The QGP formed in heavy ion collisions lives only an incredibly short time, about 10-23 fraction of a second. During this time it forms, reaches thermal equilibrium expands and decays. Understanding its temporal evolution is a very challenging and critical task.
Upper panel: Z-boson yield per event divided by TAA (circles) as a function of Npart and differential cross section measured in pp (diamond) plotted at Npart=2. Lower panel: RAA as a function of Npart. The band around unity indicates the ppppluminosity uncertainty of 5.4%. Bars and shaded boxes in both panels show the statistical and systematic uncertainties respectively. The TAA systematic uncertainty is shown separately. The Npart systematic uncertainty are smaller than the markers. The MC generated prediction detailed in the text is shown with the solid line.
There are uniques probes of the initial stage of the collisions: photons, Z0 and W± bosons. Formed very early in the collisions, Z0 bosons instantly decay into leptons, particles without color charge that freely traverse the QGP and therefore tell us about the very early stage of the interaction. See our results of measuring electorweak bosons: