Modelling uncertainties of $t\bar{t}W$ in multi-lepton channel

We compare fixed order and parton shower matched predictions for the process $pp\to {\mathrm{\ell }}^{+}{\nu }_{\mathrm{\ell }}{\mathrm{\ell }}^{-}{\bar{\nu }}_{\mathrm{\ell }}{\mathrm{\ell }}^{\pm }{\nu }_{\mathrm{\ell }}b\bar{b}+X$ at NLO in QCD, including the orders $\mathcal{O}({\alpha }_s^3{\alpha }^6)$ and $\mathcal{O}({\alpha }_s{\alpha }^8)$. The comparison is performed at the integrated and differential fiducial level at the LHC with $\sqrt{s}=13$ TeV. In the absence of parton shower matching procedure that includes the full off-shell effects for this process at NLO in QCD, we propose a new prescription. It enables the inclusion of approximate full off-shell effects to currently available parton shower matched predictions at NLO in QCD.