Geometry of the effective Majorana neutrino mass in the 0νββ decay

The neutrinoless double-beta (0νββ) decay is a unique process used to identify the Majorana nature of massive neutrinos, and its rate depends on the size of the effective Majorana neutrino mass 〈m〉_ee. We put forward a novel coupling-rod' diagram to describe 〈m〉_ee in the complex plane, by which the effects of the neutrino mass ordering and CP-violating phases on 〈m〉_ee are intuitively understood. We show that this geometric language allows us to easily obtain the maximum and minimum of |〈m〉_ee|. It remains usable even if there is a kind of new physics contributing to 〈m〉_ee, and it can also be extended to describe the effective Majorana masses 〈m〉_eμ, 〈m〉_eτ, 〈m〉_μμ, 〈m〉_μτ and 〈m〉_ττ which may appear in some other lepton-number violating processes