Speaker
Description
Ultra-peripheral heavy-ion collisions provide a clean environment for studying photon--nucleus and photon--nucleon interactions. We present measurements of $\phi$-meson production in ultra-peripheral Au+Au collisions at $\sqrt{s_{NN}} = 200$~GeV with the STAR experiment at RHIC, performed via the $\phi \rightarrow K^{+}K^{-}$ decay channel. Those are the first RHIC studies of coherent and incoherent $\phi$-meson photoproduction. The two production mechanisms are separated using the transverse momentum of the reconstructed $\phi$ meson. Coherent production dominates at low $p_T$ and incoherent production at higher $p_T$ due to the different momentum-transfer scales of photon--nucleus and photon--nucleon interactions. The enhanced low-momentum tracking capability provided by the iTPC upgrade enables access to coherent $\phi$ production at STAR. Differential cross sections are extracted as functions of $p_T$, $p_T^2$, and rapidity for both production modes. The incoherent-to-coherent cross-section ratio is compared with predictions from STARlight and calculations based on the impulse approximation, providing sensitivity to possible suppression effects in photon--nucleus interactions. We also report results on azimuthal angular correlations, including $\cos(2\Delta\phi)$ modulations at higher transverse momentum.
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