Combination of cross-section measurements for associated production of a single top-quark and a W boson at $\sqrt{s}=8$ TeV with the ATLAS and CMS experiments

A combination of cross-section measurements for the associated production of a top quark and a $W$ boson in proton-proton collisions at $\sqrt{s}=8$ TeV by the ATLAS and CMS experiments is presented. The two measurements are based on integrated luminosities of 20.3 fb$^{-1}$ and 12.2 fb$^{-1}$, respectively. The combined production cross section of a single top quark and a $W$ boson is determined as $\sigma_{tW} = 25.0\pm1.4$ (stat.) $\pm4.4$ (syst.) $\pm0.7$ (lumi.) pb = $25.0\pm4.7$ pb, in agreement with the NLO+NNLL expectation. A 95\% C.L. lower limit on the magnitude of the CKM matrix element $V_{tb}$ of $|V_{tb}|>0.79$ is extracted from the cross-section measurement.