The heavy ion Run has just concluded, marking the start of the extended year-end technical stop (EYETS). Reflecting on 2024, this year’s LHC Run exceeded expectations, delivering the highest accumulated data volume to date. In alignment with this achievement, CMS set a new record for data collection in a single year.
Above: A comparison of the current 3 years of LHC Run 3 proton-proton collisions in terms of delivered luminosity versus time.
2024 has been an exceptional year: CMS was delivered nearly 123 fb⁻¹ (inverse femtobarn) of proton-proton (pp) collisions data and 1.9 nb⁻¹ (inverse nanobarn) of lead-lead (PbPb) collisions from the LHC. It has also been a year of challenges, but CMS managed to sail through this intense year, with high pileup data taking and detector ageing, and successfully record good data for physics analyses.
In principle the LHC can collide more protons per second than can be analysed efficiently by the detectors, including CMS. The LHC thus performs “luminosity levelling”, to limit the number of collisions per second to a lower rate. This is done through careful adjustment of LHC beam parameters. The “levelled” luminosity can be maintained for many hours (normally 7-8 hours) before it gradually decreases and a new “LHC fill” is needed. In 2024 the luminosity levelling was made stable within a 2.5% margin, twice as stable compared to 2023, and this improves the stability of CMS data taking.
The integrated luminosity for pp collisions on the first plot and PbPb ones on the second, delivered by LHC and recorded by CMS.
In addition to the increased efficiency in taking data, the efficiency of deciding which data to keep was improved. The High Level Trigger (HLT) farm of computers, that analyse in real time the data from CMS, was expanded with several hybrid CPU+GPU nodes, increasing its computing power by 20%. The stability of the central DAQ system, combined with the experience of both the shifters and the on-call experts, led to an excellent level of system availability.
Above: A comparison of the number of interactions per crossing (pileup) distribution for different years of operations, from Run 1 until Run 3.
For heavy-ion data taking, a dedicated set of servers with solid-state drives was installed, adding some 250 TB of buffer space, and allowing a DAQ throughput at times exceeding 32 GB/s. This almost doubles the throughput with respect to the original Run-3 DAQ design and, once again, increased CMS efficiency at taking good data.
This year CMS also inaugurated the new control room, while the old one is now being transformed to a DAQ room, in preparation for the High-Luminosity LHC. In addition many Remote Operation Centers have been working efficiently for Data Quality Monitoring (DQM) and Trigger shifts from outside Point 5.
Above: The new CMS control room being finalised. The new control room will be used until the end of Run 3 and throughout the HL-LHC phase.
In numbers we had almost 160 days of pp collisions at 13.6 TeV, 6 days for pp collision reference run (the 5.3 TeV centre of mass energy pp collisions in preparation for lead-lead collisions), and 17 days of PbPb collisions.