Constraints on a very light CP-odd Higgs of the NMSSM and other axion-like particles

TL;DR

This paper investigates the constraints on a very light CP-odd Higgs particle in the NMSSM and similar axion-like particles, focusing on how light it can be while remaining consistent with experimental bounds.

Contribution

It provides a detailed analysis of experimental bounds on a light CP-odd Higgs below the muon threshold, establishing lower mass limits and coupling constraints.

Findings

CP-odd Higgs must be heavier than 210 MeV or have couplings much weaker than the Standard Model Higgs.

The analysis applies broadly to pseudoscalar couplings to matter.

Constraints are derived from meson decays, g-2, beam dump, and reactor experiments.

Abstract

In the NMSSM, a light CP-odd Higgs arises due to spontaneous breaking of approximate symmetries such as Peccei-Quinn or R-symmetry and is motivated by string theory. The case when it is heavier than two muons is well studied and constrained. We analyze various meson decay, g-2, beam dump and reactor bounds on the CP-odd Higgs with mass below the muon threshold, in particular, addressing the question how light a CP-odd Higgs can be. We find that it has to be heavier than 210 MeV or have couplings to fermions 4 orders of magnitude below those of the Standard Model Higgs. Our analysis applies more generally to couplings of a light pseudoscalar to matter.