First Cross-Section Measurements of Highly Lorentz-Boosted Higgs Bosons in the di-tau Channel at ATLAS

Abstract

This thesis presents the first measurements of the Standard Model Higgs boson production cross section in the highly Lorentz-boosted regime using the ATLAS detector at the Large Hadron Collider. The analysis targets Higgs boson decays to a pair of fully hadronically decaying $\tau$ leptons with Higgs boson transverse momentum $p_T^H > 300$~GeV. In this regime, the two $\tau$ leptons become highly collimated, rendering standard single-$\tau$ reconstruction inefficient. To address this challenge, a dedicated reconstruction algorithm is employed to reconstruct the boosted di-$\tau$ system. Additionally, a new di-$\tau$ identification algorithm is developed that significantly improves background rejection. The analysis uses the full Run~2 dataset of proton-proton collisions at $\sqrt{s} = 13$~TeV together with a partial Run~3 dataset at $\sqrt{s} = 13.6$~TeV. The combined Run~2 and Run~3 fit yields an observed signal significance of $3.8 \sigma$, constituting evidence for Higgs boson production in the boosted regime. The inclusive cross sections are $\sigma_{H\to\tau\tau}^{p_T^H > 300} = 9.0^{+7.5}_{-5.8}$~fb (Run~2) and $25.2^{+10.6}_{-8.8}$~fb (Run~3), in good agreement with the Standard Model predictions of 14~fb and 15.6~fb respectively. Results are also reported within the Simplified Template Cross-Section framework, with separate measurements in bins of $p_T^H$ for gluon-fusion production and inclusively for electroweak $qqH$ production, as well as a differential cross-section measurement in three $p_T^H$ intervals. This work extends ATLAS $H\to\tau\tau$ measurements into a previously unexplored high-$p_T^H$ regime and provides important input for global precision studies of Higgs boson measurements.