Disentangling Flow Contributions from the Chiral Magnetic Effect in U+U Collisions with Forward-Backward Multiplicity Asymmetry

Abstract

The observation of the Chiral Magnetic Effect (CME) in heavy-ion collisions remains challenging because of large flow-induced backgrounds and experimental constraints. We demonstrate that the forward-backward multiplicity asymmetry (FBMA) provides a robust and experimentally accessible control parameter to separate the flow background from CME signal in the collisions of deformed nuclei, such as prolate uranium where FBMA is naturally enhanced and correlated with the initial-state geometry. Monte Carlo Glauber simulations indicate that varying FBMA within a fixed centrality class modulates ellipticity largely independently of the magnetic-field correlator, establishing FBMA as a practical tool for disentangling CME signals from flow driven background.

Figures (4)

• Figure 1: Top panels: Certain geometries of U+U collisions lead to an enhanced FBMA in the final state, arising from an increased relative angle between the major axes of the two nuclei at a fixed impact parameter. The beam direction is defined along the $z$-axis. Bottom panels: Pseudorapidity dependence of the charged-particle multiplicity scaled by its mid-rapidity value for the collision configurations shown, at impact parameters b = 0 and 4 fm.
• Figure 2: Probability distribution of FBMA for minimum-bias Au+Au 200 GeV (red histogram) and U+U 193 GeV (blue histogram). The blue histogram with the largest asymmetry corresponds to U+U body–tip configurations.
• Figure 3: FBMA versus FBSA for (a) Au+Au at 200 GeV and (b) U+U at 193 GeV. Results from MCGM (shMCGM) are shown by red (blue) lines. Solid and dashed lines denote 0–10% and 50–60% centrality bins, respectively.
• Figure 4: Correlator $\gamma^{B}$ (with $\Psi_{RP}$ = $\Psi_2^{PP}$, the second order participant plane) at the center of the participant zone versus $\varepsilon_{2}$ for (a) Au+Au at 200 GeV and (b) U+U at 193 GeV, for various centrality and FBMA bins. Solid symbols connected by a black dashed line represent the FBMA = 0 bins for different centralities, illustrating a proportional increase of $\gamma_B$ with $\varepsilon_2$ across centrality.