The results of a search for charged Higgs bosons decaying to a $\tau$ lepton and a neutrino, $H^{\pm} \rightarrow \tau^{\pm} \nu$, are presented. The analysis is based on 19.5 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV collected by the ATLAS experiment at the Large Hadron Collider. Charged Higgs bosons are searched for in events consistent with top-quark pair production or in associated production with a top quark. The final state is characterised by the presence of a hadronic $\tau$ decay, missing transverse momentum, $b$-tagged jets, a hadronically decaying $W$ boson, and the absence of any isolated electrons or muons with high transverse momenta. The data are consistent with the expected background from Standard Model processes. A statistical analysis leads to 95\% confidence-level upper limits on the product of branching ratios $ {\cal B}(t\rightarrow bH^\pm) \times {\cal B} (H^\pm\rightarrow \tau^{\pm}\nu) $, between 0.23% and 1.3% for charged Higgs boson masses in the range 80-160 GeV. It also leads to 95% confidence-level upper limits on the production cross section times branching ratio, $\sigma(pp \rightarrow tH^{\pm} + X) \times {\cal B}(H^{\pm} \rightarrow \tau^{\pm} \nu)$, between 0.76 pb and 4.5 fb, for charged Higgs boson masses ranging from 180 GeV to 1000 GeV. In the context of different scenarios of the Minimal Supersymmetric Standard Model, these results exclude nearly all values of $\tan\beta$ above one for charged Higgs boson masses between 80 GeV and 160 GeV, and exclude a region of parameter space with high $\tan\beta$ for $H^{\pm}$ masses between 200 GeV and 250 GeV.
The measured B(t->H+)xB(H+->tau+ nu) limit.
The measured P P --> t H+ --> t tau+nu limit.
This paper describes a search for dark photons ($\gamma_d$) in Higgs boson decay ($H \to \gamma\gamma_d$) produced in proton-proton collisions through the $ZH$ production mode at the Large Hadron Collider at $\sqrt{s}=13$ TeV. The transverse mass of the photon and the missing transverse momentum from the non-interacting $\gamma_d$ would present a distinctive signature at the Higgs boson mass resonance. The results presented use the total Run-2 integrated luminosity of 139 fb$^{-1}$, recorded by the ATLAS detector at the LHC . The dominant reducible background processes have been estimated using data-driven techniques. A Boosted Decision Tree (BDT) technique was adopted to enhance the sensitivity of the search. Given that no excess is observed with respect to the Standard Model predictions, an observed (expected) upper limit on the branching ratio BR$(H\to \gamma\gamma_d)$ of 2.28$\%$ (2.82$^{+1.33}_{-0.84}\%$) is set at 95$\%$ CL for massless $\gamma_d$. For higher dark photons masses up to 40 GeV, the observed (expected) upper limits at 95$\%$ CL are found to be within the [2.19-2.52]$\%$ ([2.71-3.11]$\%$) range.
Distribution of the BDT classifier response for data and for the expected SM background before the background-only fit. The expectations for the signal are also shown for the massless dark photon and for dark photon mass values of 20 GeV and 40 GeV, assuming BR(H$\to\gamma\gamma_d$) = 5%. Uncertainties shown are statistical for data, while for backgrounds include statistical and systematic sources.
Distribution of the BDT classifier response for data and for the expected SM background after the background-only fit. The expectations for the signal are also shown for the massless dark photon and for dark photon mass values of 20 GeV and 40 GeV, assuming BR(H$\to\gamma\gamma_d$) = 5%. Uncertainties shown are statistical for data, while for backgrounds include statistical and systematic sources determined by the multiple-bin fit.
Background, data and signal yields in bins of BDT, in SR and VV$\gamma$ CR, after the background-only fit. The expectations for the signal are shown for the massless dark photon and for dark photon mass values of 20 GeV and 40 GeV, assuming BR(H$\to\gamma\gamma_d$) = 5%. Uncertainties are statistical for data, while for backgrounds include statistical and systematic sources.
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