Invisible Z-boson decays at [Formula Presented] colliders

The measurement of the invisible Z-boson decay width at [Formula Presented] colliders can be done “indirectly”, by subtracting the Z-boson visible partial widths from the Z-boson total width, or “directly”, from the process [Formula Presented] Both procedures are sensitive to different types of new physics and provide information about the couplings of the neutrinos to the Z boson. At present, measurements at CERN LEP and CHARM II are capable of constraining the left-handed [Formula Presented] coupling, [Formula Presented] while the right-handed one is only mildly bounded, [Formula Presented] We show that measurements at a future [Formula Presented] linear collider at different center-of-mass energies, [Formula Presented] and [Formula Presented] would translate into a markedly more precise measurement of the [Formula Presented] couplings. A statistically significant deviation from standard model predictions will point toward different new physics mechanisms, depending on whether the discrepancy appears in the direct or the indirect measurement of the invisible Z width. We discuss some scenarios which illustrate the ability of different invisible Z-boson decay measurements to constrain new physics beyond the standard model.