LHC Run I Bounds on Minimal Lepton Flavour Violation in Type-III See-saw: A Case Study

TL;DR

This paper analyzes LHC Run I data to set bounds on minimal lepton flavor violation in Type-III see-saw models, excluding triplet fermions lighter than 300 GeV and up to 480 GeV under certain neutrino mass assumptions.

Contribution

It provides the first comprehensive LHC-based constraints on minimal lepton flavor violation in Type-III see-saw models using specific final state signatures.

Findings

Triplet fermions lighter than 300 GeV are ruled out at 95% CL.

Triplet masses up to 480 GeV are excluded for normal neutrino mass ordering.

Lepton charge and flavor information are crucial for setting these bounds.

Abstract

We study the bounds on minimal lepton flavour violation in the context of Type-III see-saw imposed by LHC Run I search for events which contain two charged leptons (either electron or muons of equal or opposite sign), two jets from a hadronically decaying $W$ boson and large missing transverse momentum. In this scenario the flavour structure of the couplings of the triplet fermions to the Standard Model leptons can be reconstructed from the neutrino mass matrix and lepton number violation is very suppressed. We find that using the information on charge and flavour of the leptons in the above final state it is possible to unambiguously rule out this scenario with triplet masses lighter than 300 GeV at 95% CL. The same analysis allows to exclude triplet masses masses up to 480 GeV at 95% CL for normal ordering of neutrino masses and specific values of a Majorana CP phase currently undetermined by neutrino physics.