Speaker
Description
Photonuclear collisions are one of the simplest processes that can happen in a heavy-ion collision. They occur when one nucleus emits a quasi-real photon which interacts with the other colliding nucleus, similar to an $e+A$ collision except that the photon tends to have a much smaller virtuality. Photonuclear collisions can be used to study bulk properties of the medium such as collectivity due to initial-state effects and hadron chemistry. Results are presented for identified $\pi^\pm$, $K^\pm$, and $p(\bar{p})$ spectra in photonuclear collisions at STAR for $\rm{Au}\rm{+}\rm{Au}$ collisions at $\sqrt{s_{NN}} = 54.4~\rm{GeV}$. Significant baryon stopping and rapidity asymmetry is observed at low transverse momentum, which could indicate the existence of a baryon junction within the nucleon, a nonperturbative Y-shaped configuration of gluons which carries the baryon number and is attached to all three valence quarks. Measuring the same spectra using the 2019 $\rm{Au}\rm{+}\rm{Au}$ dataset at $\sqrt{s_{NN}} = 200~\rm{GeV}$ shows how these effects change as a function of beam energy. Measurements of particle spectra and their rapidity dependence in photonuclear events will give insight into the origin of small-system collectivity, the gluon structure of the nucleon and will help inform future measurements using particle identification at the Electron Ion Collider.
