Measurement of the cross section for top quark pair production in association with a W or Z boson in proton-proton collisions at $$ \sqrt{s}=13 $$ TeV

A measurement is performed of the cross section of top quark pair production in association with a W or Z boson using proton-proton collisions at a center-of-mass energy of 13 TeV at the LHC. The data sample corresponds to an integrated luminosity of 35.9 fb⁻¹, collected by the CMS experiment in 2016. The measurement is performed in the same-sign dilepton, three- and four-lepton final states. The production cross sections are measured to be $$ \sigma \left(\mathrm{t}\overline{\mathrm{t}}\mathrm{W}\right)={0.77}_{-0.11}^{+0.12}{\left(\mathrm{stat}\right)}_{-0.12}^{+0.13}\left(\mathrm{syst}\right)\mathrm{p}\mathrm{b} $$ σ t t ¯ W = 0.77 − 0.11 + 0.12 stat − 0.12 + 0.13 syst p b and $$ \sigma \left(\mathrm{t}\overline{\mathrm{t}}\mathrm{Z}\right)={0.99}_{-0.08}^{+0.09}{\left(\mathrm{stat}\right)}_{-0.10}^{+0.12}\left(\mathrm{syst}\right)\mathrm{p}\mathrm{b} $$ σ t t ¯ Z = 0.99 − 0.08 + 0.09 stat − 0.10 + 0.12 syst p b . The expected (observed) signal significance for the $$ \mathrm{t}\overline{\mathrm{t}}\mathrm{W} $$ t t ¯ W production in same-sign dilepton channel is found to be 4.5 (5.3) standard deviations, while for the $$ \mathrm{t}\overline{\mathrm{t}}\mathrm{Z} $$ t t ¯ Z production in three- and four-lepton channels both the expected and the observed significances are found to be in excess of 5 standard deviations. The results are in agreement with the standard model predictions and are used to constrain the Wilson coefficients for eight dimension-six operators describing new interactions that would modify $$ \mathrm{t}\overline{\mathrm{t}}\mathrm{W} $$ t t ¯ W and $$ \mathrm{t}\overline{\mathrm{t}}\mathrm{Z} $$ t t ¯ Z production.