Heavy ion collisions allow the study of the quark-gluon plasma (QGP), a state of quantum chromodynamic (QCD) matter with deconfined partons that is thought to have filled the early universe between $10^{-12}$ and $10^{-6}$ seconds of age. The top quark constitutes a novel and theoretically precise probe of the nuclear parton distribution functions (nPDFs) in the poorly explored high Bjorken-x region as well as of the properties of the produced QGP. On the one hand, precise knowledge of nPDFs is a key prerequisite to extract detailed information on the QGP properties from the experimental data. On the other hand, top quarks decay on a timescale similar to the formation and evolution of the QGP, hence offering a unique opportunity to study its time structure. The first evidence for the production of the top quark in PbPb collisions recorded by the CMS experiment at the CERN LHC is discussed in this seminar.
