  We demonstrate that rare decays of the Standard Model $Z$ boson can be used
to discover and characterize the nature of new hidden-sector particles. We
propose new searches for these particles in soft, high-multiplicity leptonic
final states at the Large Hadron Collider. The proposed searches are sensitive
to low-mass particles produced in $Z$ decays, and we argue that these striking
signatures can shed light on the hidden-sector couplings and mechanism for mass
generation.
