Measurements of long-range correlations in small systems have resulted in the observation of very similar signatures as in large systems. The 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 interpretations 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 proceeding presents the results of a two-particle correlation analysis that explicitly rejects particles from low-$p_\mathrm{T}$ jets. The sensitivity of the correlations to the presence of $Z$-boson production in the event is also tested in $pp$ collisions.
To further understand the origin of collective behavior of small systems, it is essential to study a diverse set of systems. This proceeding presents the observation of long-range correlations in photo-nuclear collisions.
This behavior is possibly the result of the hadronic nature of the photon wave function. The process studied here is a subset of those available at the future Electron Ion Collider.