Combination of ATLAS and CMS top quark pair cross section measurements in the $e\mu$ final state using proton-proton collisions at $\sqrt{s}$= 8 TeV

A combination of top-quark pair production cross-section ($\sigma_{\rm t\bar t}$) measurements at a centre-of-mass energy of 8 TeV performed by the ATLAS and CMS experiments at the LHC in the di-lepton channels is presented. The $\sigma_{\rm t\bar t}$ measurements use between 5.3 and 20.3 fb$^{-1}$ of proton-proton collisions. The combined preliminary LHC measurement of the top-quark pair production cross-section is $\sigma_{\rm t\bar t} = 241.4 \pm 1.4~\mathrm{(stat.)} \pm 5.7~\mathrm{(syst.)} \pm 6.2~\mathrm{(lumi.)}~\mathrm{pb} = 241.4 \pm 8.5~\mathrm{pb}$ for a top-quark mass of $172.5\mathrm{\ GeV}$, corresponding to a total uncertainty of 3.5%. The result is in agreement with the standard model prediction.