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Description
Abstract
With the increasing experimental precision achieved in top quark physics, modelling uncertainties have become a dominant contribution to the total uncertainty of many measurements. Therefore it is necessary to incorporate recent theoretical advances. One major improvements over the Powheg+Pythia8 $t\bar{t}$ sample, often used as nominal Monte-Carlo sample in data analyses, is available through the $bb4\ell$ Powheg process, which incorporates the correct treatment of off-shell top-quarks and $t\bar{t}/tW$ interference. Uncertainties related to these effects can be dominant in the $t\bar{t}+tW$ prediction, especially for extreme but interesting phase spaces as e.g. the high $m_{\ell b}$ region, which is often used as a proxy for the invariant mass of the top-quark. To benefit from these improvements in physics analyses, a complete recipe of the systematic uncertainties needs to be present for the $bb4\ell$ sample. The $bb4\ell$ Powheg process will be described and studies towards the systematic uncertainty prescription for the ATLAS $bb4\ell$ sample at $\sqrt{s}=13$ TeV will be presented, in particular the comparison to the $t\bar{t}+tW$ DR Powheg+Pythia8 samples as well as the study of the variation of Powheg and Pythia8 parton-shower parameters related to the matching of the $bb4\ell$ hard-process events to the parton shower.
Speakers
Tomas Jezo is a member of Academic Council at University of Münster, a POWHEG author and a member of the nCTEQ collaboration.
Katharina Voß is a Ph.D. student in the ATLAS Collaboration at the University of Siegen working in top-quark physics and in the physics modelling group in ATLAS.
