Higgs boson production in association with top quarks in the POWHEG BOX

TL;DR

The paper addresses precise modelling of $pp\to t\bar{t}H$ production at the LHC by providing a public NLO-QCD calculation matched to parton showers within the POWHEG BOX, including top-quark decays with spin correlations. It assesses theoretical uncertainties from renormalization/factorization scales and shower algorithms by comparing PYTHIA6, PYTHIA8 and HERWIG at $\sqrt{s}=8$ TeV, and studies how spin correlations affect leptonic observables. The implementation relies on established NLO amplitudes and a production–decay correlation strategy, enabling a consistent treatment of production and decay stages. Overall, the work delivers a publicly available, validated tool for experimental analyses of the $t\bar{t}H$ channel and informs choices of scale and shower in predictions.

Abstract

We present results from the analytic calculation of top+antitop+Higgs hadronic production at Next-to-Leading Order in QCD interfaced with parton-shower Monte Carlo event generators in the POWHEG BOX framework. We consider kinematic distributions of the top quark and Higgs boson at the 8 TeV Large Hadron Collider and study the theoretical uncertainties due to specific choices of renormalization/factorization scales and parton-showering algorithms, namely PYTHIA and HERWIG. The importance of spin-correlations in the production and decay stages of a top/antitop quark is discussed on the example of kinematic distributions of leptons originating from the top/antitop decays. The corresponding code is now part of the public release of the POWHEG BOX.

Figures (7)

• Figure 1: The $p_T$ (left) and $\eta$ (right) distributions of the Higgs boson at NLO-QCD with no parton shower (solid, black), and with parton shower as obtained through POWHEG+ PYTHIA6 (long-dashed, red), POWHEG+ PYTHIA8 (short-dashed, orange), and POWHEG+ HERWIG (dot-dashed, blue) respectively, for a fixed-scale choice (see text). The lower panels show the ratios: $R=d\sigma(\mathrm{NLO})/d\sigma({\tt PYTHIA6}{})$ (black), $R=d\sigma({\tt HERWIG}{})/d\sigma({\tt PYTHIA6}{})$ (red), and $R=d\sigma({\tt HERWIG}{})/d\sigma({\tt PYTHIA8}{})$ (blue). The error bars indicate the statistical uncertainties of the Monte-Carlo integration.
• Figure 2: The $p_T$ (left) and $\eta$ (right) distributions of the top quark at NLO-QCD with no parton shower (solid, black), and with parton shower as obtained through POWHEG+ PYTHIA6 (long-dashed, red), POWHEG+ PYTHIA8 (short-dashed, orange), and POWHEG+ HERWIG (dot-dashed, blue) respectively, for a fixed-scale choice (see text). The lower panels show the ratios: $R=d\sigma(\mathrm{NLO})/d\sigma({\tt PYTHIA6}{})$ (black), $R=d\sigma({\tt HERWIG}{})/d\sigma({\tt PYTHIA6}{})$ (red), and $R=d\sigma({\tt HERWIG}{})/d\sigma({\tt PYTHIA8}{})$ (blue). The error bars indicate the statistical uncertainties of the Monte-Carlo integration.
• Figure 3: The $p_T$ distributions of the $t\bar{t}$ pair (left) and of the $t\bar{t} H$ system (right) at NLO-QCD with no parton shower (solid, black), and with parton shower as obtained through POWHEG+ PYTHIA6 (long-dashed, red), POWHEG+ PYTHIA8 (short-dashed, orange), and POWHEG+ HERWIG (dot-dashed, blue) respectively, for a fixed-scale choice (see text). The lower panels show the ratios: $R=d\sigma(\mathrm{NLO})/d\sigma({\tt PYTHIA6}{})$ (black), $R=d\sigma({\tt HERWIG}{})/d\sigma({\tt PYTHIA6}{})$ (red), and $R=d\sigma({\tt HERWIG}{})/d\sigma({\tt PYTHIA8}{})$ (blue). The error bars indicate the statistical uncertainties of the Monte-Carlo integration.
• Figure 4: The $p_T$ (left) and $\eta$ (right) distributions of the Higgs boson obtained with POWHEG+ PYTHIA6 for fixed ($fix$) and dynamical ($dyn$) renormalization/factorization scales. The lower panels show the respective ratios $R=d\sigma(\xi\mu_0)/d\sigma(\mu_0)$ for $\xi=(0.5;2)$.
• Figure 5: The $p_T$ (left) and $\eta$ (right) distributions of the top quark obtained with POWHEG+ PYTHIA6 for fixed ($fix$) and dynamical ($dyn$) renormalization/factorization scales. The lower panels show the respective ratios $R=d\sigma(\xi \mu_0)/d\sigma(\mu_0)$ for $\xi=(0.5;2)$.