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

TL;DR

This paper introduces a new method to test the unitarity of the CKM matrix at colliders by analyzing the high-energy behavior of $W^+W^-$ production, providing constraints complementary to flavor physics.

Contribution

It proposes a novel collider-based test for CKM unitarity using $W^+W^-$ cross sections and explores future collider sensitivities.

Findings

Current data constrains CKM unitarity violation.

High-energy $W^+W^-$ cross section growth signals unitarity violation.

Future colliders can improve these bounds.

Abstract

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