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