Measurement of the Top Quark Mass with the CMS Experiment

The standard model of particle physics describes an astonishing number of phenomena. Yet at the same time it is incomplete: it does not describe e.g. gravitation. Finding explicit weaknesses in the standard model predictions has proved to be difficult, and hence precision measurements are currently one of the most promising methods towards this goal. One of the most intriguing precision measurements is that of the top quark mass (mt), which is connected for instance to the question about the meta-stability of the universe. This thesis strives multilaterally towards a more precise measurement and interpretation of the top quark mass. The work begins with efforts towards a more precise jet calibration at the CMS. Then, the possible weaknesses of a D0 mt analysis are reviewed. Finally, a mt measurement at the CMS is constructed for the legacy 2017–2018 datasets. The jet energy corrections are the most important experimental factor in the uncertainties of the top quark mass. Hence, they are closely linked with the mt measurement. The work on jets in this thesis aims for an exceptionally precise jet energy calibration for the CMS Run 2 legacy datasets. The author has made several important contributions towards the jet energy corrections in the Run 2 legacy reconstruction. The re-assessment of a D0 top quark mass measurement is performed outside of the CMS and D0 affiliations. The D0 top quark mass value is an important outlier in the top quark mass world combination, and a better understanding of the reasons behind this is desirable. In an earlier study it was shown that there are possible discrepancies in the flavor-dependent jet energy corrections at D0. In this thesis we demonstrate that these discrepancies (if they can be confirmed) shift the D0 top quark mass measurement to a value that is more in line with the other major measurements from CMS, ATLAS and CDF. The work culminates in the design and validation of the first direct CMS lepton+jets mt measurement on the 2017–2018 datasets. The analysis is executed using a new profile likelihood method, where the collected data can constrain systematic uncertainties in situ. Agreement between data and simulation is verified within the systematic uncertainties using control plots. The impact of an extensive set of systematic uncertainties on the mt measurement is assessed using simulations. Also the full effects of limited statistics in simulations are demonstrated using toy experiments. It is confirmed that the limiting systematic uncertainty in the current mt measurements is the modelling of b quark jets. This challenge can be encountered either by enhancing the b jet energy corrections or by performing the measurement on a larger amount of data. In profile likelihood analyses, the latter is also a valid approach.