Expected Sensitivity to Invisible Higgs Boson Decays at the ILC with the SiD Detector (A Snowmass White Paper)

In the Standard Model (SM) of particle physics, the branching ratio for Higgs boson decays to a final state which is invisible to collider detectors, $H \rightarrow ZZ^{\star} \rightarrow \nu \bar{\nu} \nu \bar{\nu}$, is order 0.10%. In theories beyond the SM (BSM), this branching ratio can be enhanced by decays to undiscovered particles like dark matter (DM). At the Large Hadron Collider (LHC), the current best upper limit on the branching ratio of invisible Higgs boson decays is 11% at 95% confidence level. We investigate the expected sensitivity to invisible Higgs decays with the Silicon Detector (SiD) at the International Linear Collider (ILC). We conclude that at $\sqrt{s}=250$ GeV with 900 fb$^{-1}$ integrated luminosity each for $e_{L}^-e_{R}^+$ and $e_{R}^-e_{L}^+$ at nominal beam polarization fractions, the expected upper limit is 0.16% at 95% confidence level.