  Neutrinoless double beta decay is the textbook example of lepton number
violation, often claimed to be a probe of neutrino Majorana mass. However, it
could be triggered by new physics; after all, neutrino Majorana mass requires
physics beyond the Standard Model. If at least one electron were right-handed,
it would automatically signify new physics rather than neutrino mass. In case
both electrons were left-handed, the situation would become rather complicated,
and additional effort would be needed to untangle the source for this process.
We offer a comprehensive study of this issue from both the effective operator
approach and the possible UV completions, including the Pati-Salam quark-lepton
unification. While neutrino exchange is natural and physically preferred, our
findings show that new physics can still be responsible for the neutrinoless
double beta decay. In particular, the Pati-Salam theory can do the job,
consistently with all the phenomenological and unification constraints, as long
as the unification scale lies above 10^12 GeV, albeit at the price of
fine-tuning of some scalar masses.
