Only experimental data will distinguish between them. All experimental attempts to look for irrelevant couplings beyond the Standard Model, such as those that give rise to proton decay, flavor changing neutral currents, Peskin–Takeuchi parameters and nonuniversal couplings have so far come up only with upper bounds. However, the observation of neutrino oscillations could be explained by adding a Majorana neutrino mass, which is an irrelevant coupling that could arise, e.g., from the seesaw mechanism.