Testing the CKM unitarity at high energy via the W^+W^- production at the LHC and future colliders

We propose a novel test to assess the unitarity of the Cabibbo-Kobayashi-Maskawa matrix, V ₂₊₌, at present and future collider experiments. Our strategy makes use of the W^+W^- production cross section to directly probe the V ₂₊₌^ V ₂₊₌ product, which regulates the high-energy behavior of the observable. The violation of unitarity is signalled by an anomalous behavior of the cross section that grows quadratically with the W^+W^- invariant mass with respect to the Standard Model prediction. By using the recent ATLAS measurements of the W^+W^- cross section we are able to constrain the maximal unitarity violation allowed by current data, producing a bound complementary to the results of flavor physics experiments. Forecasts for the high luminosity phase of the LHC and for the future 100 TeV hadron collider are also discussed.