B. Brahmachari et al., MASS LIMITS OF INVISIBLY DECAYING HIGGS PARTICLES FROM THE CERN LEP DATA, Physical review. D. Particles and fields, 48(9), 1993, pp. 4224-4229
In the Majoron models the SU(2) Higgs doublet can decay invisibly into
a Majoron pair via its mixing with a singlet. An analysis of the CERN
LEP data shows the invisible decay mode to be more visible than the S
M decay. For these models, the dominantly doublet Higgs field H is sho
wn to have a mass limit within +/-6 GeV of the SM limit irrespective o
f the model parameters. But the dominantly singlet one S can be arbitr
arily light for a sufficiently small mixing angle.