Measurement of the cross section of top quark pair production with additional charm jets using the dileptonic final state in pp collisions at 13 TeV

For the first time, a measurement of the inclusive cross section of top quark pair ($\text{t}\overline{\text{t}}$) production with two additional charm jets is presented. The measurement is performed using the dileptonic decay channel of the top quark pairs produced in proton-proton collisions at a centre-of-mass energy of $13~\mathrm{TeV}$. The used dataset corresponds to an integrated luminosity of $41.5~\mathrm{fb}^{-1}$, collected with the CMS detector at the LHC. A neural network is trained to distinguish between top quark pair events with additional c jets ($\text{t}\overline{\text{t}}\text{c}\overline{\text{c}}$), b jets ($\text{t}\overline{\text{t}}\text{b}\overline{\text{b}}$), and light-flavour jets ($\text{t}\overline{\text{t}}\text{LF}$), based on observables related to dedicated charm jet identification algorithms, as well as kinematic properties of the event. By means of a template fitting procedure using simulated and observed neural network outputs, the inclusive $\text{t}\overline{\text{t}}\text{c}\overline{\text{c}}$, $\text{t}\overline{\text{t}}\text{b}\overline{\text{b}}$, and $\text{t}\overline{\text{t}}\text{LF}$ cross sections are simultaneously extracted, together with their ratios to the inclusive $\text{t}\overline{\text{t}}$ + two jets cross section. This results in a fully coherent treatment of different additional jet flavours in the production of $\text{t}\overline{\text{t}}$ + two jets, and provides the first measurement of the $\text{t}\overline{\text{t}}\text{c}\overline{\text{c}}$ process with a measured cross section of 0.152 $\pm$ 0.022 (stat.) $\pm$ 0.019 (syst.) pb in the fiducial phase space and 7.43 $\pm$ 1.07 (stat.) $\pm$ 0.95 (syst.) pb in the full phase space.