Can One have Significant Deviations from Leptonic $3\times 3$ Unitarity in the Framework of Type I Seesaw Mechanism?

TL;DR

This paper explores how significant deviations from the standard leptonic mixing matrix unitarity can occur within the Type I Seesaw mechanism, especially when some heavy neutrinos are at the TeV scale, potentially detectable in future experiments.

Contribution

It demonstrates that notable unitarity deviations are possible in the Type I Seesaw framework with TeV-scale heavy neutrinos, challenging conventional assumptions.

Findings

Deviations can be large enough to be experimentally detectable.

At least one heavy neutrino must be at the TeV scale.

Deviations can be consistent with current experimental bounds.

Abstract

We address the question of deviations from $3\times 3$ unitarity of the leptonic mixing matrix showing that, contrary to conventional wisdom, one may have significant deviations from unitarity in the framework of type I seesaw mechanism. In order for this scenario to be feasible, at least one of the heavy neutrinos must have a mass at the TeV scale, while the other two may have much larger masses. We present specific examples where deviations from $3\times 3$ unitarity are sufficiently small to conform to all the present stringent experimental bounds but are sufficiently large to have the potential for being detectable at the next round of experiments.