Photo of LHCb collaboration members gathered in the control room in April 2024.

The LHCb experiment achieved significant milestones by the end of 2023, despite the challenges related to the LHC vacuum incident in the Vertex Locator (VELO) volume and the premature stop of the proton run in July. When the LHC restarted for the heavy-ion run, LHCb recorded an unprecedented sample of lead-lead (PbPb) collisions, down to a centrality never achieved before due to the power of the newly upgraded detectors, as shown in Figure 1. Additionally, by injecting gas into the VELO vessel via the SMOG system, collisions between lead ions and argon atoms (PbAr) in a fixed-target mode occurred simultaneously with the primary ion interactions.

Building on the progress of 2022 year-end and 2023, this year was shaping up to be a critical and demanding time for the LHCb experiment. The collaboration faced the task of quickly getting the upgraded detector systems fully operational and calibrated to collect a vast amount of data.

Fig.1: Example of a peak in invariant mass, indicating the presence of a particle containing a charm quark, as reconstructed by the LHCb experiment using data from 2023 PbPb collisions. This data is illustrated in LHCb-FIGURE-2024-004.

One of the major challenges was ensuring that the VELO detector, which had been successfully repaired over the Year-End Technical Shutdown, could operate in a fully closed position around the collision region at the required interaction rates, five times larger than those of the previous generation of the LHCb experiment. Additionally, the Upstream Tracker (UT), whose commissioning could not be completed in 2023 due to the shortened proton-proton run, needed to be swiftly integrated into global data-taking along with the other subdetectors. The online selection systems, receiving data at unprecedented rates of 4 TB/s from the new readout architecture, had to handle this load effectively for the first time at the planned instantaneous luminosity. To acquire data of high quality, online alignment and calibration processes had to be commissioned, executed in real-time, and in an automated way. Another significant challenge was ensuring the entire data chain, from online data acquisition to offline processing, was up to speed.

The first weeks after the restart of the LHC have been extremely intense: every minute of proton-proton collisions was used to perform calibration scans to fine-tune the subdetectors' performance, complete the commissioning of the UT, and push the system to operate at the nominal instantaneous luminosity.

The first major milestone of 2024 was already achieved in April when the UT was successfully integrated with the other subdetectors for data acquisition. Moreover, the selected data began to be transferred continuously to offline storage, making it available for analysis. This success, which brought a sense of optimism for the rest of the year, was met with celebration in the LHCb control room as shown on the cover photo. 

The prompt feedback from data analysts to the control room team was essential in improving reconstruction and alignment algorithms and fine-tuning the selection of the data to save on disk. As the LHC entered its luminosity production phase, the LHCb experiment—bolstered by further progress in automation—was able to operate efficiently at a luminosity close to its design target. The data collected during this phase revealed that the performance of the various subsystems surpassed that of the previous generation, even under the increased number of interactions per bunch crossing. One of the key innovations in the LHCb upgrade, the removal of the hardware-level trigger, proved to be highly effective. This change led to a significant improvement in the selection efficiency for many processes of interest, as shown in Figure 2.

Fig.2: Graph showing the improved online selection efficiency for mesons containing a beauty quark in 2024 (red points) compared to Run 2 (black points). This improvement was achieved by removing the hardware level trigger, as shown in LHCb-FIGURE-2024-014.

Following the June Technical Stop, significant progress in the calibration of the UT allowed it to be consistently included in global operations, further enhancing the physics opportunities of the experiment. Owing to an extremely high data-taking efficiency, LHCb has by now collected a proton-proton data sample corresponding to an integrated luminosity close to 8/fb in 2024, putting the experiment on track to meet the expectations set for Run 3, despite the challenges of the previous years. When comparing the integrated luminosity with that of LHCb during LHC Run 1 and 2 (up to 2018), the difference is striking.

In August, new online resources were added to boost the LHCb experiment’s capabilities, including enhancements to the trigger system. In the remainder of 2024, the goal is to make these upgrades operational to increase the instantaneous luminosity delivered to LHCb by 20%, allowing the experiment to run at full capacity. Looking further ahead, there is great anticipation for the year-end lead-lead run. For the first time, LHCb will record PbPb and PbAr collisions with both the VELO fully closed and the UT actively participating in the reconstruction of events. Additionally, with the VELO fully closed, the gas will be injected by the SMOG2 system in a storage cell, significantly increasing its interaction rate with the lead ions.

Figure 3. Curves showing the integrated luminosity recorded by LHCb, with the steepest curve representing data collected in 2024 up to the beginning of September.

The experience gained during the LHCb upgrade and this first year of data collection in nominal conditions will also be crucial for future generations of the experiment. Plans are already underway for another major upgrade that aims to increase the luminosity of LHCb by about 10 times in 10 years, further pushing the boundaries of what can be discovered at the HL-LHC.

In summary, 2024 has showcased the full potential of the new LHCb experiment, setting the stage for exciting new discoveries and advancements in the field of flavour physics and in the search for new physics at the LHC.