A Novel Reaction Plane Detector for the ATLAS Experiment*

The CERN LHC began Run 3 physics data taking in July 2022. During the three years of Long Shutdown 2 (LS2), LHC experiments had the opportunity to refurbish their existing apparatus, as well as expand their physics capabilities with the implementation of new sub-systems. 

In this period, the Nuclear Physics Laboratory at the University of Illinois at Urbana-Champaign (UIUC) has developed a novel reaction plane detector (RPD) to be installed in the LHC together with the ATLAS Zero Degree Calorimeter (ZDC) for heavy ion (HI) data taking. 

The detector consists of 256 fused silica fibers that are grouped in a total of 16 channels and read out by Photo-Multiplier Tubes (PMTs). In HI collisions, the correlated transverse deflection of spectator neutrons is directly related to the reaction plane (RP) characterizing the event. The RPD enables the RP measurement in HI collisions by mapping the transverse profile of the showers generated by the spectators in the ZDC. 

In this contribution, we present the design of the new ATLAS RPD built at UIUC for the Run 3 HI program. Details about detector design, radiation hardness and integration with ATLAS DAQ system and electronics will be discussed. The RPD was tested at the SPS in July and initial results from the test will be shown.