CERN Accelerating science

LHC Run 3: A Marathon of Progress and Perseverance

Federico Alessio (CERN) and Christopher Young (CERN) 19th Sep 2024

Earlier this month, the Large Hadron Collider reached an unprecedented milestone of 100 inverse femtobarns – equivalent to 10 million billion collisions – delivered to the ATLAS and CMS experiments in 2024, with 28 days of proton-proton collisions remaining. This marks a historic first in a single LHC run.

During the past two years, the LHC accelerator and all LHC experiments have consistently run at a steady pace, in the form of continuous improvements in operations, data-taking, and data quality, while also overcoming a series of obstacles, both at the level of the accelerator and the experiments. It is only fair to say that such obstacles were tackled by a relentless mix of expertise, dedication, and experience. In a daring parallel between the operational experience of Run 3 at the LHC at CERN and the spirit of the Olympic Games, the running period of 2023 and 2024 may be compared to a steeplechase race, where athletes race in a fast-paced race, overcoming obstacles of varying difficulty at roughly regular intervals to reach the end of the race successfully.

Undoubtedly, the LHC operational difficulties encountered in 2023 resulted in a degree of uncertainty about what the rest of Run 3 would look like. With the impending HL-LHC projects and the major Phase-2 upgrades of ATLAS and CMS, it became critical that a large wealth of statistics be collected by the experiments to fully exploit the Run 1+2+3 potential before the long shutdown. Such difficulties mostly refer to the issues with the RF finger modules, which limited the peak intensity per bunch to 1.6 x 10¹¹ throughout 2024, as well as a major leak that took place in the Inner Triplet to the left of IP8. Notably, the Inner Triplet incident meant the end of proton physics production for 2023 already in July, leaving the experiments with no data-taking throughout the summer and less than half of the target integrated luminosity for 2023. In the picture to the left, you can see some of the top experts of the LHC keeping the mood high in the LHC control room.

Nevertheless, a longer-than-initially-planned Lead Ion physics run took place once the Inner Triplet repair was completed in record time. However, this data-taking period was also plagued by operational limits. High background in the Inner Tracker of ALICE, limitations in the ion per bunch populations, and radiation effects in the electronics of the Quench Protection System (QPS) prevented reaching the leveled luminosity in ATLAS/CMS/ALICE to the desired value. Due to this, only ~66% of the initial targets were achieved. To aim for a more efficient ion run in 2024, a full replacement campaign of the QPS electronics has been carried out to cope with higher radiation. More targeted work in the injectors will be undertaken to reach the required intensity per bunch, and extensive simulation studies are concluding to find the best possible mitigation regarding the high backgrounds in ALICE.

All experiments had to cope with such unexpected events, 2023 being the first full production year of Run 3. Experiments generally aim at performing vast amounts of calibration runs, trigger studies, and improvements, ironing out operational aspects, organizing and training shifters—which often relies on actual live operations to be completed—and, last but not least, taking the first look at Run 3 data. By the end of the year, it became crucial that 2024 and the following years be high-physics-production years. The attention of our LHC colleagues, together with inputs from the experiments, shifted to finding the right running parameters that would allow for stable production, protection of machine elements, and consolidation of non-functioning parts for 2024 and beyond. In the picture to the right, you can see the tough discussions taking place between the Run Coordinators of the four experiments and the LHC Programme Coordinators, discussing the best possible data-taking scenarios for 2024. Morale and trust in the excellent capabilities of the LHC and the experiments remained very high!

Decisions were taken at the Chamonix 2024 LHC performance workshop that shaped the running conditions and scenarios for 2024, and by now, it’s clear how such decisions were instrumental in the success of physics production. At the time of writing this newsletter, ATLAS and CMS have reached ~100 fb⁻¹ of integrated luminosity, LHCb is close to 9 fb⁻¹, which corresponds to the entirety of Run 1+2 integrated luminosity, and ALICE around ~50 pb⁻¹, which is close to the full 2024 target. It is the current belief that such good performance can be transferred to the lead ion run, currently scheduled for November 2024, hopefully completing a very fulfilling year of data-taking. It is however fair to admit that the current optics scenario penalized the forward physics experiments FASER and SND, which have been suffering higher background rates due to the change of optics. This is aimed at preserving the machine hardware around IP1, while still collecting a good wealth of data. The current goal is to make sure a suitable solution will be found for the remainder of Run 3.

In the picture below, you can see the Run Coordinators, LHC Machine Coordinators, LHC Engineers in Charge and LHC Programme Coordinators celebrating some achievements of 2024: the current integrated luminosity counter for 2024 for ATLAS and CMS that has just surpassed the incredible number of 100 fb-1! Such smiling faces and incredible numbers give us confidence that the year(s) to come will reserve even greater successes.

Note from the editor:

More news from the Run Controls of the four big LHC experiments can be found in the following articles, published in the same issue:

Feel free to explore these links for more detailed insights.