Motivation and detectability of an invisibly decaying Higgs boson at the Fermilab Tevatron

A Higgs boson with a mass below 150 GeV has a total decay width of less than 20 MeV into accessible standard model states. This narrow width means that the usual branching fractions for such a light Higgs boson are highly susceptible to any new particles to which it has unsuppressed couplings. In particular, there are many reasonable and interesting theoretical ideas that naturally imply an invisibly decaying Higgs boson. The motivations include models with light supersymmetric neutralinos, spontaneously broken lepton number, radiatively generated neutrino masses, additional singlet scalar(s), or right-handed neutrinos in the extra dimensions of TeV gravity. We discuss these approaches to model building and their implications for Higgs boson phenomenology in future Fermilab Tevatron runs. We find, for example, that the Tevatron with 30 ${\mathrm{fb}}^{-1}$ integrated luminosity can make a 3$\sigma$ observation in the $l^{+}{l}^{-}+E_T$ channel for a 125 GeV Higgs boson that is produced with the same strength as the standard model Higgs boson but always decays invisibly. We also analyze the $b\overline{b}+E_T$ final state signal and conclude that it is not as sensitive, but it may assist in excluding the possibility of an invisibly decaying Higgs boson or enable confirmation of an observed signal in the dilepton channel. We argue that a comprehensive Higgs boson search at the Tevatron should include the possibility that the Higgs boson decays invisibly.