A New Reaction Plane Detector for the ATLAS and CMS Heavy Ion Program at the HL-LHC

The upcoming Run 4 of the Large Hadron Collider (LHC), enhanced by the High-Luminosity (HL) upgrade, will provide unique opportunities for significant advancements in heavy-ion collision experiments. This upgrade will include subsequent upgrades to the ATLAS and CMS experiments, increasing sensitivity and precision and enabling a more profound exploration of the strong nuclear force through heavy-ion interactions. Among these detectors, there is the Reaction Plane Detector (RPD) within the Zero Degree Calorimeters (ZDC) used by ATLAS and CMS. Integrated into the ZDC, the RPD will improve the characterization of collision geometry in Pb-Pb interactions, vital for studying quark-gluon plasma (QGP)—a state of matter believed to have existed shortly after the Big Bang. Positioned 127 meters from the ATLAS detector, the RPD estimates the reaction plane of interacting particles in heavy-ion collisions, aiding investigations into QGP expansion dynamics. In this contribution, we present the upgraded design of the RPD for Run 4, featuring improvements to withstand HL-LHC's higher radiation, simplified fiber routing, and a novel LED calibration system. These innovations will ensure stable RPD performance and facilitate precision studies of fundamental interactions, offering new insights into the early universe.