Speaker
Description
The Zero Degree Calorimeters (ZDC) of the ALICE experiment at the LHC were designed to characterize the event and monitor the luminosity in heavy-ion measurement. The ZDC readout system developed for Run 3, based on a commercial 1 GSps 12 bit digitizer assembled on an FPGA Mezzanine Card, is able to operate in self-triggered mode allowing the acquisition of all collisions without dead time. The architecture of the readout system and the ZDC performance during ion-ion collisions are presented, particularly the 2024 Pb–Pb results, where the readout rate for the channels of the most exposed calorimeters was ≈ 1.4 Mevents/s.
Summary (500 words)
This contribution presents the upgrade of the readout electronics and its performance during ion-ion collisions of the Zero Degree Calorimeters (ZDC) of the ALICE experiment at LHC. The system is made of 8 readout modules, each one capable of reading 4 channels. Each module is based on a commercial 12 bit FMC digitizer (ADC_3112, 1 Gsps, 10 bit ENOB, 1 Vpp) mounted on a IFC_1211 VME FPGA-based board. Data produced by each module is handled by a Xilinx Kintex UltraScale FPGA (KU040) and is then sent to the ALICE Common Readout Unit (CRU) using two 4.8 Gbps optical links (GBT) developed at CERN. The signals produced by the ZDC channels are continuously digitized and a custom self-triggering algorithm flags for readout the relevant portion of the waveform and extracts information such as timing, baseline average and real time event rate. An automatic reset logic monitors the status of the link and self-recovers in case of failure. The operating conditions in Run 3 ion-ion are extremely challenging due to the high rate (nearly 20 times the hadronic one) of ElectroMagnetic Dissociation (EMD) processes producing a signal above threshold in the ZDC. The readout system was designed to run in self-triggered mode (ALICE continuous readout) without dead time at an event rate up to 2.5 Mevents/s. A crucial aspect of the ZDC operation in Run 3 is the acquisition of events with a reduced bunch spacing of 50 ns (lower than the length of the signal of 60 ns) in the presence of a large signal dynamics, from a single neutron (8 mV) to 60 neutrons (600 mV) that requires a refined signal processing in order to correct for pile-up. During the 2023 and 2024 Pb–Pb data taking ALICE, thanks to the new continuous readout mode and the LHC increased luminosity, acquired 80 times more collisions than Run 1 and Run 2 combined (almost 24 billion collisions). At full intensity the ZDC produces 1.4 GBps of raw data. The readout system digitizes and saves each relevant waveform allowing precise timing and energy measurements. Figure 1 depicts an example of the digitized raw data; the single and double neutron shapes are clearly distinguishable. In Run 1 and 2 the resolution of the single neutron (1n) peak was around 20%, while with the new readout the resolution improved to a 16%; an energy spectrum example is shown figure 2. The ZDC time resolution allows ALICE to reject parasitic collision. In figure 3 in red nominal interactions, in blue main–satellite and satellite–satellite interactions. The ZDC timing resolution allows to identify the 2.5 ns spaced bunches due to the LHC 400 MHz RF working frequency. During summer 2025 the ALICE experiment is expected to acquire data in O–O and p–O collisions. These collisions systems will allow the investigation of the oxygen beam fragmentation and the centrality determination in a new configuration. The readout system, the performance of the Pb–Pb 2023 and 2024 and preliminary 2025 results will be presented.
