Deciphering the Seesaw Nature of Neutrino Mass from Unitarity Violation

TL;DR

This paper explores how different neutrino mass generation mechanisms, especially the inverse and linear seesaw, lead to potentially observable unitarity violations in the neutrino mixing matrix, introducing a new lopsided seesaw variation.

Contribution

It introduces a new lopsided seesaw mechanism and analyzes its implications for unitarity violation in neutrino mixing.

Findings

Unitarity violation is negligible in canonical seesaw.

Inverse and linear seesaws can produce significant unitarity violation.

A concrete model based on U(1)_N is discussed.

Abstract

If neutrino masses are obtained via the canonical seesaw mechanism, based on an underlying 2X2 mass matrix, unitarity violation of the neutrino mixing matrix is unavoidable, but its effect is extremely small. On the other hand, in the inverse (and linear) seesaw mechanisms, based on an underlying 3X3 mass matrix, it can be significant and possibly observable. This 3X3 matrix is examined in more detail, and a NEW variation (the lopsided seesaw) is proposed which has features of both mechanisms. A concrete example based on U(1)_N is discussed.