Search for Resonant and Nonresonant Higgs Boson Pair Production in the $b\overline{b}{\tau }^{+}{\tau }^{-}$ Decay Channel in $pp$ Collisions at $\sqrt{s}=13\mathrm{TeV}$ with the ATLAS Detector

A search for resonant and nonresonant pair production of Higgs bosons in the $b\overline{b}{\tau }^{+}{\tau }^{-}$ final state is presented. The search uses $36.1{\mathrm{fb}}^{-1}$ of $pp$ collision data with $\sqrt{s}=13\mathrm{TeV}$ recorded by the ATLAS experiment at the LHC in 2015 and 2016. Decays of the $\tau$-lepton pairs with at least one $\tau$ lepton decaying to final states with hadrons and a neutrino are considered. No significant excess above the expected background is observed in the data. The cross-section times branching ratio for nonresonant Higgs boson pair production is constrained to be less than 30.9 fb, 12.7 times the standard model expectation, at 95% confidence level. The data are also analyzed to probe resonant Higgs boson pair production, constraining a model with an extended Higgs sector based on two doublets and a Randall-Sundrum bulk graviton model. Upper limits are placed on the resonant Higgs boson pair production cross-section times branching ratio, excluding resonances $X$ in the mass range $305\mathrm{GeV}<m_X<402\mathrm{GeV}$ in the simplified hMSSM minimal supersymmetric model for $\tan \beta =2$ and excluding bulk Randall-Sundrum gravitons $G_{\mathrm{KK}}$ in the mass range $325\mathrm{GeV}<m_{G_{\mathrm{KK}}}<885\mathrm{GeV}$ for $k/{\overline{M}}_{\mathrm{Pl}}=1$.

Figure 1

Distributions of the BDT score for NR $HH$ signal (left) and bulk RS signal with $m_{G_{\mathrm{KK}}}=500\mathrm{GeV}$ and $k/{\overline{M}}_{\mathrm{Pl}}=1$ (right) in the (a),(b) ${\tau }_{\mathrm{lep}}{\tau }_{\mathrm{had}}$ single-lepton trigger (SLT), (c),(d) $\mathrm{lepton}+{\tau }_{\mathrm{had}}$ trigger (LTT) and (e),(f) ${\tau }_{\mathrm{had}}{\tau }_{\mathrm{had}}$ channels. Distributions are shown after the fit to the background-only hypothesis and the signal is scaled to approximately the expected limit. The hatched band indicates the combined statistical and systematic uncertainty in the background. The ratio of the data to the sum of the backgrounds is shown in the lower panel.

Figure 2

Observed and expected limits at 95% C.L. on the cross sections of a generic narrow-width scalar $X$ (top) and RS $G_{\mathrm{KK}}$ (bottom) times the branching fraction to two $CP$-even Higgs bosons $H$, when combining the ${\tau }_{\mathrm{lep}}{\tau }_{\mathrm{had}}$ and ${\tau }_{\mathrm{had}}{\tau }_{\mathrm{had}}$ channels. The expected cross section for the hMSSM scalar $X$ production at $\tan \beta =2$ and the bulk RS graviton production with $k/{\overline{M}}_{\mathrm{Pl}}=1.0$ are also shown in the respective plots. In the hMSSM case, the bump in the theory prediction around 350 GeV corresponds to the threshold for $X$ decaying into $t\overline{t}$ pairs.