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 CKM , at present and future collider experiments. Our strategy makes use of the W + W − production cross section to directly probe the V CKM † V CKM product, which regulates the high-energy behavior of the observable. The violation of unitarity is signaled 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.