Measurement of $ϕ$-meson production in Cu$+$Au at √s_NN=200 GeV and U$+$U at √s_NN=193 GeV

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The PHENIX experiment reports systematic measurements at the Relativistic Heavy Ion Collider of $ϕ$-meson production in asymmetric Cu$+$Au collisions at √s_NN=200 GeV and in U$+$U collisions at √s_NN=193 GeV. Measurements were performed via the ϕ→ K⁺K⁻ decay channel at midrapidity $|η|<0.35$. Features of $ϕ$-meson production measured in Cu$+$Cu, Cu$+$Au, Au$+$Au, and U$+$U collisions were found to not depend on the collision geometry, which was expected because the yields are averaged over the azimuthal angle and follow the expected scaling with nuclear-overlap size. The elliptic flow of the $ϕ$ meson in Cu$+$Au, Au$+$Au, and U$+$U collisions scales with second-order-participant eccentricity and the length scale of the nuclear-overlap region (estimated with the number of participating nucleons). At moderate pT, $ϕ$-meson production measured in Cu$+$Au and U$+$U collisions is consistent with coalescence-model predictions, whereas at high pT the production is in agreement with expectations for in-medium energy loss of parent partons prior to their fragmentation. The elliptic flow for $ϕ$ mesons measured in Cu$+$Au and U$+$U collisions is well described by a (2+1)D viscous-hydrodynamic model with specific-shear viscosity $η/s=1/4π$.