Measurement of the ttbar production cross section in the emu channel in pp collisions at 7 and 8 TeV

The inclusive cross section for top quark pair production is measured in proton-proton collisions at $\sqrt{s} = 7\rm~TeV$ and $\sqrt{s} = 8\rm~TeV$ with the CMS experiment at the CERN LHC using the full data samples collected in 2011 and 2012. The measurement is performed with events containing an opposite charge electron-muon pair. The cross sections are extracted using a binned likelihood fit to multi-differential final state distributions related to identified b quark and other jets in the event. The measured cross sections are: \begin{eqnarray*} \sigma_{{\rm t\bar{t}}} & = & 174.5 \pm 2.1 \rm{(stat)} \pm^{4.5}_{4.0} \rm{(syst)}\pm 3.8{ \rm (lumi)} \mbox{ pb} \quad \mbox{at $\sqrt{s}=7$ TeV and} \\ \sigma_{{\rm t\bar{t}}} & = & 245.6 \pm 1.3 \rm{(stat)} \pm^{6.6}_{5.5}\rm{(syst)}\pm 6.5{ \rm (lumi)} \mbox{ pb} \quad \mbox{at $\sqrt{s}=8$ TeV,} \end{eqnarray*} in good agreement with recent QCD calculations at next-to-next-to-leading-order accuracy. The ratio of the cross sections for the two different values of $\sqrt{s}$ is determined as well as cross sections in fiducial regions, defined by the acceptance requirements on the two charged leptons in the final state. The inclusive cross section results are used to determine the top quark pole mass via the dependence of the theoretically predicted cross section on the mass, giving a result of 173.6$\pm^{1.7}_{1.8}$ GeV for the PDF set NNPDF3.0. The 8 TeV data are also used to set limits on the pair production of supersymmetric top squarks with masses close to the top quark mass, as a function of the neutralino mass.