Early EM fields in PbPb collisions via their effect on dimuon decays of Z bosons and top quark production with CMS

In this article, we discuss a selection of measurements of initial-state probes conducted by CMS, the Z boson and top quark, to study early-stage aspects of heavy ion collisions. The Z boson has a very short lifetime and its decay to dimuons can be precisely measured by CMS. Since the Z is not sensitive to strong interactions, it is unmodified in heavy ion collisions. These properties make it an invaluable tool for studying the initial state of heavy ion collisions. It has been proposed that the momentum of muons resulting from a Z decay, including the experimentally observed mass and width of the Z, may be modified by the presence of immense initial-state electromagnetic fields that are thought to be created in the aftermath of these collisions. Additionally, top quark production in heavy ion collisions provides an avenue to investigate nuclear parton distribution functions. With its short life-time, the top predominantly decays into a W boson and b quark pair, before hadronizing. Leptonic final states from the subsequent W decay are effectively electroweak probes of the medium they traverse before reaching the detector. Evidence of top quark pair (tt̅) production, using √sNN = 5.02 TeV lead-lead (PbPb) collision data recorded during Run 2 in 2018, has been reported by the CMS experiment.