Speaker
Description
Measurements of two-particle correlations in $pp$ collisions show features that are strikingly similar to those seen in heavy-ion collisions, suggesting that a tiny droplet of the QGP is produced even in such collisions. In the $pp$ collisions models that attribute the correlations to semi-hard processes can also qualitatively reproduce the measurements. In this talk, we report on a series of ATLAS measurements exploring detailed properties of flow in small, medium, and large collision systems.
New ATLAS measurements of two-particle correlations with active selection on particles associated with jets from the event are performed to elucidate the origin of the long-range correlations. If the correlations are indeed generated by semi-hard processes, then the long-range correlations between particles associated with jets would be stronger than the inclusive hadron correlations, while removing jet-associated particles would weaken the correlations.
Additionally, measurements of the azimuthal anisotropy in $p$+Pb collisions reaching the transverse momentum of charged particles up to 50 GeV, in minimum-bias and jet-triggered events are presented. In the jet-triggered events, $v_2$ is non-zero over the entire kinematic range of the measurement, and is $\approx 2$-$3$% at $p_\mathrm{T} \approx 50$ GeV.
In large collision systems, we focus on understanding the longitudinal structure of the initial-state in heavy-ion collisions as a key for modeling the early-time dynamics. This talk presents results of flow decorrelations in Xe+Xe and Pb+Pb collisions.
In $AA$ collisions, the decorrelations for $v_2$ show a strong centrality and $p_{\mathrm{T}}$ dependence, while no such dependencies are observed for $v_3$ and $v_4$ decorrelations. Decorrelations in Xe+Xe collisions, when compared to Pb+Pb collisions, are found to be larger for $v_2$, but smaller for $v_3$. These system-dependent trends are not reproduced in current initial-state models when coupled with hydrodynamic evolution.
