In view of the searches at the LHC for scalar particle resonances in addition
to the 125 GeV Higgs boson, we present the cross section for a CP-even scalar
produced via gluon fusion at N3LO in perturbative QCD assuming that it couples
directly to gluons in an effective theory approach. We refine our prediction by
taking into account the possibility that the scalar couples to the top-quark
and computing the corresponding contributions through NLO in perturbative QCD.
We assess the theoretical uncertainties of the cross section due to missing
higher-order QCD effects and we provide the necessary information for obtaining
the cross section value and uncertainty from our results in specific scenarios
beyond the Standard Model. We also give detailed results for the case of a 750
GeV scalar, which will be the subject of intense experimental studies.