Constraints on the Yukawa couplings of a light scalar mixing with the Higgs from decays to pseudoscalars*

A hadronic state’s form factor encodes the complicated details of the state with an external current. In the case of states involving pseudoscalars, in particular pions and kaons, its structure is reliably given at low energies by chiral perturbation theory and at high energies by perturbative QCD. In the gap between these regimes it is common to use experimental inputs in dispersive approaches, based on analyticity and unitarity. In the quest for new particles whose mass may lie in this intermediate range, it is necessary to determine the nature of processes at this scale with precision. The abundance of models involving new scalars motivates continuing investigation of the pion and kaon scalar form factors, which are included in the most common final states. We present a dispersive, coupled channel analysis of these form factors and discuss the constraints thus imposed on the Yukawa couplings of a new scalar. We carefully examine the uncertainty propagation from experimental inputs as well as from theoretical uncertainties arising from the low-energy matching to chiral perturbation theory.