Probing the nuclear and electromagnetic structure of heavy nuclei at STAR

Abstract

Diffractive vector meson photoproduction has long been recognized as an unparalleled probe of the gluon distribution within nuclei, potentially key to elucidating non-linear QCD effects that lead to universal states of dense gluonic matter. $\phi$~meson photoproduction is particularly useful for studying small-$x$ dynamics, especially gluon saturation, due to its sensitivity to larger dipole sizes compared to, for example, $J/\psi$ and other heavy vector mesons. Compared to the $\rho^0$~meson, the $\phi$~meson has a larger mass, making it more amenable to theoretical investigation. For these reasons, the measurement of $\phi$~meson photonuclear production in A+A collisions has been long anticipated. In this talk, we present the measurement of exclusive diffractive photonuclear production of the $\phi$~meson via the $K^+K^-$ decay channel from Au+Au collisions and utilize this newly obtained measurement to compare theoretical calculations incorporating gluon saturation effects across orders of magnitude in dipole size, thereby illuminating the small-$x$ gluon distribution within nuclei. Furthermore, just as diffractive vector meson production has been employed in high-energy collisions to probe the spatial distribution of gluons within the nucleus, it has recently been demonstrated that photon photon interactions enable mapping of the photon Wigner distribution within heavy nuclei---a multidimensional image of the electromagnetic fields of high-energy nuclei. To this end, we present the measurement of $\gamma + \gamma \rightarrow e^+ e^-$ from U+U collisions at $\sqrt{s_{NN}} = 193~\mathrm{GeV}$ and investigate various approaches for constraining the nuclear (electromagnetic) structure of uranium.