On 4 November at 10.20 p.m. the LHC Engineer in Charge dumped the last proton fill of 2025, which brought the integrated luminosity for ATLAS and CMS up to 125.4 fb⁻¹, exceeding the 2025 target by 5.4 fb⁻¹ and surpassing the 2024 record by 1.5 fb⁻¹. Notably, this year’s integrated luminosity was achieved in about 14 days fewer than in 2024, bringing the total luminosity delivered since the start of LHC operations to over 500 fb⁻¹.
This record performance was mirrored by the other experiments, with the LHC delivering 12.6 fb⁻¹ to LHCb for a target of 12 fb⁻¹ and 58.9 nb⁻¹ to ALICE for a target of 50 nb⁻¹. With the proton run now complete, the LHC is preparing for the lead-ion run, which will take us to the end of the 2025 run at 6.00 a.m. on 8 December.
With the final proton fill dumped, the fourth machine development (MD) block began, shortened by a day and a half compared to the initial plan. The reason for this was the non-compliant RF finger module, which limits the efficiency of some planned MD studies. Since this non-compliance will be repaired during the year-end technical stop, the MD experts decided to postpone the affected studies to 2026, ensuring more effective use of the scarce MD time and avoiding any risk of equipment failure that could compromise the upcoming lead-ion run.
Despite the adjustment, ten studies were successfully completed, amounting to 66 hours of MD time, 55 hours of which were directly related to the HL-LHC beam performance.
In the morning of 8 November, the MD programme was concluded, followed by two days of initial lead-ion beam commissioning. Then, on 10 November, a three-day technical stop began, mainly to install additional detectors – the Zero Degree Calorimeters (ZDCs) – in the experiments. The ZDCs detect neutral particles emitted close to the beamline, providing key information on collision centrality and enabling studies of electromagnetic interactions between ions. They are only inserted for low-intensity runs, such as lead-ion operation, since high-intensity proton beams could damage them.
The final two days of lead-ion beam commissioning, on 13 and 14 November, will complete the machine set-up, paving the way for the start of lead-ion physics with a short intensity ramp-up on 15 November.
The LHC can only deliver lead-ion beams once its injector chain, consisting of Linac3, LEIR, the PS and the SPS, has been properly configured for ion operation. This preparation took place over the past months and culminated on 5 November with the final dedicated lead-ion beam commissioning block in the SPS, during which the slip-stacking process was fine-tuned.
To produce the LHC lead-ion beam, the SPS receives 14 batches of four bunches each from the PS, resulting in 56 bunches of particles in total. Within each batch, the bunches are spaced by 100 ns. During momentum slip-stacking, 28 of these bunches are shifted to interleave with the other 28, reducing the bunch spacing from 100 ns to 50 ns. This effectively doubles the number of bunches that can be injected into the LHC, thereby increasing the collision rate in the experiments.
Once LHC lead-ion physics is under way, the SPS will also complete preparations for the lead-ion beam for the North Area experimental areas. The proton physics run in the North Area will end on the morning of 24 November, after which lead ions will be sent to the area to set up and tune the beamlines between the SPS and the experiments. The lead-ion physics run in the North Area is scheduled to begin on 28 November and to conclude at 6.00 a.m. on 8 December.