A Search for Decays of the 125 GeV Higgs Boson in the bb Final State with the ATLAS Detector using p-p Collisions at s=13 TeV

Abstract

This thesis presents a search for decays of the 125 GeV Higgs boson (h) to a pair of new (pseudo)scalar particles (a), h → aa, where the a-bosons decay to a b-quark pair and tau lepton (τ) pair. Adding a new (pseudo)scalar to the Standard Model is a simple extension which produces a wide range of phenomenology. The addition of a new (pseudo)scalar is also motivated by dark matter, which could couple to ordinary matter via the a-boson, and by recent experimental excesses in the muon anomalous magnetic moment and W boson mass, which both are modulated by a new scalar.

The search is performed in the mass range 12 < ma < 60 GeV in different analysis channels defined by the decay products of the tau pair. The search uses 139 fb−1 of proton-proton collision data with √s = 13 TeV recorded by the ATLAS experiment at the LHC between 2015 and 2018. The analysis searches in three decay channels of the ττ system and makes use of both leptonic (e or μ) and hadronic (τhad) tau lepton decays: eμ, μτhad, and eτhad. A further categorization is performed by the experimental signature of the heavy-flavor jets, including a boosted double b-jet tagger. Background processes are modeled with Monte Carlo for event topologies with two prompt leptons or hadronic taus and a data-driven technique is used to estimate the contribution from non-prompt leptons and hadronic tau decays. Control regions are employed to constrain different background processes: high transverse mass for tt, high mvis(ττ) for Drell-Yan, and same-sign visible tau decay products for non-prompt leptons and taus. A Parameterized Neural Network is employed to discriminate between signal and background in the low-mass (mvis < 60 GeV) regime in which h → aa decays are kinematically allowed.

Limits on the branching ratio for the h → aa → bbττ process are set and no evidence of any beyond the Standard Model phenomena is observed. This thesis sets the most sensitive limits worldwide on exotic Higgs decays in the bbττ final state. The branching ratio is limited to less than 1.5% over the mass range from 12 to 60 GeV for the mass of the new psuedoscalar ma.

A measurement of the resolution, efficiency, and noise rate of drift tube chambers for the ATLAS Phase-II upgrade is also presented. Notably, a novel treatment of the multiple Coulomb scattering correction is included in the drift tube resolution measurement.