Disentangling new physics effects on nonresonant Higgs boson pair production from gluon fusion

We study the kinematic characteristics of the loop-induced effects on nonresonant Higgs pair production via gluon fusion in a type-II two Higgs doublet model with heavy vectorlike quarks. First, we performed a comprehensive analysis of the model phenomenology, suggesting an ansatz to satisfy theoretical and experimental constraints. Contrary to the usual expectation, the loop-induced effects can highly enhance the di-Higgs rate if the Higgs sector is extended into the wrong-sign limit (κₜ=-κb=1): the total cross section can be about three times as large as the SM expectation. As anomalous Higgs trilinear self-coupling of λₕₕₕ/λₕₕₕSM=-0.5, 5.5 similarly enhances the di-Higgs rate, we made a comparative study focusing on the kinematic distributions. The threshold effects from the vectorlike quarks generate the correlated bumps in the distributions of the invariant mass of the Higgs boson pair and the transverse momentum of a Higgs boson, located at Mₕₕ2MVLQ and pTʰMVLQ. Each bump has a long tail stretching toward the high region. The anomalous λₕₕₕ cases shift the distributions into low regions. In addition, we show the kinematic difference between the loop-induced effects and an s-channel wide resonance. Therefore, the kinematic regimes with high Mₕₕ and high pTʰ would have high potential to probe the loop-induced effects on the di-Higgs process, which is shown through full HL-LHC simulations in the bb̄bb̄ and bb̄γγ final states.