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

We present results from the analytic calculation of $t\overline{t}H$ 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.

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 $\mathrm{POWHEG}+\mathrm{PYTHIA}6$ (long-dashed, red), $\mathrm{POWHEG}+\mathrm{PYTHIA}8$ (short-dashed, orange), and $\mathrm{POWHEG}+\mathrm{HERWIG}$ (dot-dashed, blue) respectively, for a fixed-scale choice (see text). The lower panels show the ratios: $R=d\sigma (\mathrm{NLO})/d\sigma (\mathrm{PYTHIA}6)$ (black), $R=d\sigma (\mathrm{HERWIG})/d\sigma (\mathrm{PYTHIA}6)$ (red), and $R=d\sigma (\mathrm{HERWIG})/d\sigma (\mathrm{PYTHIA}8)$ (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 $\mathrm{POWHEG}+\mathrm{PYTHIA}6$ (long-dashed, red), $\mathrm{POWHEG}+\mathrm{PYTHIA}8$ (short-dashed, orange), and $\mathrm{POWHEG}+\mathrm{HERWIG}$ (dot-dashed, blue) respectively, for a fixed-scale choice (see text). The lower panels show the ratios: $R=d\sigma (\mathrm{NLO})/d\sigma (\mathrm{PYTHIA}6)$ (black), $R=d\sigma (\mathrm{HERWIG})/d\sigma (\mathrm{PYTHIA}6)$ (red), and $R=d\sigma (\mathrm{HERWIG})/d\sigma (\mathrm{PYTHIA}8)$ (blue). The error bars indicate the statistical uncertainties of the Monte Carlo integration.

Figure 3

The $p_T$ distributions of the $t\overline{t}$ pair (left) and of the $t\overline{t}H$ system (right) at NLO QCD with no parton shower (solid, black) and with parton shower as obtained through $\mathrm{POWHEG}+\mathrm{PYTHIA}6$ (long-dashed, red), $\mathrm{POWHEG}+\mathrm{PYTHIA}8$ (short-dashed, orange), and $\mathrm{POWHEG}+\mathrm{HERWIG}$ (dot-dashed, blue) respectively, for a fixed-scale choice (see text). The lower panels show the ratios: $R=d\sigma (\mathrm{NLO})/d\sigma (\mathrm{PYTHIA}6)$ (black), $R=d\sigma (\mathrm{HERWIG})/d\sigma (\mathrm{PYTHIA}6)$ (red), and $R=d\sigma (\mathrm{HERWIG})/d\sigma (\mathrm{PYTHIA}8)$ (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 $\mathrm{POWHEG}+\mathrm{PYTHIA}6$ 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 $\mathrm{POWHEG}+\mathrm{PYTHIA}6$ 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 6

The $p_T$ distributions of the $t\overline{t}$ pair (left) and of the $t\overline{t}H$ system (right) obtained with $\mathrm{POWHEG}+\mathrm{PYTHIA}6$ for fixed (fix) and dynamic (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 7

Normalized transverse-momentum distribution of the hardest lepton (left) and azimuthal angle separation of the two hardest oppositely charged leptons (right) obtained with $\mathrm{POWHEG}+\mathrm{PYTHIA}6$ with (blue lines) and without (red lines) spin correlations in the decays of the top quarks, for a dynamic-scale choice (see text). The lower panels show the ratios of the results without and with spin correlations.