PARTON SHOWER UNCERTAINTIES ON THE TOP MASS
Abstract: The non-Abelian QCD nature allows for pp collisions to contain gluon emissions off quarks as well as gluon branchings into further gluons — a structure that can be described by the parton shower model. In this project we present the results of the impact the shower has on the reconstruction of the W-boson and top quark masses using Pythia, the Lund Monte Carlo event generator. As a warm-up, the importance of showers and of multiparton interactions, as well as the value of the strong coupling constant, is investigated. We further study the results from different parton shower implementations by comparing the data of the default Pythia shower with the one from three other algorithms — the initial-state dipole recoil in Pythia, the dipole-like Dire and the antenna-based Vincia showers. Comparisons on reconstructed top masses are presented both for a common default αS value and for one roughly tuned to give the same jet profile for all models. Unexpectedly large differences have been observed, amounting to ±0.5GeV in some cases. More detailed research could either reveal the origin of these discrepancies, or conclude that there is still a long way to go until the value of the top mass can be safely extracted from data.
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