We searched for evidence of a Higgsstrahlung process in a secluded sector, leading to a final state with a dark photon U and a dark Higgs boson h', with the KLOE detector at DAFNE. We investigated the case of h' lighter than U, with U decaying into a muon pair and h' producing a missing energy signature. We found no evidence of the process and set upper limits to its parameters in the range 2mμ < mU < 1000MeV, mh' < mU.
Anastasi, A., Ceradini, F., DI CICCO, A., Loffredo, S. (2015). Search for dark Higgsstrahlung in e+e− → μ+μ− and missing energy events with the KLOE experiment. PHYSICS LETTERS. SECTION B, 747, 365-372 [10.1016/j.physletb.2015.06.015].