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

Abstract

We propose a novel test to assess the unitarity of the Cabibbo-Kobayashi-Maskawa matrix, $V_{\mathrm{CKM}}$, at present and future collider experiments. Our strategy makes use of the $W^{+}{W}^{-}$ production cross section to directly probe the $V_{\mathrm{CKM}}^{†}{V}_{\mathrm{CKM}}$ 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.

16 pages, 3 figures, journal version

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