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Search for non-resonant Higgs pair-production in the $\mathrm{b}\overline{\mathrm{b}}\mathrm{b}\overline{\mathrm{b}}$ final state with the CMS detector
Abstract: Results of a search for non-resonant production of Higgs boson pairs decaying to two $\rm{b \bar{b}}$ pairs are presented. This search uses data from proton-proton collisions at a centre-of-mass energy of 13 TeV, collected by the CMS experiment at the LHC in 2016 and corresponding to an integrated luminosity of 35.9 fb$^{-1}$. No signal is observed, and an upper limit of 847fb is set on the standard model cross section for non-resonant Higgs boson pair-production, where the Higgs bosons decay to $\rm{b \bar{b}}$ pairs. The same signature is studied in the context of theories of physics beyond the standard model that predict modified couplings of the Higgs boson, and used to extract limits on Higgs boson anomalous couplings.
Figures & Tables Summary References CMS Publications

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Figure 1:
Feynman diagrams that contribute to HH production by gluon-gluon fusion at leading order. Diagrams (a) and (b) correspond to SM-like processes, while diagrams (c), (d), and (e) correspond to pure BSM effects: (c) and (d) describe contact interactions between the H and gluons, and (e) exploits the contact interaction of two Higgs bosons with top quarks.

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Figure 2:
Left: comparison of the distribution of BDT output for real data (histogram) selected in a region of the leading versus trailing Higgs mass plane that excludes a 60-GeV-wide box around the most probable values of the di-jet masses of signal events, with the corresponding output on an artificial sample obtained from the same data set by hemisphere mixing. Right: bin-by-bin differences between data and model, in standard deviation units before (top) and after (bottom) bias correction; pull distribution for the differences, fit to a Gaussian distribution. Bias correction uncertainty is increased to take the sigma of the residuals to one.

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Figure 3:
Results of the fit to the BDT distribution for the SM HH production signal. In the bottom panel a comparison is shown between the best-fit signal and best-fit background subtracted from measured data. The band, centred at zero, shows the total uncertainty.

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Figure 4:
Cross section limits on the 13 BSM models investigated. See Table xxxxx for their respective parameter values.

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Figure 5:
Cross section limits on $\rm {H({{\mathrm {b}} {\overline {\mathrm {b}}}})H({{\mathrm {b}} {\overline {\mathrm {b}}}})}$ for values of $k_\lambda $ in the [-20;20] range, assuming $k_t = $ 1; the theoretical prediction with $k_t = $ 1 is also shown.

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Table 1:
The values of the anomalous coupling parameter for the the 13 benchmark models studied\nobreakspace {} [19]. For reference the values of the parameters in the SM are also included.

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Table 2:
Cut-flow efficiency for the SM like signal $\rm {pp} \rightarrow \rm {HH} \rightarrow \rm {b \bar{b} }$; the efficiency and the relative reduction of each successive selection is shown. The number of expected events for an integrated luminosity of 1fb$^{-1}$ is also reported.

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Table 3:
List of BDT input variables.

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Table 4:
Systematic uncertainties considered in the analysis and relative impact on the expected limit for SM HH production. The relative impact is obtained by fixing the nuisance parameters corresponding to each source and recalculating the expected limit.

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Table 5:
The expected upper limit on $\sigma (\rm {pp \rightarrow H({{\mathrm {b}} {\overline {\mathrm {b}}}})H({{\mathrm {b}} {\overline {\mathrm {b}}}})})$ in the SM at 95% confidence level using 35.9 fb$^{-1}$ of data considering the whole set of nuisance parameters.

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Table 6:
Expected and observed limits on the HH production cross section for the 13 BSM benchmark models.
This document presents a search for non-resonant HH production with both Higgs bosons decaying into $\rm{b \bar{b}}$ pairs. The SM production has been studied along with 13 BSM benchmark models, using a data set of $\sqrt{s}= $ 13 TeV proton-proton collision events corresponding to an integrated luminosity of 35.9 fb$^{-1}$ collected by the CMS detector during the 2016 LHC run. The analysis of events acquired by a hadronic multijet trigger includes the selection of events with 4 b-tagged jets and a classification using boosted decision trees targeting each of the signals considered. Limits on the HH production cross section times the square of the branching fraction for the H decay to b-quark pairs are extracted for the SM and each BSM model considered using binned likelihood fits of the shape of the BDT classifier output. The background model is derived from a novel data-driven technique that provides a multi-dimensional representation of the QCD multijet background, which makes up the major part of the data, and also models well the overall background distribution. The expected upper limit on $\sigma(\rm{pp \rightarrow H(\mathrm{b\bar{b}})H(\mathrm{b\bar{b}})})_{SM}$ is 419 fb. The observed upper limit is 847 fb, which corresponds to approximately 74 times the expected value for the SM process. Anomalous couplings of the H are also investigated. The upper limits extracted for the HH production cross section in the 13 BSM benchmark models range from 508 to 3513 fb.
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