PRL Coverpage and Figure 2-bottom in the paper. Distribution of the transverse momentum ratio x_{jZ} between the jet and the Z boson. Compared to pp data, the distribution in PbPb is shifted to left indicating that the jets lose energy after interacting with the QCD medium.
Our measurement of jet quenching using Z+jet correlations has been published in PRL and chosen as the cover of corresponding issue (See the synopsis from the PRL journal).
Properties of the quark-gluon plasma can be inferred from measurements of jets and Z bosons simultaneously produced in the ion collisions that create the plasma. Z bosons interact with the QGP much more weakly than the partons do, so they don’t lose energy as they travel through it. Thus the bosons energy is very close to the initial energy of the partons emerging from the ion collisions. And the difference between the boson and parton energies can reveal the QGP’s properties.
The excitement is that the experimental uncertainties in identifying a Z boson are smaller compared to that for an isolated photon due to the complicated procedure of photon identification in heavy ion collisions. On the other hand, Z bosons are produced much more rarely compared to isolated photons. This was why the first measurement of Z+jet correlations in PbPb collisions is so hard and it only became possible thanks to the high statistics PbPb data collected by the CMS detector at the end of 2015.