First measurement of the running of the top quark mass

In this thesis, the first experimental determination of the running of the top quarkmass is presented. The running is extracted from a measurement of the differential top quark-antiquark ($t$$\bar{t}$) production cross section as a function of the invariant mass of the $t$$\bar{t}$ system, $m$$_{t\bar{t}}$. The analysis is performed using proton-proton collision data recorded by the CMS detector at the CERN LHC in 2016, corresponding to an integrated luminosity of 35.9 fb$^{−1}$ . Candidate $t$$\bar{t}$ events are selected in the final state with an electron and a muon of opposite charge, and the differential cross section d$\sigma$$_{t\bar{t}}$/d$m$$_{t\bar{t}}$ is determined at the parton level by means of a maximum-likelihood fit to multidifferential final-state distributions. The value of the top quark mass in the modified minimal subtraction ($\overline{MS}$) renormalization scheme, $m$$_{t}$($\mu$), is determined as a function of the scale $\mu$ = $m$$_{t\bar{t}}$ by comparing the measured d$\sigma$$_{t\bar{t}}$/d$m$$_{t\bar{t}}$ to theoretical predictions at next-to-leading order, and the resulting scale dependence is interpreted as the running of the top quark mass. The observed running is found to be compatible with the one-loop solution of the corresponding renormalization group equation, up to a scale of the order of 1 TeV.