Neutrino Masses and the LHC: Testing Type II Seesaw

TL;DR

This paper proposes a systematic method to test the Type-II seesaw mechanism for neutrino mass generation at the LHC, focusing on Higgs triplet decays and their dependence on neutrino mass hierarchies and phases.

Contribution

It introduces a comprehensive approach to distinguish neutrino mass hierarchies and phases through Higgs triplet decay channels at the LHC, considering different vacuum expectation value scenarios.

Findings

Lepton number violation signals can differentiate neutrino hierarchies.

Certain Higgs decay channels are robust indicators independent of Majorana phases.

Branching ratios can measure the Higgs triplet vacuum expectation value.

Abstract

We demonstrate how to systematically test a well-motivated mechanism for neutrino mass generation (Type-II seesaw) at the LHC, in which a Higgs triplet is introduced. In the optimistic scenarios with a small Higgs triplet vacuum expectation value vd < 10^{-4} GeV, one can look for clean signals of lepton number violation in the decays of doubly charged and singly charged Higgs bosons to distinguish the Normal Hierarchy (NH), the Inverted Hierarchy (IH) and the Quasi-Degenerate (QD) spectrum for the light neutrino masses. The observation of either H+ --> tau+ nubar or H+ --> e+ nubar will be particularly robust for the spectrum test since they are independent of the unknown Majorana phases. The H++ decays moderately depend on a Majorana phase Phi2 in the NH, but sensitively depend on Phi1 in the IH. In a less favorable scenario vd > 2 10^{-4} GeV, when the leptonic channels are suppressed, one needs to observe the decays H+ --> W+ H_1 and H+ --> t bbar to confirm the triplet-doublet mixing which in turn implies the existence of the same gauge-invariant interaction between the lepton doublet and the Higgs triplet responsible for the neutrino mass generation. In the most optimistic situation, vd approx 10^{-4} GeV, both channels of the lepton pairs and gauge boson pairs may be available simultaneously. The determination of their relative branching fractions would give a measurement for the value of vd.