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Search for Higgs boson pair production in the two bottom quarks plus two photons final state in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

The collaboration
CERN-EP-2021-180, 2021.

Abstract (data abstract)
CERN-LHC, ATLAS. Limits on pair production of Higgs bosons in the final state with two bottom quarks plus two photons. The search is based on 139 $fb^{-1}$ of pp collisions at $\sqrt{s}$ = 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. Data include exactly two b-tagged jets with a 77% working point and two well-identified isolated photons. The search is sensitive to both resonant and nonresonant production of Higgs boson pairs. Both the nonresonant and resonant searches employ multivariate analysis techniques to select events. Events are selected if they satisfy a common set of preselection requirements; then they are required to fulfill different requirements for the nonresonant and the resonant search. These further requirements are based on the BDT scores shown in tables 1 to 6. $m^{*}_{b\bar{b}\gamma\gamma} = m_{b\bar{b}\gamma\gamma} - m_{b\bar{b}} - m_{\gamma\gamma} + 250$ GeV is used to implement selection criteria for both the nonresonant and resonant analyses. Preselection: - there are at least two photons passing the tight and isolated object selection criteria; - the di-photon invariant mass, built with the two leading photons, satisfies 105 GeV < $m_{\gamma\gamma}$ < 160 GeV; - the leading (sub-leading) photon $p_{T}$ is larger than 35% (25%) of the mass of the di-photon system; - there are exactly two b-tagged jets at 77% working point; - no electrons or muons are present; - fewer than six central (|$\eta$|< 2.5) jets are required, to help rejecting $t\bar{t}H$ events where the top quarks decay hadronically. Non-resonant selection: Following the preselection, events are divided into two regions using the value of $m^{*}_{b\bar{b}\gamma\gamma}$. High mass region with $m^{*}_{b\bar{b}\gamma\gamma} > 350$ GeV targets the SM signal, while low mass region with $m^{*}_{b\bar{b}\gamma\gamma} < 350$ GeV targets BSM signals. In each mass region, a dedicated BDT is trained using XGBoost, and two categories are defined based on the BDT score. Events with a BDT score below 0.881 in the low mass region or below 0.857 in the high mass region are discarded. Resonant selection: Using the TMVA toolkit, two BDTs are trained to better separate the signal from backgrounds of different nature (the ${\gamma\gamma}$ plus the ${t\bar{t}\gamma\gamma}$ backgrounds and the single Higgs boson background, where ZH and ${t\bar{t}H}$ production modes are the dominant resonant backgrounds). The combined BDT score of an event ($BDT_{tot}$) is obtained by combining the two BDT scores in quadrature. Only events passing a minimum requirement on the value of the $BDT_{tot}$ are considered in the analysis. Events with a BDT score below 0.85 for $m_{X}$ = 300 GeV or below 0.75 for $m_{X}$ = 500 GeV are discarded. The BDT thresholds for other $m_{X}$ points are listed in Table 31.

• #### Table 1

Data from figure 6(a)

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The BDT distribution of the di-Higgs ggF signal for two different values of $\kappa_{\lambda}$ and the main backgrounds in the...

• #### Table 2

Data from sub-plot of figure 6(a)

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The BDT distribution (with x-axis zoomed in) of the di-Higgs ggF signal for two different values of $\kappa_{\lambda}$ and the...

• #### Table 3

Data from figure 6(b)

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The BDT distribution of the di-Higgs ggF signal for two different values of $\kappa_{\lambda}$ and the main backgrounds in the...

• #### Table 4

Data from sub-plot of figure 6(b)

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The BDT distribution (with x-axis zoomed in) of the di-Higgs ggF signal for two different values of $\kappa_{\lambda}$ and the...

• #### Table 5

Data from figure 7(a)

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The $BDT_{tot}$ score for the benchmark signal $m_{X}$ = 300 GeV and for the main backgrounds. Distributions are normalized to...

• #### Table 6

Data from figure 7(b)

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The $BDT_{tot}$ score for the benchmark signal $m_{X}$ = 500 GeV and for the main backgrounds. Distributions are normalized to...

• #### Table 7

Data from figure 9(a)

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Distributions of $m_{\gamma\gamma}$ in high mass BDT tight category for the nonresonant $HH$ search. The data-derived fractions of nonresonant $\gamma\gamma$,...

• #### Table 8

Data from figure 9(b)

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Distributions of $m_{\gamma\gamma}$ in high mass BDT loose category for the nonresonant $HH$ search. The data-derived fractions of nonresonant $\gamma\gamma$,...

• #### Table 9

Data from figure 9(c)

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Distributions of $m_{\gamma\gamma}$ in low mass BDT tight category for the nonresonant $HH$ search. The data-derived fractions of nonresonant $\gamma\gamma$,...

• #### Table 10

Data from figure 9(d)

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Distributions of $m_{\gamma\gamma}$ in low mass BDT loose category for the nonresonant $HH$ search. The data-derived fractions of nonresonant $\gamma\gamma$,...

• #### Table 11

Data from figure 10(a)

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Distributions of $m_{\gamma\gamma}$ for the selections used for the resonance mass point $m_{X}$ = 300 GeV for the resonant search....

• #### Table 12

Data from figure 10(b)

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Distributions of $m_{\gamma\gamma}$ for the selections used for the resonance mass point $m_{X}$ = 500 GeV for the resonant search....

• #### Table 13

Data from table 6

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The number of data events observed in the 120 GeV < $m_{\gamma\gamma}$ < 130 GeV window, the number of $HH$...

• #### Table 14

Data from figure 12

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Observed and expected limits at 95% CL on the cross section of nonresonant Higgs boson pair production as a function...

• #### Table 15

Data from figure 13

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Values of the negative log-profile-likelihood ratio ($-2ln\Lambda$) as a function of $\kappa_{\lambda}$ evaluated for the combination of all the categories...

• #### Table 16

Data from table 7

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The number of events observed in the 120 < $m_{\gamma\gamma}$ < 130 GeV window in data, the number of events...

• #### Table 17

Data from figure 15

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Observed and expected limits at 95% CL on the production cross section of a narrow-width scalar resonance $X$ as a...

• #### Table 18

Data from table 8

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Breakdown of the dominant systematic uncertainties. The impact of the uncertainties is defined according to the statistical analysis described in...

• #### Table 19

Data from table 9 in auxiliary materials

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Cutflow for nonresonant di-Higgs ggF signal sample, yields are normalized to 139 $fb^{-1}$.

• #### Table 20

Data from table 10 in auxiliary materials

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Cutflow for resonant signal sample, with $m_{X}$ = 300 GeV, yields are normalized to 139 $fb^{-1}$.

• #### Table 21

Data from table 11 in auxiliary materials

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Cutflow for resonant signal sample, with $m_{X}$ = 500 GeV, yields are normalized to 139 $fb^{-1}$.

• #### Table 22

Data from figure 18 in auxiliary materials

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Comparison of $m_{b\bar{b}}$ distributions when applying the specific b-jet energy calibration and the nominal jet energy calibration. The distributions are...

• #### Table 23

Data from figure 18 in auxiliary materials

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Fit results of $m_{b\bar{b}}$ distributions when applying the specific b-jet energy calibration and the nominal jet energy calibration. The distributions...

• #### Table 24

Data from figure 29 in auxiliary materials

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The relative amount (purity) of expected events from SM $HH$ and single Higgs boson production processes for each of the...

• #### Table 25

Data from figure 30 in auxiliary materials

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• #### Table 31

Data from table 14 in auxiliary materials

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Minimum BDT value of the events passing the selection criteria of the resonant search. The combined BDT score is formed...