The measurement of the invisible Z-boson decay width at e+e - colliders can be done "indirectly," by subtracting the Z-boson visible partial widths from the Z-boson total width, or "directly," from the process e+e -→yvv̄. 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 Zvv̄ 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 e+e- linear collider at different center-of-mass energies√s=mz and √s≈170 GeV, would translate into a markedly more precise measurement of the Zvv̄ 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.
ASJC Scopus subject areas
- Physics and Astronomy (miscellaneous)