Right-Handed Neutrinos at LHC and the Mechanism of Neutrino Mass Generation

TL;DR

This work investigates the possibility of detecting right-handed (RH) neutrinos with masses near the electroweak scale at colliders and how such a discovery would illuminate the neutrino-mass-generation mechanism. It shows that large LH–RH mixing requires cancellations among light-neutrino contributions, typically enforced by lepton-number conservation or discrete flavor symmetries like $A_4$, and it analyzes perturbations that generate realistic light-neutrino masses. The study finds that lepton-number-violating signals are generically suppressed unless fine-tuning occurs, while lepton-flavor-violating signals can be sizeable but are constrained by low-energy bounds; collider observables largely decouple from the light-mass mechanism except in tightly constrained models. Observing RH neutrinos would still be valuable, and potential correlations between neutrino masses and collider signatures may emerge in minimal scenarios, testable at future facilities such as the ILC.

Abstract

We consider the possibility to detect right-handed neutrinos, which are mostly singlets of the Standard Model gauge group, at future accelerators. Substantial mixing of these neutrinos with the active neutrinos requires a cancellation of different contributions to the light neutrino mass matrix at the level of 10^{-8}. We discuss possible symmetries behind this cancellation and argue that they always lead to conservation of total lepton number. Light neutrino masses can be generated by small perturbations violating these symmetries. In the most general case, LHC physics and the mechanism of neutrino mass generation are essentially decoupled; with additional assumptions, correlations can appear between collider observables and features of the neutrino mass matrix.

Figures (1)

• Figure 1: Feynman diagrams for lepton-number- (left) and lepton-flavour-violating processes (right) involving heavy neutrinos at the LHC.