Speaker
Description
A subset of heavy-ion collisions are ultra-peripheral collisions (UPC), where the ions collide with a large impact parameter, causing them to interact primarily through their intense electromagnetic fields. These interactions allow for the study of various photon-induced processes at high energies. The products of exclusive UPC interactions are typically characterized by very low transverse momenta products, which present several challenges to efficiently select with a trigger. In Run 3, the ATLAS Transition Radiation Tracker (TRT) FastOR trigger provided a novel tool to trigger on these processes at Level 1. The TRT trigger was, for the first time, adapted for use in heavy-ion collisions, allowing it to trigger on low-$p_\mathrm{T}$ particles ($p_\mathrm{T} > 200$ MeV), including muons from coherent J/Psi decays. Another feature of many UPC collisions is single-sided nuclear breakup or multiple photon exchange, which causes one or both of the colliding ions to emit forward neutrons.
The Zero-Degree Calorimeter (ZDC) detects these neutrons and provides a fully digital trigger. This tool allows for triggering on many distinct neutron topologies within the Level 1 system. In Run 4, the new joint ATLAS-CMS HL-ZDC will be installed, providing similar triggering capabilities in the more challenging environment of the HL-LHC. The combination of the ZDC and TRT along with other calorimeter systems allows for the efficient collection of data for many distinct UPC processes.
| Category | Experiment |
|---|---|
| Collaboration (if applicable) | ATLAS Collaboration |
