Measuring charge transport in high-energy nuclear collisions with an energy scan of isobaric collisions

We present a method to measure electric-charge transport in high-energy nuclear collisions using a beam-energy scan of isobaric systems. Comparing collisions of nuclei with identical mass number but different atomic numbers allows the charge difference ( Q Δ Q ) to be extracted with a double-ratio technique that suppresses most experimental systematic uncertainties. By varying the beam energy, the rapidity gap ( y Δ y ) over which electric charge is transported can be systematically scanned. Simulations of Ru+Ru and Zr+Zr collisions at √sNN=19.6 s NN = 19.6 –200 GeV with UrQMD and Pythia 8 Angantyr show that midrapidity Q Δ Q decreases exponentially with increasing y Δ y , with the slope parameter exhibiting strong model dependence. Comparisons to baryon-number transport reveal distinct patterns. In both UrQMD and Pythia 8 Angantyr (with and without final-state baryon junctions), where baryon number is carried solely by valence quarks, the rapidity slope for baryon transport is larger than that for electric-charge transport. In contrast, scenarios that include baryon junctions in the initial state are expected to produce the opposite trend. This demonstrates that an isobar beam-energy scan provides a sensitive probe of electric-charge transport and offers new constraints on the microscopic mechanisms governing conserved-charge redistribution in QCD matter.