Top-mass observables: all-orders behaviour, renormalons and NLO + Parton Shower effects

Abstract

In this thesis we focus on the theoretical subtleties of the top-quark mass ($m_t$) determination, issue which persists in being highly controversial. In the first part of the thesis, we investigate the presence of linear renormalons in observables that can be employed to determine the top mass. We considered a simplified toy model to describe $W^{\ast }\to t\bar{b}\to Wb\bar{b}$. The computation is carried out in the limit of a large number of flavours ($n_f$), using a new method that allows to easily evaluate any infrared safe observable at order ${\alpha }_s({\alpha }_s{n}_f{)}^n$ for any $n$. The second part of the thesis is devoted to the comparison of several NLO generators for top-pair production, implemented in the POWHEG BOX framework, that differ by the level of accuracy employed to describe the top decay. The impact of the shower Monte Carlo programs, used to complete the NLO events generated by POWHEG BOX, is also studied. In particular, we discuss the two most widely used shower Monte Carlo programs, i.e. Pythia 8.2 and Herwig 7.1, and we present a method to interface them with processes that contain decayed emitting resonances.

arXiv commentPhD thesis: 152 pages, 50 figures, 34 tables

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