  New physics beyond the Standard Model is required to give mass to the light
neutrinos. One of the simplest ideas is to introduce new heavy, gauge singlet
fermions that play the role of right-handed neutrinos in a seesaw mechanism.
They could have large Yukawa couplings to the Higgs boson, affecting the Higgs
couplings and in particular the triple Higgs coupling $\lambda_{HHH}^{}$, the
measure of which is one of the major goals of the LHC and of future colliders.
We present a study of the impact of these heavy neutrinos on $\lambda_{HHH}^{}$
at the one-loop level, first in a simplified 3+1 model with one heavy Dirac
neutrino and then in the inverse seesaw model. Taking into account all possible
experimental constraints, we find that sizeable deviations of the order of 35%
are possible, large enough to be detected at future colliders, making the
triple Higgs coupling a new, viable observable to constrain neutrino mass
models. The effects are generic and are expected in any new physics model
including TeV-scale fermions with large Yukawa couplings to the Higgs boson,
such as those using the neutrino portal.
