Measurement of the ttbb cross section and the ratio ttbb/ttjj in the lepton+jets final state at 8 TeV with CMS detector

The associated production of a top-quark pair with at least two additional jets is studied in the single lepton+jets final state for an integrated luminosity of 19.6 fb$^{-1}$ at a center-of-mass energy of $\sqrt{s}$=8 TeV. The jets from the top-quark decay are identified using a constrained kinematic fit and multivariate classifiers in different categories split by the jet multiplicity. The contributions of $\rm{t\bar{t}jj}$ and $\rm{t\bar{t}b\bar{b}}$ with at least two additional jets of any flavor or b jets to the inclusive $\rm{t\bar{t}+jets}$ production are extracted with a simultaneous template fit of b-tag discriminants. The measured cross sections $\sigma(\rm{t\bar{t}b\bar{b}}) =271.0 \pm 103.0(stat)\pm 32.2(syst)\pm 7.0(lumi)$ fb, $\sigma(\rm{t\bar{t}jj}) = 23.1 \pm 2.3(stat)\pm 2.9(syst)\pm 0.6(lumi)$ pb and the cross section ratio $\sigma(\rm{t\bar{t}b\bar{b}})$/$\sigma(\rm{t\bar{t}jj}) = 0.0117 \pm 0.0040(\rm{stat}) \pm 0.0003(\rm{syst})$ correspond to the phase space of additional jets defined at generator-level as: $p_{\rm{T{aj}}}> 40\,$GeV/c, $|\eta_{\rm aj}|<$2.5, $\Delta \rm R_{\rm ajj} >$ 0.5. The flavor of generated jets is defined by the flavor of the leading quark in the jet. The obtained results are in good agreement with NLO calculations and with the CMS measurement in the dilepton channel when the same jet definition is used.