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

## TL;DR

This thesis explores the theoretical aspects of top-quark mass measurements, analyzing renormalon effects in observables and comparing NLO generators with different decay accuracy levels, including the impact of parton shower models.

## Contribution

It introduces a new method to evaluate infrared safe observables at all orders in a large flavor number limit and compares various NLO generators and shower programs for top-pair production.

## Key findings

- Linear renormalons are present in top-mass observables.
- Different NLO generators show varying effects on top decay modeling.
- Parton shower programs significantly influence top mass measurements.

## 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 $W^{*} \rightarrow t \bar{b} \to Wb \bar{b}$. 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.