Importance of Loop Effects in Probing Lepton Number Violation

Abstract

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 ($0\hat{I}½{\hat{I}}^2{\hat{I}}^2$). 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 $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ contributions induced by dimension-7 and loop-level dimension-5 SMEFT operators.

Minor changes and typos corrected, 6+2 pages, 3+1 figures