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CMS-HIG-20-010 ; CERN-EP-2022-117
Search for nonresonant Higgs boson pair production in final state with two bottom quarks and two tau leptons in proton-proton collisions at $\sqrt{s} = $ 13 TeV
Phys. Lett. B 842 (2023) 137531
Abstract: A search for the nonresonant production of Higgs boson pairs (HH) via gluon-gluon and vector boson fusion processes in final states with two bottom quarks and two tau leptons is presented. The search uses data from proton-proton collisions at a center-of-mass energy of $\sqrt{s} = $ 13 TeV recorded with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb$^{-1}$. Events in which at least one tau lepton decays hadronically are considered and multiple machine learning techniques are used to identify and extract the signal. The data are found to be consistent, within uncertainties, with the standard model (SM) predictions. Upper limits on the HH production cross section are set to constrain the parameter space for anomalous Higgs boson couplings. The observed (expected) upper limit at 95% confidence level corresponds to 3.3 (5.2) times the SM prediction for the inclusive HH cross section and to 124 (154) times the SM prediction for the vector boson fusion HH cross section. At 95% confidence level, the Higgs field self-coupling is constrained to be within $-$1.7 and 8.7 times the SM expectation, and the coupling of two Higgs bosons to two vector bosons is constrained to be within $-$0.4 and 2.6 times the SM expectation.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Feynman diagrams contributing to HH production via gluon-gluon fusion in the SM at leading order. The different H interactions are labelled by the coupling modifiers $\kappa $.

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Figure 2:
Feynman diagrams contributing to Higgs boson pair production via vector boson fusion in the SM at leading order. The different H interactions are labelled with the coupling modifiers $\kappa $.

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Figure 3:
Distributions of the reconstructed mass of the bb (upper left), $\tau \tau $ (upper right) and HH (lower) pairs in the most sensitive category of the analysis (res2b). Events are shown in the ${\tau _{\mathrm{e}} {\tau _\mathrm {h}}}$ (upper left), ${\tau _{\mu} {\tau _\mathrm {h}}}$ (upper right) and ${{\tau _\mathrm {h}} {\tau _\mathrm {h}}}$ (lower) channels for the full 2016-2018 data set, after the selection on the reconstructed masses of the $\tau \tau $ and bb pairs, as described in Section 5. The shaded band in the plots represents the statistical uncertainty only.

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Figure 3-a:
Distribution of the reconstructed mass of the bb pairs in the most sensitive category of the analysis (res2b). Events are shown in the ${\tau _{\mathrm{e}} {\tau _\mathrm {h}}}$ channel for the full 2016-2018 data set, after the selection on the reconstructed masses of the $\tau \tau $ and bb pairs, as described in Section 5. The shaded band represents the statistical uncertainty only.

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Figure 3-b:
Distribution of the reconstructed mass of the $\tau \tau $ pairs in the most sensitive category of the analysis (res2b). Events are shown in the ${\tau _{\mu} {\tau _\mathrm {h}}}$ channel for the full 2016-2018 data set, after the selection on the reconstructed masses of the $\tau \tau $ and bb pairs, as described in Section 5. The shaded band represents the statistical uncertainty only.

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Figure 3-c:
Distribution of the reconstructed mass of the HH pairs in the most sensitive category of the analysis (res2b). Events are shown in the ${{\tau _\mathrm {h}} {\tau _\mathrm {h}}}$ channel for the full 2016-2018 data set, after the selection on the reconstructed masses of the $\tau \tau $ and bb pairs, as described in Section 5. The shaded band represents the statistical uncertainty only.

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Figure 4:
The postfit DNN distributions in the ${{\tau _\mathrm {h}} {\tau _\mathrm {h}}}$ channel in 2018 for the most sensitive category in the ggF (left) and VBF (right) searches. The shaded band in the plots represents the statistical plus systematic uncertainty. The expected SM distributions for the ggF and VBF HH signals are shown superimposed on the figures.

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Figure 4-a:
The postfit DNN distribution in the ${{\tau _\mathrm {h}} {\tau _\mathrm {h}}}$ channel in 2018 for the most sensitive category in the ggF search. The shaded band represents the statistical plus systematic uncertainty. The expected SM distributions for the ggF and VBF HH signals are shown superimposed on the figure.

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Figure 4-b:
The postfit DNN distribution in the ${{\tau _\mathrm {h}} {\tau _\mathrm {h}}}$ channel in 2018 for the most sensitive category in the VBF search. The shaded band represents the statistical plus systematic uncertainty. The expected SM distributions for the ggF and VBF HH signals are shown superimposed on the figure.

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Figure 5:
Combination of bins of all postfit distributions, ordered according to the expected signal-to-square-root-background ratio, where the signal is the SM HH signal expected in that bin and the background is the prefit background estimate in the same bin, separately for the ${\tau _{\mathrm{e}} {\tau _\mathrm {h}}}$ channel (upper left), the ${\tau _{\mu} {\tau _\mathrm {h}}}$ channel (upper right), and ${{\tau _\mathrm {h}} {\tau _\mathrm {h}}}$ channel (lower). The ratio also shows the signal scaled to the observed exclusion limit (as shown in Table 2).

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Figure 5-a:
Combination of bins of all postfit distributions, ordered according to the expected signal-to-square-root-background ratio, where the signal is the SM HH signal expected in that bin and the background is the prefit background estimate in the same bin, for the ${\tau _{\mathrm{e}} {\tau _\mathrm {h}}}$ channel. The ratio also shows the signal scaled to the observed exclusion limit (as shown in Table 2).

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Figure 5-b:
Combination of bins of all postfit distributions, ordered according to the expected signal-to-square-root-background ratio, where the signal is the SM HH signal expected in that bin and the background is the prefit background estimate in the same bin, for the ${\tau _{\mu} {\tau _\mathrm {h}}}$ channel. The ratio also shows the signal scaled to the observed exclusion limit (as shown in Table 2).

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Figure 5-c:
Combination of bins of all postfit distributions, ordered according to the expected signal-to-square-root-background ratio, where the signal is the SM HH signal expected in that bin and the background is the prefit background estimate in the same bin, for the ${{\tau _\mathrm {h}} {\tau _\mathrm {h}}}$ channel. The ratio also shows the signal scaled to the observed exclusion limit (as shown in Table 2).

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Figure 6:
Combination of bins of all postfit distributions, ordered according to the expected signal-to-square-root-background ratio, where the signal is the SM HH signal expected in that bin and the background is the prefit background estimate in the same bin, separately for the background contribution split into physics processes (left), and split into the three considered final state channels (right). The ratio also shows the signal scaled to the observed exclusion limit (as shown in Table 2).

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Figure 6-a:
Combination of bins of all postfit distributions, ordered according to the expected signal-to-square-root-background ratio, where the signal is the SM HH signal expected in that bin and the background is the prefit background estimate in the same bin, for the background contribution split into physics processes. The ratio also shows the signal scaled to the observed exclusion limit (as shown in Table 2).

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Figure 6-b:
Combination of bins of all postfit distributions, ordered according to the expected signal-to-square-root-background ratio, where the signal is the SM HH signal expected in that bin and the background is the prefit background estimate in the same bin, for the background contribution split into the three considered final state channels. The ratio also shows the signal scaled to the observed exclusion limit (as shown in Table 2).

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Figure 7:
The expected and observed limits on the ratio of experimentally estimated ggF plus VBF ($\sigma ({{\mathrm{p}} {\mathrm{p}}} \to {\mathrm{H} \mathrm{H}})$, left) and VBF only ($\sigma ({{\mathrm{p}} {\mathrm{p}}} \to \mathrm{q} \mathrm{q} {\mathrm{H} \mathrm{H}})$, right) HH production cross section and the expectation from the SM ($\sigma _{\text {Theory}}$) at 95% CL, separated into different years and combined for the full 2016-2018 data set.

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Figure 7-a:
The expected and observed limits on the ratio of experimentally estimated ggF plus VBF ($\sigma ({{\mathrm{p}} {\mathrm{p}}} \to {\mathrm{H} \mathrm{H}})$) HH production cross section and the expectation from the SM ($\sigma _{\text {Theory}}$) at 95% CL, separated into different years and combined for the full 2016-2018 data set.

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Figure 7-b:
The expected and observed limits on the ratio of experimentally estimated VBF only ($\sigma ({{\mathrm{p}} {\mathrm{p}}} \to \mathrm{q} \mathrm{q} {\mathrm{H} \mathrm{H}})$) HH production cross section and the expectation from the SM ($\sigma _{\text {Theory}}$) at 95% CL, separated into different years and combined for the full 2016-2018 data set.

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Figure 8:
(left) Observed and expected upper limits at 95% CL as functions of $\kappa _{\lambda}$ on the ggF plus VBF HH cross section times the ${\mathrm{b} \mathrm{b} \tau \tau}$ branching fraction. (right) Observed and expected upper limits at 95% CL as functions of $\kappa _{2{\mathrm{V}}}$ on the VBF only HH cross section times the ${\mathrm{b} \mathrm{b} \tau \tau}$ branching fraction. In both cases all other couplings are set to their SM expectation. The red solid line shows the theoretical prediction for the HH production cross section and its uncertainty (red shaded band).

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Figure 8-a:
Observed and expected upper limits at 95% CL as functions of $\kappa _{\lambda}$ on the ggF plus VBF HH cross section times the ${\mathrm{b} \mathrm{b} \tau \tau}$ branching fraction. All other couplings are set to their SM expectation. The red solid line shows the theoretical prediction for the HH production cross section and its uncertainty (red shaded band).

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Figure 8-b:
Observed and expected upper limits at 95% CL as functions of $\kappa _{2{\mathrm{V}}}$ on the VBF only HH cross section times the ${\mathrm{b} \mathrm{b} \tau \tau}$ branching fraction. All other couplings are set to their SM expectation. The red solid line shows the theoretical prediction for the HH production cross section and its uncertainty (red shaded band).

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Figure 9:
(left) Two-dimensional exclusion regions as a function of the $\kappa _{\lambda}$ and $\kappa _t$ couplings for the full 2016-2018 combination, with both $\kappa _{2{\mathrm{V}}}$ and $\kappa _{{\mathrm{V}}}$ are fixed to unity. (right) Two-dimensional exclusion regions as a function of $\kappa _{2{\mathrm{V}}}$ and $\kappa _{{\mathrm{V}}}$, with both $\kappa _{\lambda}$ and $\kappa _t$ are set to unity. Expected uncertainties on exclusion boundaries are inferred from uncertainty bands of the limit calculation, and are denoted by dark and light-grey areas. The blue area marks parameter combinations that are observed to be excluded. For visual guidance, theoretical cross section values are illustrated by thin, labeled contour lines with the SM prediction denoted by a red diamond.

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Figure 9-a:
Two-dimensional exclusion regions as a function of the $\kappa _{\lambda}$ and $\kappa _t$ couplings for the full 2016-2018 combination, with both $\kappa _{2{\mathrm{V}}}$ and $\kappa _{{\mathrm{V}}}$ are fixed to unity. Expected uncertainties on exclusion boundaries are inferred from uncertainty bands of the limit calculation, and are denoted by dark and light-grey areas. The blue area marks parameter combinations that are observed to be excluded. For visual guidance, theoretical cross section values are illustrated by thin, labeled contour lines with the SM prediction denoted by a red diamond.

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Figure 9-b:
Two-dimensional exclusion regions as a function of $\kappa _{2{\mathrm{V}}}$ and $\kappa _{{\mathrm{V}}}$, with both $\kappa _{\lambda}$ and $\kappa _t$ are set to unity. Expected uncertainties on exclusion boundaries are inferred from uncertainty bands of the limit calculation, and are denoted by dark and light-grey areas. The blue area marks parameter combinations that are observed to be excluded. For visual guidance, theoretical cross section values are illustrated by thin, labeled contour lines with the SM prediction denoted by a red diamond.

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Figure 10:
Observed likelihood scan as a function of $\kappa _{\lambda}$ (left) and $\kappa _{2{\mathrm{V}}}$ (right) for the full 2016-2018 combination. The dashed lines show the intersection with threshold values one and four, corresponding to 68 and 95% confidence intervals, respectively.

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Figure 10-a:
Observed likelihood scan as a function of $\kappa _{\lambda}$ for the full 2016-2018 combination. The dashed lines show the intersection with threshold values one and four, corresponding to 68 and 95% confidence intervals, respectively.

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Figure 10-b:
Observed likelihood scan as a function of $\kappa _{2{\mathrm{V}}}$ for the full 2016-2018 combination. The dashed lines show the intersection with threshold values one and four, corresponding to 68 and 95% confidence intervals, respectively.
Tables

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Table 1:
Summary of selections applied to the $\tau \tau $ candidate pair. Trigger thresholds in parentheses refer to the 2017-2018 data-taking period when different from 2016.

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Table 2:
Expected and observed upper limits at 95% CL for the SM point ($\kappa _{\lambda}=$ 1), where $\sigma _{{\mathrm{g} \mathrm{g} \text {F}} +\text {VBF}}^{\text {SM}}=$ 32.776 fb represents the sum of the ggF plus VBF HH cross sections.

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Table 3:
Expected and observed upper limits at 95% CL for the SM point ($\kappa _{2{\mathrm{V}}}=$ 1), where $\sigma _{\text {VBF}}^{\text {SM}}=$ 1.726 fb represents the VBF only HH cross section.
Summary
A search for nonresonant Higgs boson pair (HH) production via gluon-gluon fusion (ggF) and vector boson fusion (VBF) processes in final states with two bottom quarks and two tau leptons was presented. The search uses the full 2016-2018 data set of proton-proton collisions at a center-of-mass energy of $\sqrt{s} = $ 13 TeV recorded with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb$^{-1}$. The three decay modes of the $\tau\tau$ pair with the largest branching fraction have been selected, requiring one tau lepton to be always decaying hadronically and the other one either leptonically or hadronically. Upper limits at 95% confidence level (CL) on the inclusive ggF plus VBF HH production cross section are set, as well as on the VBF only HH production cross section.

This analysis benefits from an improved trigger strategy as well as from a series of techniques developed especially for this search: among others, several neural networks to identify the b jets from the H decay, to categorize the events, and to perform signal extraction. Moreover, this analysis builds up on the improvements made by the CMS Collaboration in the jet and tau lepton identification and reconstruction algorithms. All these techniques enable the achievement of particularly stringent results on the HH production cross sections.

The observed (expected) 95% CL upper limit on HH total production cross section corresponds to 3.3 (5.2) times the theoretical SM prediction. The observed (expected) 95% CL upper limit for the VBF only HH SM cross section corresponds to 124 (154) times the theoretical SM prediction.

The observed (expected) 95% CL constraints on $\kappa_{\lambda}$ and $\kappa_{2\mathrm{V}}$, derived from limits on the HH production cross section times the ${\mathrm{b}\mathrm{b}\tau\tau}$ branching fraction, are found to be $-$1.7 $< \kappa_{\lambda} <$ 8.7 ($-$2.9 $ < \kappa_{\lambda} < $ 9.8) and $-$0.4 $ < \kappa_{2\mathrm{V}} < $ 2.6 ($-$0.6 $ < \kappa_{2\mathrm{V}} < $ 2.8), respectively.
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