The quartic and trilinear Higgs field couplings to an additional real scalar are renormalizable, gauge and Lorentz invariant. Thus, on general grounds, one expects such couplings between the Higgs and an inflaton in quantum field theory. In particular, the (often omitted) trilinear coupling is motivated by the need for reheating the Universe after inflation, whereby the inflaton decays into the Standard Model (SM) particles. Such a coupling necessarily leads to the Higgs-inflaton mixing, which could stabilize the electroweak vacuum by increasing the Higgs self-coupling. We find that the inflationary constraints on the trilinear coupling are weak such that the Higgs-inflaton mixing up to order one is allowed, making it accessible to colliders. This entails an exciting possibility of a direct inflaton search at the LHC.

12 pages, 2 figures