First combination of Tevatron and LHC measurements of the top-quark mass

We present a combination of measurements of the mass of the top quark, $m_{\rm top}$, performed by the CDF and D0 experiments at the Tevatron collider and the ATLAS and CMS experiments at the Large Hadron Collider (LHC). The Tevatron data correspond to an integrated luminosity of up to 8.7 fb${^{-1}}$ of proton-antiproton collisions from Run II of the Tevatron at a centre-of-mass energy of 1.96 TeV. The LHC data correspond to an integrated luminosity of up to 4.9 fb${^{-1}}$ of proton-proton collisions from the run at a centre-of-mass energy of 7 TeV. The combination includes measurements in the $t\bar t\to$ lepton+jets, $t\bar t\to$ dilepton, $t\bar t\to$ all jets and $t\bar t\to$ $E_{\rm T}^{\rm miss}$+jets final states. The resulting combined measurement of $m_{\rm top}$ is $173.34 \pm 0.27 \mbox{(stat)} \pm 0.71 \mbox{(syst)}$ GeV, with a total uncertainty of $0.76$ GeV.