Discovering an invisibly decaying Higgs boson at hadron colliders

A Higgs boson lighter than 2m(W) 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, h(inv), 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+h(inv) channel, we show that the LHC can yield a discovery signal above 5 sigma 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 h(inv) mass from production cross sections at the LHC, and find that combining WBF and Z+h(inv) allows a relatively model-independent determination of the h(inv) mass with an uncertainty of 35-50 GeV (15-20 GeV) with 10 (100) fb(-1). At the Tevatron, a 3 sigma observation of a 120 GeV h(inv) 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+h(inv) channel allows a 3 sigma observation of h(inv) with 7 fb(-1) per detector. Because of overwhelming irreducible backgrounds, h(inv)+j is not a useful search channel at either the Tevatron or the LHC, despite the larger production rate.