Invisible Z - Boson Decays at e + e − Colliders

The measurement of the invisible Z-boson decay width at ee colliders can be done “indirectly,” by subtracting the Z-boson visible partial widths from the Z-boson total width, or “directly,” from the process ee → γνν̄. 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 LEP and CHARM II are capable of constraining the left-handed Zνν̄-coupling, 0.45 ∼ gL ∼ 0.5, while the right-handed one is only mildly bounded, |gR| ≤ 0.2. We show that measurements at a future ee linear collider at different center-of-mass energies, √ s = mZ and √ s ≈ 170 GeV, would translate into a markedly more precise measurement of the Zνν̄-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 Zwidth. We discuss some scenarios which illustrate the ability of different invisible Z-boson decay measurements to constrain new physics beyond the Standard Model.