Combination of ATLAS and CMS top quark pair cross section measurements in the emu final state using proton-proton collisions at 8 TeV

A combination of ATLAS and CMS measurements of the top quark pair production cross section ($\sigma_{\mathrm{t \bar t}}$) using proton-proton collisions at a centre-of-mass energy of 8~TeV is presented. The measurements make use of top quark pair events in the dilepton decay channel with exactly one electron and one muon in the final state. The integrated luminosities of the two measurements are 20.3 and 5.3~fb$^{-1}$ for ATLAS and CMS, respectively. The combined cross section is determined to be $\sigma_{\mathrm{t \bar t}} = 241.5 \pm 1.4~\mathrm{(stat.)} \pm 5.7~\mathrm{(syst.)} \pm 6.2~\mathrm{(lumi.)}~\mathrm{pb} = 241.5 \pm 8.5~\mathrm{pb}$ for a top quark mass reference value of 172.5~GeV. The total uncertainty of the combined result is 3.5\%. An uncertainty on the LHC beam energy of 0.66\% translates into an additional uncertainty of 4.2 pb on $\sigma_{\mathrm{t \bar t}}$. The combined cross section depends on the assumed top quark mass value $m_{\mathrm {t}}$ with a slope of $1/\sigma_{\mathrm{t \bar t}} {\rm d}\sigma_{\mathrm{t \bar t}}/{\rm d}{m_{\mathrm {t}}} = -0.46$\%/GeV around the reference value. The result is in good agreement with the standard model prediction calculated in perturbative QCD to next-to-next-to-leading order.