Large gauge invariant non-standard neutrino interactions

TL;DR

This paper analyzes gauge invariance constraints on non-standard neutrino interactions, classifies effective operators, and discusses conditions for model building with new mediators, focusing on dimension six and eight operators.

Contribution

It provides a systematic decomposition of non-standard neutrino interaction operators based on tree-level mediators and explores the constraints for viable model building.

Findings

Only diagonal tau-neutrino interactions can arise from dimension six operators.

Dimension eight operators can produce large interactions in all flavors, requiring at least two mediators.

Cancellation conditions are necessary for the couplings to satisfy gauge invariance constraints.

Abstract

Theories beyond the Standard Model must necessarily respect its gauge symmetry. This implies strict constraints on the possible models of non-standard neutrino interactions, which we analyze. The focus is set on the effective low-energy dimension six and eight operators involving four leptons, decomposing them according to all possible tree-level mediators, as a guide for model building. The new couplings are required to have sizeable strength, while processes involving four charged leptons are required to be suppressed. For non-standard interactions in matter, only diagonal tau-neutrino interactions can escape these requirements and can be allowed to result from dimension six operators. Large non-standard neutrino interactions from dimension eight operators alone are phenomenologically allowed in all flavour channels and shown to require at least two new mediator particles. The new couplings must obey general cancellation conditions both at the dimension six and eight levels, which result from expressing the operators obtained from the mediator analysis in terms of a complete basis of operators. We illustrate with one example how to apply this information to model building.