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\title{Probing the Nuclear and Electromagnetic Structure of Heavy Nuclei at STAR}
\author{Xihe Han$^{[1]}$, Nicholas Jindal$^{[2]}$}
\date{}
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\begin{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 longer lifetime and 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. We 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.

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 $\sqrt{s_{NN}}=193$ GeV and investigate various approaches for constraining the nuclear (electromagnetic) structure of uranium.

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