We study the contributions of colorless vectorlike fermions to the triple gauge couplings $W^{+}{W}^{-}\gamma$ and $W^{+}{W}^{-}{Z}^0$. We consider models in which their coupling to the Standard Model Higgs boson is allowed or forbidden by quantum numbers. We assess the sensitivity of the future accelerators FCC-ee, ILC, and CLIC to the parameters of these models, assuming they will be able to constrain the anomalous triple gauge couplings with precision $\delta {\kappa }_V\sim \mathcal{O}({10}^{-4})$, $V=\gamma ,Z^0$. We show that the combination of measurements at different center-of-mass energies helps to improve the sensitivity to the contribution of vectorlike fermions, in particular when they couple to the Higgs. In fact, the measurements at the FCC-ee and, especially, the ILC and the CLIC, may turn the triple gauge couplings into a new set of precision parameters able to constrain the models better than the oblique parameters or the $H\to \gamma \gamma$ decay, even assuming the considerable improvement of the latter measurements achievable at the new machines.

• Received 23 June 2017

DOI:https://doi.org/10.1103/PhysRevD.96.035035