First measurement of the cross section for top quark pair production with additional charm jets using dileptonic final states in pp collisions at $\sqrt{s} = $ 13 TeV

The first measurement of the inclusive cross section for top quark pairs ($\mathrm{t\bar{t}}$) produced in association with two additional charm jets is presented. The analysis uses the dileptonic final states of $\mathrm{t\bar{t}}$ events produced in proton-proton collisions at a centre-of-mass energy of 13 TeV. The data correspond to an integrated luminosity of 41.5 fb$^{-1}$, recorded by the CMS experiment at the LHC. A new charm jet identification algorithm provides input to a neural network that is trained to distinguish among $\mathrm{t\bar{t}}$ events with two additional charm ($\mathrm{t\bar{t}c\bar{c}}$), bottom ($\mathrm{t\bar{t}b\bar{b}}$), and light-flavour or gluon ($\mathrm{t\bar{t}LL}$) jets. By means of a template fitting procedure, the inclusive $\mathrm{t\bar{t}c\bar{c}}$, $\mathrm{t\bar{t}b\bar{b}}$, and $\mathrm{t\bar{t}LL}$ cross sections are simultaneously measured, together with their ratios to the inclusive $\mathrm{t\bar{t}}$ + two jets cross section. This provides measurements of the $\mathrm{t\bar{t}c\bar{c}}$ and $\mathrm{t\bar{t}b\bar{b}}$ cross sections of 10.1 $\pm$ 1.2 (stat) $\pm$ 1.4 (syst) pb and 4.54 $\pm$ 0.35 (stat) $\pm$ 0.56 (syst) pb, respectively, in the full phase space. The results are compared and found to be consistent with predictions from two different matrix element generators with next-to-leading order accuracy in quantum chromodynamics, interfaced with a parton shower simulation.