First measurements of the $\rm{t\bar{t}}$ cross section in LHC pp and pPb collisions at 5.02 and 8.16 TeV and determination of the absolute luminosity in the CMS experiment

Soon after the discovery of the bottom quark, the quest for the top quark had begun. The search carried out for nearly 20 years because the mass of the top quark turned out to be unexpectedly large, around 40 times the mass of the bottom quark. The discovery of the top quark in nuclear collisions had to further wait another 20 years for the 2016 proton-nucleus run at the CERN LHC. The first top quark measurement in nuclear collisions is demonstrated with the observation of the top quark pair process, using $174\pm6\,\mathrm{nb}^{-1}$ of proton-nucleus collisions at 8.16 TeV with the CMS experiment. The measurement is performed by analyzing events with exactly one isolated electron or muon and at least four jets. The inclusive cross section that is simultaneously measured in the two final states is $45\pm8\,\mathrm{nb}$, consistent with perturbative QCD calculations as well as the expectations from scaled proton-proton data. Measurements of top quark pair production at various energies probe different values of $x$, the fractional momentum of the proton carried by the partons. Using a data sample of $27.4\pm0.6\,\mathrm{pb}^{-1}$ collected by the CMS experiment during the proton-proton run at 5.02 TeV in 2015, the first measurement of the inclusive top quark pair cross section is also presented for events with one or two leptons (electrons or muons), and at least two jets. A moderate decrease of the uncertainty in the gluon distribution is observed in the less-explored kinematic range of $x \gtrsim 0.1$. To determine the luminosity and the associated uncertainty the van der Meer scan technique is used, a purely experimental method.