We investigate charm-quark dynamics in asymmetric $\mathrm{Cu}+\mathrm{Au}$ collisions at top energies available at the BNL Relativistic Heavy Ion Collider using a Langevin approach embedded in a realistic hydrodynamic background. The intrinsic asymmetry of the colliding nuclei leads to a spatially lopsided initial energy-density profile, which generates a dipolar flow structure in the transverse plane even at midrapidity. As charm quarks propagate through this medium, they acquire a finite directed flow, $v_1$. We find that the $p_T$-integrated heavy-quark $v_1$ is approximately an order of magnitude larger than that of charged hadrons. In addition, the $p_T$-differential $v_1$ exhibits strong sensitivity to the initial spatial distribution of heavy quarks, highlighting the potential impact of preequilibrium modifications of the heavy-quark phase-space distribution on final-state anisotropies. Beyond geometric effects, $v_1$ also provides direct sensitivity to medium interactions through the temperature-dependent drag coefficient. Its pronounced dependence on this transport input indicates that precision measurements of heavy-flavor directed flow could place meaningful constraints on heavy-quark transport coefficients, thereby improving Langevin-based descriptions and predictive power for heavy-flavor observables in heavy-ion collisions.

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• Particle & resonance production
• Quark-gluon plasma
• Relativistic heavy-ion collisions
• Heavy mesons
• Langevin equation