Optimizing a New Reaction Plane Detector for ATLAS and CMS in Heavy-Ion Run 4*

Studying heavy ion collisions provides insights into the nature of quark-gluon plasma (QGP), a state of matter that existed shortly after the Big Bang. The ATLAS experiment at CERN uses a Zero Degree Calorimeter (ZDC) with a Reaction Plane Detector (RPD) to characterize the event geometry in heavy ion collisions and investigate the properties of QGP. For Run 4 and beyond, a new High-Luminosity (HL) ZDC will be constructed.

To advance the RPD, integrating a new LED calibration system is crucial. Development and testing of this system includes pulsing photo-multiplier tubes (PMT) directly rather than using routing fibers from the LED board. Experience from the ATLAS Run 3 RPD aids in determining necessary design modifications to enhance radiation hardness and accommodate Run 4's smaller space constraints. Methods to optimize the RPD include creating physical detector mock-ups to ensure feasibility and analyzing experimental data of PMT responses to refine the overall inner configuration. These efforts aim to improve the performance and reliability of the RPD in the high-radiation environment of the HL-ZDC.

This contribution presents the current status and future plans for upgrading the RPD section of the HL-ZDC, with a focus on optimizing the active area design and assembly.