The most precise measurement of the \(t\bar{t}\) cross-section (\(\sigma _{t\bar{t}}\)) in proton–proton collisions at \(\sqrt{s}=8\) \(\,\mathrm{TeV}\) from the ATLAS Collaboration was made using events with an opposite-charge electron–muon pair and one or two b-tagged jets [1], and used a preliminary calibration of the integrated luminosity. The luminosity calibration has been finalised since [2] with a total uncertainty of 1.9%, corresponding to a substantial improvement on the previous uncertainty of 2.8%. Since the uncertainty on the integrated luminosity contributed 3.1% of the total 4.3% uncertainty on the \(\sigma _{t\bar{t}}\) measurement reported in [1], a significant improvement in the measurement is possible by using the new luminosity calibration, as documented in this Addendum.

The new calibration corresponds to an integrated luminosity of 20.2 fb\(^{-1}\) for the \(\sqrt{s}=8\) \(\,\mathrm{TeV}\) sample, a decrease of 0.2%. The cross-section was recomputed taking into account the effects on both the conversion of the \(t\bar{t}\) event yield to a cross-section, and the background estimates, giving a result of:

$$\begin{aligned} \sigma _{t\bar{t}}= 242.9\pm 1.7\pm 5.5\pm 5.1\pm 4.2\,\mathrm pb, \end{aligned}$$

where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, knowledge of the integrated luminosity, and of the LHC beam energy, giving a total uncertainty of 8.8 pb (3.6 %). The result is consistent with the theoretical prediction of \(252.9^{+13.3}_{-14.5}\) pb, calculated at next-to-next-to-leading-order with next-to-next-to-leading-logarithmic soft gluon terms with the top++ 2.0 program [3] as discussed in detail in Ref. [1].

The updated value of the ratio of cross-sections \(R_{t\bar{t}}=\) \(\sigma _{t\bar{t}}\)(8 \(\,\mathrm{TeV}\))/\(\sigma _{t\bar{t}}\)(7 \(\,\mathrm{TeV}\)) is:

$$\begin{aligned} R_{t\bar{t}}=1.328\pm 0.024 \pm 0.015 \pm 0.038 \pm 0.001, \end{aligned}$$

with uncertainties defined as above, adding in quadrature to a total of 0.047. The largest uncertainty comes from the uncertainties on the integrated luminosities, considered to be uncorrelated between the \(\sqrt{s}=7\) \(\,\mathrm{TeV}\) and \(\sqrt{s}=8\) \(\,\mathrm{TeV}\) datasets. This result is \(2.1\sigma \) below the expectation of \(1.430\pm 0.013\) calculated from top++ 2.0 as discussed in Ref. [1].

The updated fiducial cross-sections, for a \(t\bar{t}\) decay producing an \(e\mu \) pair within a given fiducial region, are shown in Table 1, updating Table 5 of Ref. [1]. The results are given both for the analysis requirements of \(p_{\mathrm {T}}>25\,\,\mathrm{GeV} \) and \(|\eta |<2.5\) for both leptons, and for a reduced acceptance of \(p_{\mathrm {T}}>30\,\,\mathrm{GeV} \) and \(|\eta |<2.4\). They are given separately for the two cases where events with either one or both leptons coming from \(t\rightarrow W\rightarrow \tau \rightarrow \ell \) rather than the direct decay \(t\rightarrow W\rightarrow \ell \) (\(\ell =e\) or \(\mu \)) are included, or where the contributions involving \(\tau \) decays are subtracted. The results shown for the \(\sqrt{s}=7\) \(\,\mathrm{TeV}\) data sample are unchanged with respect to those in Ref. [1]. The results for the top quark pole mass and limits on light supersymmetric top squarks presented in Ref. [1] are derived from \(\sqrt{s}=7\) \(\,\mathrm{TeV}\) and \(\sqrt{s}=8\) \(\,\mathrm{TeV}\) cross-section measurements taken together, and would be only slightly improved by the luminosity update described here.

Table 1 Fiducial cross-section measurement results at \(\sqrt{s}=7\,\mathrm{TeV}\) and \(\sqrt{s}=8\,\mathrm{TeV}\), for different requirements on the minimum lepton \(p_{\mathrm {T}}\) and maximum lepton \(|\eta |\), and with or without the inclusion of leptons from \(W\rightarrow \tau \rightarrow \ell \) decays, with the final 2012 luminosity calibration. In each case, the first uncertainty is statistical, the second due to analysis systematic effects, the third due to the integrated luminosity and the fourth due to the LHC beam energy