except in a small region close to 1200 GeV) is excluded. These limits on the bulk graviton and the radion are the most stringent to date, over the mass range explored.
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Compact Muon Solenoid
LHC, CERN