Search for Heavy Higgs Bosons Decaying into Two Tau Leptons with the ATLAS Detector Using $pp$ Collisions at $\sqrt{s}=13\mathrm{TeV}$

A search for heavy neutral Higgs bosons is performed using the LHC Run 2 data, corresponding to an integrated luminosity of $139{\mathrm{fb}}^{-1}$ of proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ recorded with the ATLAS detector. The search for heavy resonances is performed over the mass range 0.2–2.5 TeV for the ${\tau }^{+}{\tau }^{-}$ decay with at least one $\tau$-lepton decaying into final states with hadrons. The data are in good agreement with the background prediction of the standard model. In the $M_h^{125}$ scenario of the minimal supersymmetric standard model, values of $\tan \beta >8$ and $\tan \beta >21$ are excluded at the 95% confidence level for neutral Higgs boson masses of 1.0 and 1.5 TeV, respectively, where $\tan \beta$ is the ratio of the vacuum expectation values of the two Higgs doublets.

Figure 1

The $m_T^{\mathrm{tot}}$ for the $b$-veto (left) and $b$-tag (right) categories of the ${\tau }_{\mathrm{lep}}{\tau }_{\mathrm{had}}$ channel (top) and ${\tau }_{\mathrm{had}}{\tau }_{\mathrm{had}}$ channel (bottom). The binning displayed is that entering into the fit. The predictions and uncertainties for the background processes are obtained from the fit assuming the background-only hypothesis. Expectations from signal processes are superimposed. Overflows are included in the last bin of the distributions.

Figure 2

The observed and expected 95% C.L. upper limits on the production cross section times branching fraction for a scalar boson ($\varphi$) produced via (a) ggF and (b) $b$-associated production. The limits are calculated from a statistical combination of the ${\tau }_{\mathrm{lep}}{\tau }_{\mathrm{had}}$ and ${\tau }_{\mathrm{had}}{\tau }_{\mathrm{had}}$ channels. The excluded region from the 2015–2016 data ATLAS search [24] is depicted by the dotted pink line. The 95% C.L. upper limits on $\tan \beta$ as a function of $m_A$ in the $M_h^{125}$ scenario is shown (c). The lowest value of $\tan \beta$ considered for the $M_h^{125}$ scenario is 0.5. In the small lower-left region shown in solid blue, the mass splitting between $A$ and $H$ bosons is above 50% of the mass resolution and therefore the simple addition of the cross sections is not valid. However, this region of parameter space in the $M_h^{125}$ scenario provides predictions that are incompatible with the measured mass value of the observed Higgs boson by more than $3\sigma$. The exclusion limit around $m_A=350\mathrm{GeV}$ reflects the behavior of the $A\to \tau \tau$ branching fraction close to the $A\to t\overline{t}$ kinematic threshold for low $\tan \beta$. The hatched area defines which side of the curve is excluded by the search.