Constraining Early-Time Electromagnetic Fields in Heavy Ion Collisions through Z Boson Dimuon Decays with CMS

In non-central ultrarelativistic heavy-ion collisions, the generation of immense electromagnetic fields has garnered significant interest, particularly regarding their potential effects during the quark-gluon plasma (QGP) phase. Despite extensive studies, the time evolution of these fields within the QGP remains an open question. This uncertainty has motivated investigations into phenomena such as the chiral magnetic effect, spin polarization splitting, and modifications to directed flow (v₁). In this talk, we present the very first search using the Z boson as a novel probe of the initial electromagnetic field in heavy-ion collisions. Owing to its electroweak nature, early production time, and prompt decay, the Z boson is unaffected by the QGP evolution and thus provides direct access to the early-time dynamics of the electromagnetic field. Using CMS PbPb data from Run 2 at √s_NN = 5.02 TeV, we employed three distinct methods to investigate potential modifications to the Z boson mass and width relative to a pp baseline. These measurements were compared to model-dependent theoretical predictions, offering new insights into the interaction between the residual electromagnetic field and the QGP, and helping to further constrain theoretical models.