The ATLAS Zero Degree Calorimeter and High-Luminosity Zero Degree Calorimeter
The ATLAS Zero Degree Calorimeter (Run 1-3)#
The ATLAS Zero Degree Calorimeter (ZDC) is the most forward detector of the ATLAS experiment at the CERN Large Hadron Collider. Located approximately 140 m from the interaction point inside the Target Absorber for Neutrals (TAN), it is designed to detect neutral particles emitted at pseudorapidities beyond |η| > 8.3. The ZDC plays a unique role in the ATLAS heavy-ion program by measuring forward spectator neutrons produced in nuclear collisions. These measurements provide essential information on collision geometry and centrality, while also enabling the identification and study of ultra-peripheral collisions (UPCs), in which nuclei interact through their electromagnetic fields rather than via direct hadronic interactions.
Originally installed for the first LHC heavy-ion run in 2010, the ATLAS ZDC has operated throughout Runs 1, 2, and 3 during all heavy-ion data-taking campaigns, including A+A and p+A collisions, as well as dedicated low-μ pp runs used for heavy-ion reference measurements. The detector has also participated in dedicated forward-physics runs in conjunction with the LHCf experiment, providing efficient identification of forward neutrons relevant for cosmic-ray studies.
Since the beginning of LHC heavy-ion operations, the ZDC has been an essential component of the ATLAS heavy-ion program. In hadronic nuclear collisions, it measures spectator neutrons—nucleons that do not participate in the interaction—which provide a direct probe of the collision geometry. In ultra-peripheral collisions, the ZDC is used to tag electromagnetic interactions through the pattern of forward neutron emission, enabling measurements of photon-induced processes. The detector also contributes to heavy-ion luminosity determination and supports dedicated studies of forward neutron production.
Ahead of Run 3, the detector underwent a major upgrade to significantly enhance its performance. The upgrade included new radiation-hard fused-silica Cherenkov radiators, improved optical coupling and faster signal transmission, fully digital readout electronics, an integrated LED calibration system, and the installation of the new Reaction Plane Detector (RPD). The RPD provides fine transverse segmentation of the forward energy deposition, enabling event-plane measurements and improving the detector’s capability to characterize the geometry of heavy-ion collisions. Together, these upgrades substantially enhanced the detector’s precision, timing performance, and operational reliability for the Run 3 heavy-ion physics program.
The ATLAS High-Luminosity Zero Degree Calorimeter Project (Run 4++)#
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