Measurement of the $t\bar{t}$ production cross-section in $pp$ collisions at $\sqrt{s}=8$ TeV using $e\mu$ events with $b$-tagged jets

This note describes a measurement of the inclusive top-pair production cross-section ($\sigma_{t\bar{t}}$) with the full 2012 ATLAS data sample of 20.3 fb$^{-1}$ of proton-proton collision data at a centre-of-mass energy of $\sqrt{s}=8$ TeV, using $t\bar{t}$ events with an opposite-sign $e\mu$ pair in the final state. Jets containing $b$ quarks were tagged using an algorithm based on track impact parameters and reconstructed secondary vertices. The numbers of events with exactly one and exactly two $b$-tagged jets were counted and used to simultaneously determine $\sigma_{t\bar{t}}$ and the efficiency to reconstruct and $b$-tag a $b$-jet from a top quark decay, thereby minimising the associated systematic uncertainties. The cross-section was measured to be: $\sigma_{t\bar{t}}=237.7\pm 1.7 (stat) \pm 7.4 (syst) \pm 7.4 (lumi) \pm 4.0 (beam\ energy)$ pb, where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, the integrated luminosity, and LHC beam energy, giving a total relative uncertainty of 4.8 %. The result is consistent with recent theoretical QCD calculations at next-to-next-to-leading order.