Search for a CP-Odd Higgs Boson Decaying to $Zh$ in $pp$ Collisions at $\sqrt{s}=13$ TeV and Development of a ${b}$-Jet Tagging Calibration Method for ${c}$ Jets at the ATLAS Experiment

In this thesis a search for a CP-odd Higgs boson $A$ decaying into a $Z$ boson and a neutral CP-even Higgs boson $h$ with a mass of 125 GeV is described. The search is based on proton-proton collision data at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector in the years 2015 and 2016 corresponding to an integrated luminosity of 36 fb$^{-1}$. Final states, where the $Z$ boson decays into pairs of electrons, muons or neutrinos and the $h$ boson into bottom-quark pairs, are considered. The presence of neutrinos is inferred by a momentum imbalance in the transverse plane, the bottom-quark pairs are reconstructed either as two small-radius or one large-radius calorimeter jet(s). The reconstructed invariant or transverse mass distributions of the $Zh$ candidates are searched for the presence of localised excesses above the expectation from known Standard Model processes. No evidence for an $A$-boson signal is found. Upper limits on the $A$-boson production cross-section times the branching fractions for $A$-boson decays into $Zh$ and $h$-boson decays to bottom-quark pairs are derived at the 95% confidence level for $A$ bosons with masses between 220 GeV and 2 TeV; they range between 0.0019 pb and 0.81 pb. The results are interpreted in the context of Two-Higgs-Doublet Models. Furthermore, a novel method to calibrate the $b$-tagging efficiency for $c$-quark jets is described. It is demonstrated for the standard $b$-tagging algorithm of the ATLAS experiment in Run 1 using proton-proton collision data at a centre-of-mass energy of 7 TeV collected with the ATLAS detector in the year 2011. The calibration uses a sample of $c$ jets obtained from events, where a $W$ boson is produced in association with a single $c$ quark; the resulting $c$ jet is identified by a muon stemming from the semileptonic $c$-hadron decay. The measured $c$-jet tagging efficiencies are presented relative to the corresponding efficiencies in simulated $W$+$c$ events as scale factors. The measured scale factors are extrapolated using a simulation-based and data-supported procedure to obtain scale factors valid for inclusive samples of $c$ jets. Their uncertainties range between 5% and 13 %.