When heavy nuclei collide in accelerators, a multitude of particles is produced, and physicists want to understand how electric charge moves through this chaos. Two trains crash into each other: the passengers (quarks) scatter, but their luggage (charge) may end up far from the crash site. To measure how far the charge is carried, the authors propose comparing collisions of ruthenium (Ru) and zirconium (Zr) nuclei. They have the same number of nucleons (particles in the nucleus) but different numbers of protons, so the difference in charge is known precisely. This trick allows subtracting many measurement uncertainties.
By varying the collision energy, one can change the "window" in rapidity — a quantity related to the velocity of particles along the beam. The larger the rapidity difference between two points, the longer the path the charge must travel to get there. Simulations show that the amount of charge at the center of the collision falls exponentially with this difference, but the rate of falloff strongly depends on the model. This means the experiment can distinguish between different theoretical descriptions.
This approach is reminiscent of studying river flow by tracking the spread of dye dumped into it: if you know how much dye was released at one point, you can infer the flow speed and turbulence from its concentration downstream. Similarly, knowing the charge difference between isobar nuclei, physicists can trace how charge "spreads" in rapidity, thereby extracting information about the properties of the quark-gluon plasma — the state of matter that existed in the first moments after the Big Bang.
Ruthenium and zirconium nuclei have nearly the same mass but differ in charge by 4 units: ruthenium has 44 protons, zirconium has 40. This small difference is enough to observe the effect in an experiment.