Measurements of $W^+W^-$ production cross sections in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Measurements of $W^+W^-\rightarrow e^\pm \nu \mu^\mp \nu$ production cross-sections are presented, providing a test of the predictions of perturbative quantum chromodynamics, parton distribution functions, and the electroweak theory. The measurements are performed using $pp$ collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015-2018 at $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 140 fb$^{-1}$. The number of events due to top-quark pair production, the largest background, is reduced by rejecting events containing jets with $b$-hadron decays. An improved methodology of estimating the remaining top quark background enables a precise measurement of $W^+W^-$ cross-sections with no additional requirements on jets. The fiducial $W^+W^-$ cross section is determined in a maximum-likelihood fit with an uncertainty of 3.1%. The measurement is extrapolated to the full phase space, resulting in a total $W^+W^-$ cross-section of $127\pm4$ pb. Differential cross-sections are measured as a function of twelve observables that comprehensively describe the kinematics of $W^+W^-$ events. The measurements are compared to state-of-the art theory calculations and excellent agreement with predictions is observed.