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

Abstract

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 number allows the charge difference ($ΔQ$) to be extracted with a double-ratio technique that suppresses most experimental systematic uncertainties. By varying the beam energy, the rapidity gap ($Δy$) over which electric charge is transported can be systematically scanned. Simulations of Ru+Ru and Zr+Zr collisions at $\sqrt{s_{\rm NN}}$=19.6-200GeV with UrQMD and PYTHIA Angantyr show that midrapidity $ΔQ$ decreases exponentially with increasing $Δy$, with the slope parameter exhibiting strong model dependence. Comparisons with the baryon number transport reveal distinct patterns. In both UrQMD and PYTHIA 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.

Figures (7)

• Figure 1: UrQMD simulations of rapidity distributions of $\pi^{\pm}$, $K^{\pm}$, $p$, and $\bar{p}$ for 0--20% (left) and 60--80% (right) centrality classes in Ru+Ru collisions at $\sqrt{s_{\rm NN}}$ = 200 (top) and 19.6 (bottom) GeV.
• Figure 2: Centrality dependence of $\Delta Q$ within $|y|<0.5$ for (i) all charged particles, (ii) charged $\pi$, $K$, $p$, and (iii) charged $\pi$, $K$, $p$ based on the double ratio method, for 200 GeV Ru+Ru and Zr+Zr collisions simulated with UrQMD.
• Figure 3: Rapidity distributions of $\Delta Q = Q_{\rm Ru+Ru} - Q_{\rm Zr+Zr}$ for 0--20% and 60-80% centrality classes at $\sqrt{s_{\rm NN}}$ = 200 (left) and 19.6 (right) GeV.
• Figure 4: Distributions of $\Delta Q$ within $|y|<0.5$ against the rapidity gap ($\Delta y$), over which the electric charge is transported, for 0--20% (right) and 60-80% (right) Ru+Ru and Zr+Zr collisions simulated with UrQMD. Since $\Delta Q$ is evaluated around midrapidity, $y_{Q} = 0$. Similar distributions for $Q_{\rm Ru+Ru}$ and $Q_{\rm Zr+Zr}$ are also shown for comparison.
• Figure 5: Centrality dependence of the slopes for the $\Delta y$ dependence of $Q_{\rm Ru+Ru}$, $Q_{\rm Zr+Zr}$, and $\Delta Q$ within $|y|<0.5$ for Ru+Ru and Zr+Zr collisions. Different panels correspond to different event generators or generator configurations. Vertical bars around data points indicate fit errors on slopes.