Where should we look for the deviation from quantum mechanics?

We argue that the experimental measurement of the lifetime and decay law of metastable helium is a promising way to find a deviation from quantum mechanics. We analyze the decay of the metastable state for a quartic deformation of quantum mechanics. We find that the absolute change of the mean lifetime can be significant even if the modification is small. We show that, in the Wigner--Weisskopf approximation, the deviation from the exponential law in the modified theory can occur on time scales comparable with the lifetime.