Studies for the measurement of the jet mass in fully merged hadronic top quark decays

This thesis presents studies for a measurement of the invariant mass of jets containing all decay products of a fully hadronic top quark decay given by the decay channel $t$ $\to$ $Wb$ $\to$ $qq'b$. At high transverse momentum $_{pT}$ of the top quark, the decay products are collimated and can be reconstructed in one jet. The invariant mass of these jets is a fundamental jet substructure variable, used in top tagging algorithms to distinguish between jets from top quark decays and jets from light quarks or gluons. Jet substructure is well suited to test the modeling of parton showers and hadronization in Monte Carlo generators. In the special case of jets from very collimated top quark decays the invariant mass of jets is sensitive to the mass of the top quark itself. Moreover, this jet mass is in principle calculable from first principle in an effective field theory, providing the opportunity to compare the measurement directly to those calculations. This could lead to a measurement of the top quark mass in a well defined renormalization scheme. The main result of this thesis is a preliminary measurement of the jet mass of the leading jet in collimated top quark decays for $_{pT}$ > 400 GeV. The measurement is corrected to the level of stable particles. Corrections for detector effects are applied using a regularized unfolding. A second measurement has been done for jets with $_{pT}$ > 500 GeV, where jets with 400 GeV < $_{pT}$ < 500 GeV are taken into account in the migration matrix of the unfolding. This measurement shows a reduced dependence on the simulation used to calculate the migration matrix.