Importance of Loop Effects in Probing Lepton Number Violation

The discovery of the lepton number violation would be a smoking gun signal for physics beyond the Standard Model, and its most sensitive probe is the search for neutrinoless double beta decay ($0\nu \beta \beta$). Working in the framework of the Standard Model Effective Field Theory (SMEFT), we show that one-loop effects can remarkably improve the tree-level bounds on the new-physics scales for several dimension-7 operators. Using ultraviolet model examples, we then showcase the competition among $0\nu \beta \beta$ contributions induced by dimension-7 and loop-level dimension-5 SMEFT operators.