Interference and resonance effects in gluon-fusion Higgs-pair production in the Yukawa-aligned two-Higgs-doublet model

Higgs-pair production through gluon fusion is sensitive both to the Higgs self-coupling and to additional scalars exchanged in the s channel. We compute gg → hh at leading order in QCD in the CP-conserving two-Higgs-doublet model with Yukawa alignment (A2HDM), keeping the full top-quark mass dependence and every interference term among the box, the light-Higgs triangle and the heavy-scalar triangle. Previous A2HDM studies of heavy scalars decaying to Higgs pairs used the narrow-width approximation. We contrast two benchmark points. When the heavy scalar H decouples from the top-quark loop, the modified couplings of h weaken the destructive box–triangle interference, and the 13 TeV cross section rises to 1.27 times the Standard Model value without any resonance. For m_H = 300 GeV, below the ttbar threshold, the cross section reaches 3.69 times the Standard Model value; interference with the continuum changes the integrated resonant contribution by only about 4%, but turns the parton-level m_hh distribution into a peak–dip structure. The two mechanisms are therefore separated by the shapes of the m_hh and p_T spectra rather than by the rate. Both benchmarks are illustrative and satisfy a simplified set of theoretical and experimental constraints. Code: https://doi.org/10.5281/zenodo.24029211