Measurement of the differential $W\to\mu+\nu$ cross section at high transverse masses at $\sqrt{s}=13\,$TeV with the ATLAS detector

In this thesis, the measurement of the cross section of the charged-current Drell-Yan process in the muonic decay channel is presented. The cross section is measured differentially in the transverse mass of the W boson, $m_T^W$, while covering the high mass tail in a range from \SIrange{130}{2000}{\GeV}. Separate results for both charges of the W boson as well as their combination are presented. The analysis is based on proton-proton collisions at the LHC at a center-of-mass-energy of $\sqrt{s}=13\,$TeV, recorded at the ATLAS detector in 2015 and 2016. \\ The modeling of background processes plays a key role, in particular the estimation of the multijet background, which consists of fake muons. This background is obtained data-driven via the Matrix Method. The number of signal events in the data is unfolded to the born level using the technique of Iterative Bayesian Unfolding. Dedicated corrections for the detector smearing, measurement efficiency and acceptance are applied. The final calculated cross sections include statistical as well as systematic uncertainties. At high transverse masses, the statistical uncertainty dominates, whereas at low $m_T^W$ the uncertainty associated with the \met is the most significant. Finally, the results are compared to the born level of the signal \texttt{POWHEG-BOX}+\texttt{PYTHIA~8} MC. Overall, the theory and unfolded cross section are in good agreement, with the exception of the first $m_T^W$ bin, ranging from $130-170\,$GeV, where an underestimation in the MC is visible.