Signatures from an extra-dimensional seesaw model

TL;DR

This paper explores a novel extra-dimensional seesaw model where right-handed neutrinos propagate in extra dimensions, leading to small neutrino masses and distinctive collider signatures, with current bounds constraining its observable effects.

Contribution

It introduces a new extra-dimensional seesaw mechanism with a specific bulk Majorana mass structure and analyzes its phenomenological implications and experimental constraints.

Findings

Heavy Majorana neutrinos are the primary source of lepton number violation.

Mixing between Kaluza-Klein modes and light neutrinos can be significant.

Current bounds on leptonic mixing non-unitarity exclude observable signals at the LHC.

Abstract

We study the generation of small neutrino masses in an extra-dimensional model, where right-handed neutrinos are allowed to propagate in the extra dimension, while the Standard Model particles are confined to a brane. Motivated by the fact that extra-dimensional models are non-renormalizable, we truncate the Kaluza-Klein towers at a maximal extra-dimensional momentum. The structure of the bulk Majorana mass term, motivated by the Sherk-Schwarz mechanism, implies that the right-handed Kaluza-Klein neutrinos pair to form Dirac neutrinos, except for a number of unpaired Majorana neutrinos at the top of each tower. These heavy Majorana neutrinos are the only sources of lepton number breaking in the model, and similarly to the type-I seesaw mechanism, they naturally generate small masses for the left-handed neutrinos. The lower Kaluza-Klein modes mix with the light neutrinos, and the mixing effects are not suppressed with respect to the light-neutrino masses. Compared to conventional fermionic seesaw models, such mixing can be more significant. We study the signals of this model at the Large Hadron Collider, and find that the current low-energy bounds on the non-unitarity of the leptonic mixing matrix are strong enough to exclude an observation.