  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.

High Energy Physics - Phenomenology