Estimate of $\mathrm{\Gamma }(H\to gg)$ at $O({\alpha }_s^7)$ from Padé approximants with constraints from large-$n_f$

In this work we estimate the first unknown QCD correction to the decay of the Higgs boson into gluons, $H\to gg$, in the heavy-top limit using Padé approximants built to the perturbative series and to its Borel transform. In the process, we also obtain estimates for the first unknown coefficients of the QCD $\beta$ function, of the QCD decoupling relation, and of the gluonium correlator. We also provide an updated prediction for the first unknown coefficient of $\mathrm{\Gamma }(H\to b\bar{b})$. We determine the polynomial dependence of the perturbative coefficients on the number of quark flavors, $n_f$. We point out that the large-$n_f$ result for $H\to gg$ can be obtained from results for the gluonium correlator already available in the literature. The coefficient of the highest power of $n_f$ is then imposed as a constraint and is also used as a new test to discard unreliable approximants. The method is validated through the postdiction of the last known coefficient of the series. Our model-independent estimate for the yet unknown coefficient of order ${\alpha }_s^7$ of $\mathrm{\Gamma }(H\to gg)$ is $-312\pm 15$ (in the $\overline{\mathrm{M}\mathrm{S}}$ scheme with $\mu =m_H$, where $m_H$ is the Higgs mass). With our estimate the residual renormalization-scale dependence of $\mathrm{\Gamma }(H\to gg)$ is reduced by a factor of almost 1.5.