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

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.

Lattice QCD estimate of the η c (2S) → J/ψγ decay rate

“…The above form appears to be adequate to describe our data, and the result of that extrapolation is shown in figure 2. We obtain 14) in very good agreement with the experimentally established R exp 2 = 1.220(1) [37]. The parameter X R measures the shift of the continuum value with respect to the one obtained at the lattice with a = 0.085(3) fm, which appears to be in the range of 3 ÷ 5%.…”

“…Another important motivation for a more dedicated experimental study of this decay mode lies in the fact that this decay rate could be sensitive to the CP-odd light Higgs boson if its mass were very light, i.e. close to that of the η c -meson [14][15][16][17].…”

Lattice QCD study of the radiative decays J/ψ → η c γ and h c → η c γ

“…Proponents of the extensions of the Standard Model involving more than one Higgs doublet speculated that the experimentally established pseudoscalar state η b might be actually a mixture of the true η b and the light parity-odd Higgs boson A 0 [18][19][20][21]. 2 This would solve the puzzle of too large a hyperfine splitting and would give more support to a plausible solution that m A 0 ∼ 9 GeV.…”

“…Those results may be used for a cleaner exclusion of the possibility of having very light m A 0 < 2m τ (see e.g. [18][19][20][21]). …”

Updated constraints from radiative Υ decays on a light CP-odd Higgs

“…However, such minor effects will have little impact on the discussion that follows. Note also that the very-light mass region m A < ∼ 5 GeV is already essentially excluded by low-energy constraints [32], such as B s → µ + µ − or B → X s µ + µ − [33].…”