A theoretical study of top-mass measurements at the LHC using NLO+PS generators of increasing accuracy

Abstract

In this paper we study the theoretical uncertainties in the determination of the top-quark mass using next-toleading-order (NLO) generators interfaced to parton showers (PS) that have different levels of accuracy. Specifically we consider three generators: one that implements NLO corrections in the production dynamics, one that includes also NLO corrections in top decay in the narrow width approximation, and one that implements NLO corrections for both production and decay including finite-width and interference effects. Since our aim is to provide an assessment of the uncertainties of purely theoretical origin, we consider simplified top-mass related observables that are broadly related to those effectively used by experiments, eventually modelling experimental resolution effects with simple smearing procedures. We estimate the differences in the value of the extracted top mass that would occur due to the use of the three different NLO generators, to the variation of scales, to the choice of parton distribution functions and to the matching procedure. Furthermore, we also consider differences due to the shower and to the modelling of non-perturbative effects by interfacing our NLO generators to both Pythia8.2 and Herwig7.1, with various settings. We find very different results depending upon the adopted shower model. While with Pythia8.2 we find moderate differences between the different NLO+PS generators, with Herwig7.1 we find very large ones. Furthermore, the differences between Pythia8.2 and Herwig7.1 generators are also remarkably large.

Coherent showers in decays of colored resonances

“…As representative of the class of partitioned-dipole shower we use Pythia 8.240 [17] for later comparisons in section V, in part because there is already available an interface to recent versions of Powheg Box [10,12,13]. In addition since both Vincia and Pythia share the same modelling of non-perturbative physics such as hadronisation and underlying event (although they may differ in the default tuned values of parameters controlling these processes), this better allows us to isolate differences that are of perturbative origin.…”

“…We take theq-shower using Herwig 7.1.4 as representative of this class for later comparisons. Again, this is partially motivated by the presence of an existing interface to Powheg Box [13,14].…”

“…In [13] a comparison of the two resonance-aware matching methods and the much earlier hvq implementation for observables relevant to the top mass measurement was performed, using interfaces to Pythia 8.2 and the angular-orderedq-shower in Herwig 7.1. It was generally observed that the differences between these two showers for a given method much exceeded the differences resulting from the different choices for the matching method.…”

“…Where comparisons to Pythia 8.2 are Herwig 7.1 performed we closely follow the prescription in [13]. For matching to Vincia, the procedure we use is very similar to that used for Pythia.…”

“…Finally, we consider top quark production as a case study for an application of our formalism. This is a particularly well-motivated example, since it was recently noted in [13,14] that the existing approaches of Pythia 8.2 [6,[15][16][17] and Herwig 7.1 [18][19][20][21][22] exhibit substantial shape differences in their predictions for the differential distribution of the reconstructed invariant mass of the top, already at the level of the parton shower. This has potential implications for the minimum uncertainty present in the measurement of the top quark pole mass extracted through direct methods.…”