Light Axion within the Next-to-Minimal Supersymmetric Standard Model

TL;DR

The paper investigates the NMSSM Higgs sector to identify a parameter regime with a light CP-odd axion and analyzes how the SM-like Higgs can decay into axion pairs, potentially altering Higgs search strategies. It derives analytic expressions for the CP-even/CP-odd mass matrices and the h^0 A1^0 A1^0 coupling in the decoupling limit (s ≫ v) and in the large tanβ regime, complemented by numerical scans subject to collider and perturbativity constraints. The results show that a light axion is viable for GeV-scale mλ and mκ with s ≫ v, and that the h^0 → A1^0 A1^0 decay can have a substantial branching fraction (c up to ~0.05, and even larger in some regions), thereby reconfiguring Higgs phenomenology. These findings supply practical, analytic tools for interpreting Higgs signals in NMSSM scenarios and motivate broader phenomenological exploration of light CP-odd scalars in extended Higgs sectors.

Abstract

We analyze the Higgs sector in the Next-to-Minimal Supersymmetric Standard Model, emphasizing the possibility of a light CP-odd scalar (axion) in the spectrum. We compute the coupling of the Standard-Model-like Higgs boson to a pair of axions, and show that it can be large enough to modify the Higgs branching fractions, with a significant impact on the Higgs searches. We delineate the range of parameters relevant for this scenario, and also derive analytic expressions for the scalar masses and couplings in two special cases - a decoupling limit where all scalars other than the axion are heavier than the Standard-Model-like Higgs boson, and the large tan beta limit.