  If the standard electroweak gauge model is extended to include two or more
Higgs doublets, there may be a neutral Higgs boson $h$ which is light (with a
mass of say 10 GeV) but the $hZZ$ coupling is suppressed so that it has so far
escaped experimental detection. However, the effective $hhZZ$ coupling is
generally unsuppressed, hence the decay of Z into two light Higgs bosons plus a
fermion-antifermion pair may have an observable branching fraction, especially
if $h$ decays invisibly as for example in the recently proposed doublet Majoron
model.
