Upgrades of the ATLAS Zero Degree Calorimeter System for Run 3 at the Large Hadron Collider

TL;DR

This work documents the Run 3 upgrades of the ATLAS Zero Degree Calorimeter system, focusing on replacing radiation-sensitive quartz radiators with hydrogen-doped fused silica, deploying fast air-core signal cables, and implementing fully-digital triggering alongside LED-based calibration. It introduces Reaction Plane Detectors to enable event-by-event determination of the directed-flow plane, and details the integrated detector configuration, electronics (LUCROD), calibration, and Monte Carlo simulations that validate performance across 2023 and 2024 Pb+Pb runs. The results show improved energy containment, more symmetric RPD response, and robust, in-situ calibration that maintains a stable 1n peak despite significant radiation exposure. The paper also sketches future HL-LHC upgrades (HL-ZDC) that adapt the forward-calorimeter system to higher luminosities and altered TAN/TAXN geometries, ensuring continued forward physics capabilities.

Abstract

Experimental studies of ultra-relativistic heavy ion collisions at the Large Hadron Collider (LHC) depend crucially on Zero Degree Calorimeters (ZDCs) that measure neutrons produced at near-beam rapidity in nucleus-nucleus collisions. In hadronic nuclear collisions these neutrons are mainly spectator neutrons, those that do not scatter from opposing nucleons during the collision. As a result, the ZDCs provide a vital probe of heavy ion collision geometry. The ZDCs are also essential in the study of ultra-peripheral collisions that are initiated by photons associated with the electric fields of one or both nuclei. Coherent photon emission typically leaves the photon emitter intact, making the observation of no ZDC signal, on one or both sides, a tag of such processes. The ATLAS ZDCs, built prior to Run 1 were substantially upgraded for LHC Run 3. The primary upgrades included replacement of the quartz Cherenkov radiator with $\text{H}_2$-doped fused silica rods; installation of fast air-core signal cables between the ZDC and the ATLAS USA15 cavern; new LED-based calibration system; and new electronics implemented for readout and fully-digital triggering. The ZDCs were also augmented with new "Reaction Plane Detectors" (RPDs) designed to measure the transverse centroid of multi-neutron showers to allow event-by-event reconstruction of the directed-flow plane in nuclear collisions. The Run~3 ZDC detectors, including the RPDs, are described in detail with emphasis on aspects that are new for Run~3.

Figures (44)

• Figure 1: Top: A not-to-scale demonstration of the placement of the LHC TANs and the ZDCs relative to the ATLAS detector. Bottom: the ATLAS coordinate system. The blue arrows indicate possible momentum vectors in three dimensions ($\vec{p}$) or in the $x-y$ plane ($\vec{p}_T$) to illustrate the polar and azimuthal angles, respectively.
• Figure 2: Diagram showing the TAN on the A side of the ATLAS detector along with the components of the ZDC, as installed for the 2023 Pb+Pb data-taking period. The arrow indicates the direction of impinging neutral particles produced by Pb+Pb collisions. The TAN is rendered as semi-transparent to illustrate its internal structure and to allow the active portions of the ZDC to be seen.
• Figure 3: The ZDC and BRAN configuration in the C (left) and A (right) side TANs during the 2023 Heavy Ion Run.
• Figure 4: The ZDC and BRAN configuration in the C (left) and A (right) side TANs during the 2024 Heavy Ion Run.
• Figure 5: Left panel: a CAD 3D model showing the shielding nearby the TAN, as used during detector installation. Right panel: picture of the ZDC shielding, fully assembled and painted. The numbering on the picture indicates different components: front shield panels (1), (2), (3) and (4), side shielding panel (5), balcony shielding (6), built-in steps (7) and heavy-duty locking swivel casters (8).