Forward top quark pair production is studied in $pp$ collisions in the $\mu eb$ final state using a data sample corresponding to an integrated luminosity of 1.93 fb$^{1}$ collected with the LHCb experiment at a centreofmass energy of 13 TeV. The crosssection is measured in a fiducial region where both leptons have a transverse momentum greater than 20 GeV and a pseudorapidity between 2.0 and 4.5. The quadrature sum of the azimuthal separation and the difference in pseudorapidities, denoted $\Delta R$, between the two leptons must be larger than 0.1. The $b$jet axis is required to be separated from both leptons by a $\Delta R$ of 0.5, and to have a transverse momentum in excess of 20 GeV and a pseudorapidity between 2.2 and 4.2. The crosssection is measured to be $$\sigma_{t\bar{t}}= 126\pm19\,(\mathrm{stat})\pm16\,(\mathrm{syst})\pm5\,(\mathrm{lumi})\,\,\mathrm{ fb}$$ where the first uncertainty is statistical, the second is systematic, and the third is due to the luminosity determination. The measurement is compatible with the Standard Model prediction.
The muon (left) $\eta$ and (right) $p_{\mathrm{ T}}$ distribution in data compared to the expected contributions. The $ t \overline t $ signal yield is determined to be the number of selected events minus the sum of the expected backgrounds. The multiboson background is determined to be negligible. 
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The electron (left) $\eta$ and (right) $p_{\mathrm{ T}}$ distribution in data compared to the expected contributions. The $ t \overline t $ signal yield is determined to be the number of selected events minus the sum of the expected backgrounds. The multiboson background is determined to be negligible. 
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The $ b$ jet (left) $\eta$ and (right) $p_{\mathrm{ T}}$ distribution in data compared to the expected contributions. The $ t \overline t $ signal yield is determined to be the number of selected events minus the sum of the expected backgrounds. The multiboson background is determined to be negligible. 
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The combined invariant mass of the muon, electron and $ b$ jet in data compared to the expected contributions. The $ t \overline t $ signal yield is determined to be the number of selected events minus the sum of the expected backgrounds. The multiboson background is determined to be negligible. 
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Graphical comparison of the measured crosssections with the predictions from the aMC@NLO , POWHEG and MCFM generators. For the data, the inner error band represents the statistical uncertainty, and the outer the total, while for the theoretical predictions, the inner band represents the scale uncertainty and the outer represents the total. The prediction is shown (above) for the muon, electron and jet fiducial, and (below) for the top quark fiducial region. 
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Animated gif made out of all figures. 
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The efficiency to fully reconstruct and identify the candidates. 
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Summary of the systematic uncertainties on the measurement of the $t\bar{t}$ crosssection in the fiducial region defined by the requirements placed on the leptons and the $ b$ jet , expressed as a percentage of the measured crosssection. An additional uncertainty due to $ t \overline t $ modelling is considered when extrapolating to the top quark fiducial region. 
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Created on 11 December 2018.Citation count from INSPIRE on 11 December 2018.