Maximal value for trilinear Higgs coupling in a 3-3-1 EFT

Abstract: Recent efforts, both theoretical and experimental, have increasingly focused on the scalar potential of the Standard Model, with a highlight on the trilinear Higgs coupling. This parameter has long been recognized for its potential to test Beyond-Standard-Model (BSM) theories and its significance in understanding early cosmological dynamics. In order to broadly map BSM scenarios, a powerful tool is to devise its effective field theory (EFT) version for low-energies. In this work, we obtain a consistent EFT for a class of models based on the gauge group $SU(3)_c\times SU(3)_L\times U(1)_Y$. After properly matching the UV-complete theory at tree-level, we show that the EFT is a Two-Higgs-Doublet Model (2HDM), where some of the quartic couplings are naturally small. By imposing bounds from electroweak precision observables, collider, flavor, as well as theoretical considerations we obtain that the maximum value of the trilinear Higgs coupling is more than four times larger than the SM prediction, potentially testable at the LHC Hi-Lumi upgrade and other future colliders. Moreover, we find that such large values are only attainable if one considers an out-of-alignment scenario, even if the deviation is very small, which is not typically considered in the literature.