Abstract
In this paper we study the theoretical uncertainties in the determination of the top-quark mass using next-to-leading-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.
Highlights
The abundant production of top pairs at the Large Hadron Collider (LHC) provides an opportunity for detailed studies of top-quark properties, for tests of the Standard Model (SM) in the top sector, and for measurements of fundamental parameters such as the top-quark mass
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
If we switch off the matrix-element corrections (MEC) in Pythia8.2, we find that the peak position at the particle level in the hvq case is displaced by 61 MeV, while, if smearing effects are included, the shift is of 916 MeV, a rather large value, that can be disregarded as being due to the poor accuracy of the collinear approximation in b radiation when MEC corrections are off. – The jet-energy peak seems to be more sensitive to the modeling of radiation from the b quark
Summary
The abundant production of top pairs at the Large Hadron Collider (LHC) provides an opportunity for detailed studies of top-quark properties, for tests of the Standard Model (SM) in the top sector, and for measurements of fundamental parameters such as the top-quark mass. The present value of the indirect top-mass determination from electroweak precision data (176.7 ± 2.1 GeV, see [1]) is in slight tension, at the 1.6 σ level, with the direct measurements. The latest combination of the Tevatron and the LHC results [3] yields 173.34 ± 0.76 GeV, but more recent results favour even smaller values, close to 172.5 GeV, see [4,5,6,7]. It has been shown that in the Standard Model as is (i.e. assuming no new physics effects up to the Planck scale), the vacuum is stable if the top mass, mt , is below 171 GeV (i.e. very close to its present value), metastable up to 176 GeV, and unstable above this value [10,11,12,13]. The fact that the Higgs boson quartic coupling almost vanishes at the Planck scale may have some deep meaning that we are as yet unable to unveil
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