Searching for Nambu-Goldstone Bosons at the LHC

TL;DR

This paper explores the phenomenology of a minimal Standard Model extension with a Nambu-Goldstone boson, focusing on its implications for invisible Higgs decays at the LHC and proposing search strategies for such scenarios.

Contribution

It introduces a minimal extension involving a Nambu-Goldstone boson, analyzes the Higgs decay phenomenology, and develops strategies for detecting invisible decays at the LHC.

Findings

Invisible Higgs decays could dominate in the natural parameter region.

Search strategies can be improved by analyzing specific kinematic distributions.

The model's decay rates are approximated for non-Abelian symmetry variants.

Abstract

Phenomenological implications of a minimal extension to the Standard Model are considered, in which a Nambu-Goldstone boson emerges from the spontaneous breaking of a global U(1) symmetry. This is felt only by a scalar field which is a singlet under all Standard Model symmetries, and possibly by neutrinos. Mixing between the Standard Model Higgs boson field and the new singlet field may lead to predominantly invisible Higgs boson decays. The "natural" region in the Higgs boson mass spectrum is determined, where this minimally extended Standard Model is a valid theory up to a high scale related with the smallness of neutrino masses. Surprisingly, this region may coincide with low visibility of all Higgs bosons at the LHC. Monte-Carlo simulation studies of this "nightmare" situation are performed and strategies to search for such Higgs boson to invisible (Nambu-Goldstone boson) decays are discussed. It is possible to improve the signal-to-background ratio by looking at the distribution of either the total transverse momentum of the leptons and the missing transverse momentum, or by looking at the distribution of the azimuthal angle between the missing transverse momentum and the momentum of the lepton pair for the Z- and Higgs-boson associated production. We also study variations of the model with non-Abelian symmetries and present approximate formulae for Higgs boson decay rates. Searching for Higgs bosons in such a scenario at the LHC would most likely be solely based on Higgs to "invisible" decays.