Discovering an invisibly decaying Higgs boson at hadron colliders

A Higgs boson lighter than 2mW that decays mostly into invisible channels (e.g., dark matter particles) is theoretically well-motivated. We study the prospects for discovery of such an invisible Higgs, hᵢₙᵥ, at the LHC and the Tevatron in three production modes: (1) in association with a Z, (2) through weak boson fusion (WBF), and (3) accompanied by a jet. In the Z+hᵢₙᵥ channel, we show that the LHC can yield a discovery signal above 5σ with 10 fb^-1 of integrated luminosity for a Higgs mass of 120 GeV. With 30 fb^-1 the discovery reach extends up to a Higgs mass of 160 GeV. We also study the extraction of the hᵢₙᵥ mass from production cross sections at the LHC, and find that combining WBF and Z+hᵢₙᵥ allows a relatively model-independent determination of the hᵢₙᵥ mass with an uncertainty of 35--50 GeV (15--20 GeV) with 10 (100) fb^-1. At the Tevatron, a 3σ observation of a 120 GeV hᵢₙᵥ in any single channel is not possible with less than 12 fb^-1 per detector. However, we show that combining the signal from WBF with the previously studied Z+hᵢₙᵥ channel allows a 3σ observation of hᵢₙᵥ with 7 fb^-1 per detector. Because of overwhelming irreducible backgrounds, hᵢₙᵥ+j is not a useful search channel at either the Tevatron or the LHC, despite the larger production rate.