Search for Higgs Boson Pair Production in the bb̅τ⁺τ^- Final State with the ATLAS Experiment at the LHC

Abstract

A search for Higgs boson pair (<i>HH</i>) production in final states with two <i>b</i> quarks and two <i>τ</i> leptons using 139 fb^-1 of proton–proton collision data at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the Large Hadron Collider (LHC) is presented. Searches for <i>HH</i> production are vital for solidifying our understanding of electroweak symmetry breaking in the Standard Model (SM) since they serve as direct probes of the Higgs boson self-coupling. In addition, they provide sensitivity to new phenomena introduced by theories beyond the SM. <br> The search presented in this thesis targets channels with one or more hadronic <i>τ</i>-lepton decays. It relies on effective tau identification algorithms, which are algorithms that distinguish signatures of hadronic <i>τ</i>-lepton decays in the ATLAS detector from other sources, such as quark- or gluon-initiated jets. A novel tau identification algorithm based on recurrent neural networks is introduced. It exploits information from reconstructed charged-particle tracks, topological clusters of calorimeter cell signals, and purposefully constructed discriminating variables for tau identification. The new technique outperforms the method previously employed in the ATLAS experiment. <br> Three different types of Higgs boson pair production are considered in this thesis: non-resonant <i>HH</i> production predicted by the SM (SM <i>HH</i> production); resonant <i>HH</i> production via scalar, narrow-width resonances; and non-resonant <i>HH</i> production with anomalous values of the Higgs boson self-coupling constant <i>λ_HHH</i>. <br> The search for SM <i>HH</i> production targets the gluon–gluon fusion and vector boson fusion production modes. No statistically significant deviation from the background-only hypothesis is observed; therefore, upper limits are set on the SM <i>HH</i> production cross section. The search yields an upper limit of 4.7 times the SM cross section prediction at 95% confidence level (CL). <br> The search for resonant <i>HH</i> production considers intermediate resonances with masses ranging from 251 to 1600 GeV. Over the considered mass range, the largest deviation from the background-only hypothesis is observed for a mass of 1000 GeV with a local (global) significance of 3.1σ (2.0σ). Upper limits at 95% CL are set on the resonant <i>HH</i> production cross section, which range from 20 to 900 fb depending on the mass of the resonance. <br> Finally, the search for SM <i>HH</i> production is reinterpreted in the context of anomalous values of the Higgs boson self-coupling constant. Upper limits at 95% CL are set on the non-resonant <i>HH</i> production cross section as a function of the self-coupling modifier <i>κ_λ = λ_HHH / λ</i>^SM<i>_HHH</i> where <i>λ</i>^SM<i>_HHH</i> is the Higgs boson self-coupling predicted by the SM. The upper limits on the non-resonant <i>HH</i> production cross section are compared to theory predictions, providing constraints on the self-coupling modifier of -2.4 ≤ <i>κ_λ</i> ≤ 9.2.