Measurements of differential cross sections for $t\bar{t}$ production in proton-proton collisions at $\sqrt{s} = 13$ TeV using events containing two leptons with the CMS experiment

This work presents measurements of the normalized and absolute differential top-quark pair¯(tt) production cross sections in proton-proton collisions at a centre-of-mass energy of 13 TeV.The data were collected by the CMS experiment at the CERN LHC in 2016 and correspond toan integrated luminosity of 35.9 fb−1 . The measurements are performed using events with twooppositely charged leptons (e+ e− , e± µ∓ , µ+ µ− ). The differential cross sections are measured¯as a function of kinematic observables of the top quarks, the tt-system, and the top quarkdecay products, as well as of the jet multiplicity in the event. Differential measurementsare based on an event counting method and involve a regularized unfolding technique tocorrect the reconstructed distributions for acceptance and detector effects. Depending on theobservable, differential cross sections are presented in the full phase space at parton leveland/or in a fiducial phase space at particle level. The results are compared to standard modelpredictions from Monte Carlo generators at next-to-leading-order accuracy in QCD, basedon matrix element level interfaced to parton shower simulations. None of the predictionsprovide a uniformly good description of data for all measured distributions. In particular,a substantial disagreement is observed between data and predictions for the transversemomentum distributions of top quarks and their decay products. Measured differential cross¯sections are used to extract the tt and leptonic charge asymmetries, and, for the first timeat 13 TeV, to constrain the top quark chromomagnetic dipole moment in an effective fieldtheory.