Radiative decays of charged leptons as constraints of unitarity polygons for active-sterile neutrino mixing and CP violation

We calculate the rates of radiative $$\beta ^- \rightarrow \alpha ^- + \gamma $$ β - → α - + γ decays for $$(\alpha , \beta ) = (e, \mu )$$ ( α , β ) = ( e , μ ) , $$(e, \tau )$$ ( e , τ ) and $$(\mu , \tau )$$ ( μ , τ ) by taking the unitary gauge in the $$(3+n)$$ ( 3 + n ) active-sterile neutrino mixing scheme, and make it clear that constraints on the unitarity of the $$3\times 3$$ 3 × 3 Pontecorvo–Maki–Nakagawa–Sakata (PMNS) matrix U extracted from $$\beta ^- \rightarrow \alpha ^- + \gamma $$ β - → α - + γ decays in the minimal unitarity violation scheme differ from those obtained in the canonical seesaw mechanism with n heavy Majorana neutrinos by a factor 5/3. In such a natural seesaw case we show that the rates of $$\beta ^- \rightarrow \alpha ^- + \gamma $$ β - → α - + γ can be used to cleanly and strongly constrain the effective apex of a unitarity polygon, and compare its geometry with the geometry of its three sub-triangles formed by two vectors $$U^{}_{\alpha i} U^*_{\beta i}$$ U α i U β i ∗ and $$U^{}_{\alpha j} U^*_{\beta j}$$ U α j U β j ∗ (for $$i \ne j$$ i ≠ j ) in the complex plane. We find that the areas of such sub-triangles can be described in terms of the Jarlskog-like invariants of CP violation $${{\mathcal {J}}}^{ij}_{\alpha \beta }$$ J α β ij , and their small differences signify slight unitarity violation of the PMNS matrix U.