Search for new heavy charged bosons and measurement of high-mass Drell-Yan production in proton-proton collisions

The Large Hadron Collider (LHC) at CERN, at which protons collide at unprecedented center of mass energies and very high instantaneous luminosity, gives unique possibilities for precise tests of the Standard Model and to search for new physics phenomena. A precise prediction of the processes at the LHC is essential and a key role plays hereby the knowledge of the parton density functions (PDFs) of the proton. In this thesis two analyses are presented. In the first analysis a new heavy charged gauge boson, a so-called W' boson, is searched for. Those new gauge bosons are predicted by some theories extending the Standard Model gauge group to solve some of its conceptual problems. Decays of the W' boson in final states with a lepton ($\ell^\pm = e^\pm, \mu^\pm$) and the corresponding \mbox{(anti-)neutrino} are considered. Data are used which were collected by the ATLAS experiment in the year 2015 at a center of mass energy of $\sqrt{s}=13$ TeV. The collected data corresponds to an integrated luminosity of $3.2$ fb$^{-1}$. The Standard Model prediction for the expected background is estimated with Monte Carlo simulations and methods based on data. The resulting spectrum of the transverse mass is tested, using statistical methods, for differences between data and the Standard Model predictions. No significant deviation from the Standard Model predictions is found and masses of a Sequential Standard Model $\wp$ boson below $4.07$ TeV are excluded with a confidence level of $95\%$.\\ In the second analysis a measurement of the double-differential cross section of the process $pp \rightarrow Z/\gamma^* + X \rightarrow \ell^+\ell^- + X$, including also a $\gamma\gamma$ induced contribution, at a center of mass energy of $\sqrt{s} = 8$ TeV is presented. The measurement is performed in an invariant mass region of $116$ GeV to $1500$ GeV as a function of invariant mass and absolute rapidity of the $\ell^+\ell^-$-pair as well as a function of invariant mass and pseudorapidity separation of the $\ell^+\ell^-$-pair. The analyzed data were recorded by the ATLAS experiment in the year 2012 and correspond to an integrated luminosity of $20.3$ fb$^{-1}$. It is expected that the measured cross sections are sensitive to the PDFs at very high values of the Bjorken-$x$ scaling variable and to the photon structure of the proton. In this thesis the measurement of the cross sections for the decay into an $e^+e^-$-pair is discussed in detail. The measured cross sections are combined with a measurement for the decay into a $\mu^+\mu^-$-pair to reduce the systematic and statistical uncertainties. The combined cross sections are compared to theory predictions and studies of the sensitivity of the measurement are carried out. It is shown that, with help of this measurement, the uncertainty of the photon PDF can be strongly reduced.