Reconstruction and identification of boosted di-$\tau$ decays in a search for Higgs boson pairs with the ATLAS detector

The discovery of the Higgs boson opened the way for new experimental searches at the Large Hadron Collider at CERN. The resonant production of two Higgs bosons is a promising signature for searches for new physics phenomena that are predicted by extensions of the Standard Model such as the Minimal Supersymmetric Standard Model. Searches for heavy resonances with masses above 1 TeV are experimentally challenging. In this regime, the Higgs bosons are produced with high momenta. Their decay products are highly collimated to the extent that the standard techniques for particle reconstruction become inefficient. A novel method, referred to as di-τ reconstruction, is developed for the reconstruction of pairs of hadronically decaying /tauleptons with a large Lorentz boost in the ATLAS detector. Highly boosted pairs of τ leptons are reconstructed with an efficiency of more than 80%. The multi-variate di-τ identification recognizes pairs of τ leptons with an efficiency of more than 60%, whereas the probability to misidentify the background of a quark- or gluon-initiated jet is less than 0.01%. The di-τ algorithms are calibrated and validated using data of 139 fb${}^{-1}$ of proton--proton collisions at a center-of-mass energy of 13 TeV. These novel techniques are employed in a search for resonant production of Higgs boson pairs in the $b\overline{b}{\text{τ}}^{+}{\text{τ}}^{-}$ final state. No significant deviations from the background expectation are observed. Upper limits are derived on the production cross section of a narrow scalar resonance times its branching ratio to decay into a Higgs boson pair. Exclusion limits between 858 fb and 27 fb are observed at 95% confidence level for resonance masses from 1 TeV to 3 TeV.