Measuring the Ratio of σ(ttH)/σ(ttZ) in the ttH multilepton channel with the ATLAS detector

At the Large Hadron Collider (LHC) at CERN protons are collided with a center of mass energy (CME) up to 13 TeV. These energies allow the study of the Higgs boson, the discovery of which completed the Standard Model (SM) in 2012. The Higgs boson is a neutral, scalar particle with a mass of 125 GeV . It is predicted by the Higgs field, which gives mass to the elementary particles in the SM by coupling to them. The coupling to fermions, known as Yukawa coupling, is proportional to the particle masses. Therefore the Higgs boson couples stronger to the heavy particles in the SM. The coupling to the heaviest particle in the SM, the top quark, was for the first time observed this June at the ATLAS and CMS experiments at the LHC. A direct measurement of the top quark Yukawa coupling can be achieved by measuring the cross section of the associated production of a top quark antiquark pair and a Higgs boson, denoted as ttH. The coupling to a top quark pair could be sensitive to new physics beyond the SM, due to the predicted coupling strength in the order of unity. If there is new physics, a deviation from the observed coupling strength and the SM prediction is expected. Therefore a very precise and accurate measurement of the ttH rate is important in order to be sensitive to possible deviations from the SM. The ttH process is a rather rare production process (only ∼ 1% of Higgs bosons are produced through this production). Consequently, a lot of effort is put into the ttH analyses as a part of the outstanding research program made by the ATLAS collaborators. This report describes my work during my time at CERN as a Summer Student in the ATLAS ttH multilepton group. After I left CERN I continued working on the project. A short summary of the additional work that was done is given in the Appendix.