Measurements of long-range correlations in $\mathrm{\gamma A}$ and $pp$ collisions with ATLAS

Measurements of long-range correlations in small systems has resulted in the observation of very similar signatures as large systems. These correlations in large systems are known to arise from the hydrodynamic flow of the quark-gluon plasma (QGP). In small systems like \pp collisions, the interpretation of these collective signatures are much more diverse, ranging from a small droplet of QGP to initial state correlations. To test the origin and sensitivity of these correlations to semi-hard processes, this note presents the results of a two-particle correlation analysis that explicitly rejects particles from low-\pt jets. Also tested in \pp collisions, is the sensitivity of correlations of underlying-event charged particles to the presents of $Z$-boson production. To further understand the origin of collective behavior of small systems, it is essential to study a diverse set of systems. This note presents the observation of long-range correlations in photo-nuclear collisions. This observation is possibly the result of the hadronic nature of the photon wave function at a range of virtuallities of these collision which will be a subset of the photon virtuallities probed by the future Electron Ion Collider.