Measurement of the $t\bar{t}$ production cross-section in $pp$ collisions at $\sqrt{s}=13$ 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 a data sample of 78 pb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV, collected in 2015 by the ATLAS detector, using $t\bar{t}$ events with an opposite-sign electron-muon pair in the final state. Jets containing $b$-quarks are 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 determine simultaneously $\sigma_{t\bar{t}}$ and the efficiency to reconstruct and $b$-tag a jet from a top quark decay, thereby minimising the associated systematic uncertainties. The cross-section is measured to be: \[ \sigma_{t\bar{t}}= 825 \pm 49 \ (stat) \pm 60 (syst) \pm 83 (lumi)~pb, \] where the three uncertainties arise from data statistics, experimental and theoretical systematic effects, and the integrated luminosity, giving a total relative uncertainty of 14%. The result is consistent with recent theoretical QCD calculations at next-to-next-to-leading order.