  Scalar and pseudo-scalar resonances decaying to top quarks are common
predictions in several scenarios beyond the standard model (SM) and are
extensively searched for by LHC experiments. Challenges on the experimental
side require optimising the strategy based on accurate predictions. Firstly,
QCD corrections are known to be large both for the SM QCD background and for
the pure signal scalar production. Secondly, leading order and approximate
next-to-leading order (NLO) calculations indicate that the interference between
signal and background is large and drastically changes the lineshape of the
signal, from a simple peak to a peak-dip structure. Therefore, a robust
prediction of this interference at NLO accuracy in QCD is necessary to ensure
that higher-order corrections do not alter the lineshapes. We compute the exact
NLO corrections, assuming a point-like coupling between the scalar and the
gluons and consistently embedding the calculation in an effective field theory
within an automated framework, and present results for a representative set of
beyond the SM benchmarks. The results can be further matched to parton shower
simulation, providing more realistic predictions. We find that NLO corrections
are important and lead to a significant reduction of the uncertainties. We also
discuss how our computation can be used to improve the predictions for physics
scenarios where the gluon-scalar loop is resolved and the effective approach is
less applicable.
