Physics Motivation: Higgs can decay invisibly into final states as candidate dark matter particles, if there is a hidden sector which couples to Higgs field.
Search Channel: e+e- → ZH, Z→qq, H→invisible, at Ecm=500 GeV.
Detector Benchmark: in this channel, there is in principle no confusion from jet clustering, henceforth the Z mass resolution or recoil mass resolution is largely determined by jet energy resolution. Nevertheless there is potential contamination from pile-up events, such as gamma gamma → low-pt hadrons, and beam spectrum, which could affect the dependence on JER. One should try to disentangle the impact from detector performance and from beam background, in order to give the prospects that some of the contaminations might be improved in future.
final observable: 95% C.L. upper limit on Branching Ratio of H→invisible
intermediate observables: JER, Z or recoil mass resolution