Even though high energy QCD processes are accessible via perturbation theory, lower-energy effects like hadronization are still only understood from a phenomenological perspective. Since Z bosons are unable to transport color charges, the properties of the underlying event can be studied effectively in selections of Drell–Yan Z 0 → → ℓ ℓ + ℓ ℓ − − events, where the Z candidates and their decay products are ignored. What is left is the pure underlying event, insensitive to the physics of the hard Drell–Yan process. Other processes may suffer from misidentification issues, since they might also produce hadronic jets in the hard process.