Explain the origin of the small dark-matter mass splitting

Explain why the Majorana mass splitting of the inelastic dark-matter candidate should be small, specifically of order 10^{-7} relative to its Dirac mass, within the minimal model containing a new massive Z' gauge boson.

Background

The paper discusses inelastic dark matter models in which the dark matter consists of two nearly degenerate Majorana states. Interactions mediated by a new massive Z' gauge boson can produce the LZ nuclear-recoil event while allowing the mass splitting and scattering cross section to vary more independently than in the Higgsino interpretation.

Although the minimal Z' model can accommodate the required small splitting technically through a dark-matter number symmetry, the paper identifies the absence of a deeper explanation for this small parameter as an unresolved issue. The proposed Randall–Sundrum warped-extra-dimensional construction is introduced to provide such an origin by generating an exponentially suppressed Majorana splitting from UV-brane physics, so the problem is framed as a motivation that the subsequent model is intended to address.