Search for MSSM Higgs Bosons in Di-$τ$ Final States with the ATLAS Experiment

A search for neutral MSSM Higgs bosons decaying into a $\tau$ lepton pair, $A/H/h \rightarrow\tau^+\tau^−$, is presented. The search is based on proton–proton collision events recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy $\sqrt{s} = 7$ TeV, corresponding to an integrated luminosity of 2.1 fb$^{−1}$. One of the tau leptons is reconstructed in a leptonic, the other in a hadronic final state. The contributions of the main background processes have been estimated using data driven methods. For the $Z/\gamma^* → \tau^+\tau^−$ background an embedding method has been used, that replaces the muons in reconstructed Z/gamma* → mu+mu− events with simulated tau leptons. The event yield of the W + jets background has been studied in a sideband control region and the simulation has been normalized to agree with the sideband data. The contribution from QCD multi-jet processes has been estimated from three data control regions. Systematic uncertainties due to these background estimates have been identified and studied as well as systematic uncertainties affecting the simulated signal and background event samples. The dominating systematic uncertainty has been found to be due to the uncertainty on the energy scales of jets and hadronically decaying tau leptons. In total 4065 data events have been selected and the number of expected events from Standard Model background processes has been estimated to be $4151\pm66\pm796$, where the first error is statistical and the second error is systematic. As no excess over the Standard Model expectation is found, exclusion limits at the 95% confidence level are set on the production cross section of a generic Higgs boson, in dependence of its mass and on the production of the neutral MSSM Higgs bosons $A/H/h$ as a function of the parameters $\tan\beta$ and $m_A$ in the $m_{h}^{\text{max}}$ scenario. These exclusion limits represent significant improvements over those obtained by similar searches of other experiments at LEP and the Tevatron.