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

Sarah Andreas, Oleg Lebedev, Saul Ramos-Sanchez, Andreas Ringwald

TL;DR

The paper analyzes how light a CP-odd Higgs A^0 can be in NMSSM-style models by aggregating constraints from meson decays, muon g−2, and reactor/beam-dump experiments. It treats A^0 couplings to fermions via a single parameter C_Aff and explores scenarios where A^0 lies below the muon threshold, including Peccei–Quinn and R-symmetry limits that yield a largely singlet pseudoscalar. The main result is that A^0 lighter than 2m_μ is excluded unless C_Aff ≲ 10^−4, pushing viable masses above ~210 MeV, with multiple independent bounds reinforcing this conclusion. The findings are framed as general constraints on light pseudoscalars and point to collider tests like h → 2A^0 → 4μ as key probes.

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.

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

TL;DR

The paper analyzes how light a CP-odd Higgs A^0 can be in NMSSM-style models by aggregating constraints from meson decays, muon g−2, and reactor/beam-dump experiments. It treats A^0 couplings to fermions via a single parameter C_Aff and explores scenarios where A^0 lies below the muon threshold, including Peccei–Quinn and R-symmetry limits that yield a largely singlet pseudoscalar. The main result is that A^0 lighter than 2m_μ is excluded unless C_Aff ≲ 10^−4, pushing viable masses above ~210 MeV, with multiple independent bounds reinforcing this conclusion. The findings are framed as general constraints on light pseudoscalars and point to collider tests like h → 2A^0 → 4μ as key probes.

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.

Paper Structure

This paper contains 17 sections, 49 equations, 4 figures.

Table of Contents

  1. Introduction
  2. NMSSM and a light CP--odd Higgs
  3. Peccei--Quinn limit
  4. R--symmetry limit
  5. Constraints on a light CP--odd Higgs
  6. Rare B--decays $B \rightarrow K + {\rm invisible}$
  7. Rare K--decays $K \rightarrow \pi + {\rm invisible}$
  8. Rare decays $B \rightarrow K ~e^+e^-$, $K \rightarrow \pi ~e^+e^-$
  9. Rare K--decays $K^+ \rightarrow \pi^+ + X$
  10. Radiative Upsilon--decays
  11. Pion decay $\pi^0 \rightarrow e^+ e^-$
  12. Muon anomalous magnetic moment
  13. Other constraints
  14. Further bounds from reactor and beam dump experiments
  15. Reactor bounds
  16. ...and 2 more sections

Figures (4)

  • Figure 1: $\tan\beta$ dependence of BR$(A^0 \rightarrow e^+ e^-)$. The lowest curve corresponds to $\tan\beta=1$, the higher one to $\tan\beta=3$, and the highest curve to $\tan\beta=10$.
  • Figure 2: Constraints from meson decays and muon $g-2$. The colored regions are excluded. These bounds include the effect of varying BR$(A^0 \rightarrow e^+ e^-)$ with $m_{A^0}$.
  • Figure 3: Constraints from nuclear power reactors.
  • Figure 4: Constraints from the beam dump experiments.