A measurement of W+jet and Z+jet cross sections in the tau decay channel, and their ratio in the ATLAS experiment

The amount of collision data delivered by the Large Hadron Collider and collected by the ATLAS detector in Spring 2011 was sufficient enough so that a variety of important measurements could be carried out. Among them are the measurements of the W+jet and the Z+jet cross sections in the tau decay channel of the W and Z boson, and the W+jet to Z+jet cross sections ratio measurement, the so called RJET measurement. The goal of these measurements is, by comparing the theoretical predictions and the measured quantities, to investigate, whether signs of physics beyond the Standard Model can be observed in the W(→ τν)+jet or the Z(→ τ τ )+jet signatures. The RJET measurement is an extra measurement which tested the possibility of canceling some systematic uncertainties which entered both W(→ τν)+jet and Z(→ τ τ )+jet cross section measurements, and thus provide a measurement with an enhanced sensitivity. This thesis provides the W(→ τν)+jet and the Z(→ τ τ )+jet observations and cross section measurements and their RJET ratio is estimated. The W(→ τν)+jet cross section is estimated to be σW+jet = 1.08 ± 0.06(stat.) ± 0.21(syst.) ± 0.03(lumi.) nb, and the Z(→ τ τ )+jet cross section is estimated to be σZ+jet = 0.130 ± 0.015(stat.) ± 0.023(syst.) ± 0.004 (lumi.) nb. The RJET ratio is estimated to be RJET = 8.3 ± 1.0(stat.) ± 1.5(syst.). The measured cross sections as well as the RJET ratio correspond within the uncertainty with the theoretical predictions. Future improvements of the analysis are discussed in the summary of the thesis.