Finding the Higgs boson was immensely challenging, but was achieved in 2012 by ATLAS and CMS at the LHC. Here the Higgs was discovered in its decays to 2 photons, and in the 4 lepton channel.
Here we are targeting a much more difficult task - where the Higgs decays to "invisible" particles that are not detectable at the LHC. In the Standard Model, this decay can occur where the Higgs decays to 4 neutrinos. This is an extremely rare process in the Standard Model, with an expected branching ratio of only 0.1%.