Anomaly-free constraints in neutrino seesaw models

TL;DR

This paper explores how to implement anomaly-free U(1)_X gauge symmetries in neutrino mass models, revealing constraints on combining different seesaw types and proposing ways to distinguish models via collider signatures.

Contribution

It demonstrates the incompatibility of type-I and type-III seesaws with a generic anomaly-free U(1)_X, and identifies constrained charge assignments for combined models that can be tested at the LHC.

Findings

Type-I and type-III seesaws cannot be simultaneously anomaly-free unless U(1)_X is hypercharge.

Nontrivial anomaly-free charge assignments exist for combined seesaw models, but are tightly constrained.

Discovery and analysis of a new neutral gauge boson at LHC can discriminate among different seesaw models.

Abstract

The implementation of seesaw mechanisms to give mass to neutrinos in the presence of an anomaly-free U(1)_X gauge symmetry is discussed in the context of minimal extensions of the standard model. It is shown that type-I and type-III seesaw mechanisms cannot be simultaneously implemented with an anomaly-free local U(1)_X, unless the symmetry is a replica of the well-known hypercharge. For combined type-I/II or type-III/II seesaw models it is always possible to find nontrivial anomaly-free charge assignments, which are however tightly constrained, if the new neutral gauge boson is kinematically accessible at LHC. The discovery of the latter and the measurement of its decays into third-generation quarks, as well as its mixing with the standard Z boson, would allow one to discriminate among different seesaw realizations.