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, $h_{\mathrm{inv}}$, 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 $Z+h_{\mathrm{inv}}$ channel, we show that the LHC can yield a discovery signal above $5\sigma$ with $10{\mathrm{fb}}^{-1}$ of integrated luminosity for a Higgs mass of 120 GeV. With $30{\mathrm{fb}}^{-1}$ the discovery reach extends up to a Higgs mass of 160 GeV. We also study the extraction of the $h_{\mathrm{inv}}$ mass from production cross sections at the LHC, and find that combining WBF and $Z+h_{\mathrm{inv}}$ allows a relatively model-independent determination of the $h_{\mathrm{inv}}$ mass with an uncertainty of 35–50 GeV (15–20 GeV) with $10(100){\mathrm{fb}}^{-1}$. At the Tevatron, a $3\sigma$ observation of a 120 GeV $h_{\mathrm{inv}}$ in any single channel is not possible with less than $12{\mathrm{fb}}^{-1}$ per detector. However, we show that combining the signal from WBF with the previously studied $Z+h_{\mathrm{inv}}$ channel allows a $3\sigma$ observation of $h_{\mathrm{inv}}$ with $7{\mathrm{fb}}^{-1}$ per detector. Because of overwhelming irreducible backgrounds, $h_{\mathrm{inv}}+j$ is not a useful search channel at either the Tevatron or the LHC, despite the larger production rate.