We consider the collider phenomenology of a singlet Majoron model with softly broken lepton number. Lepton number is spontaneously broken when the real part of a new singlet scalar develops a vacuum expectation value. With the additional soft terms violating lepton numbers, the imaginary part of this singlet scalar becomes a massive pseudo-Majoron which can account for the dark matter. In the presence of the coupling of the pseudo-Majoron with the standard model (SM) Higgs, the SM Higgs mostly decays into a pair of pseudo-Majorons, giving rise to ${\cancel{E}}_T$ signals at a hadron collider. Since the Higgs visible decay branching fractions get reduced in the presence of this invisible decay mode, the bounds on the SM Higgs mass from the CERN LEP and Tevatron experiments get diluted, and the invisible decay channel of the Higgs becomes important for the discovery of low mass Higgs at the Large Hadron Collider.

• Received 8 June 2011

DOI:https://doi.org/10.1103/PhysRevD.84.015017