(Image: CERN) Today at 13h51, the engineer in charge of the LHC at the CERN Control Centre announced that stable beams had returned to the Large Hadron Collider (LHC), marking the start of the 2025 physics data-taking campaign, the fourth year of the LHC's third operating period (Run 3*). The 2025 campaign will start with proton collisions at 13.6 TeV, and the proposed integrated luminosity targets for the LHC's four experiments are ambitious, aiming for almost as much as in 2024, although the proton-proton programme will be shorter than last year. And the more luminosity, the more collisions for the experiments and therefore more data. The lead ion campaign will take place in October and November, and the lessons learned from the 2024 campaign could lead to further optimisation, and therefore greater integrated luminosity. New in 2025: the LHC’s first oxygen ion run will take place in July. But why doesn't the physics campaign start until May? This is because between two campaigns, the equipment in CERN's accelerator complex and their experiments is stopped for a few months to be overhauled, optimised and upgraded, in what is known as the year-end technical stop (YETS). Here we would like to take you on a tour of the LHC's four main experiments - ALICE, ATLAS, CMS and LHCb - which recorded an unprecedented amount of data in 2024, and give you an overview of the work and improvements carried out during the 2024/2025 YETS. Replacement of chambers and consolidation work on the Muon Spectrometer in the ALICE cavern. (Image: ALICE) ALICE Maintenance and small repairs were carried out on several detector systems and infrastructure elements in ALICE. The largest single project was the refurbishment of the cavern crane, which was completed during the YETS. A water leak in a magnet cooling system was also repaired. On the detector systems, a cooling leak was repaired in the field cage cooling system of the Time Projection Chamber, some chambers were replaced on the muon spectrometer, and cooling circuits in several electronic crates of the Time-of-Flight system were unclogged. For the Inner Tracking System, a new low-voltage power connection scheme was implemented. For the other detectors, EMCal, TRD, and FDD, checks and minor repairs were also performed. In addition, stability enhancements were implemented in the data acquisition farm to further improve the data recording performance. With over 98% of experiment shifts already booked, ALICE is fully prepared for the upcoming run with proton, oxygen, and lead beams. ATLAS Replacement of a Thin Gap Chamber (TGC) muon detector on ATLAS Big Wheel A. (Image: Masato Aoki/ATLAS) Key interventions across the experiment focused on ensuring detector and data quality performance in 2025 and on bringing forward work from Long Shutdown 3 (LS3). Of particular note is the replacement of some muon chambers on the muon big wheels and the installation of new ones in the region between the barrel and the end-cap. The muon system also underwent a major leak repair campaign. On top of that, consolidation and maintenance work took place on all detector systems and front-end cooling leak repairs were executed on the calorimeters and the muon detectors. The ATLAS Forward Proton (AFP) detector was refurbished and additional shielding was put in place, while the ATLAS magnet system underwent its annual maintenance. In preparation for the High-Luminosity LHC (HL-LHC), infrastructure upgrades were implemented in parallel, including the installation of CO2 cooling transfer lines, the installation and connection of one cooling plant, as well as upgrade works on electrical circuits. Installation of new muon detectors – the improved Resistive Plate Chambers (iRPC) – in one of the muon end-caps, several metres away from the collision point. (Image: CMS) CMS The detector was opened up for routine maintenance and small repairs, and to install new detectors for the BRIL (the system for Beam Radiation, Instrumentation and Luminosity) at the heart of CMS. New muon detectors were also installed to improve efficiency in the forward regions. A huge amount of infrastructure for LS3 and beyond was also successfully installed, including New Forward Shielding on both sides, some of the new CO2 cooling plants underground, a new hypoxic air plant for the drying of new detectors post-LS3, and many of the main transfer lines that will transport the CO2 to the new detectors. Following the closure of CMS, data taking resumed, with the CRUZET run (Cosmic Rays at Zero Tesla) aimed at testing all the hardware and software as well as the control and monitoring systems in CMS' new Control Room. The magnet was then re-powered and the CRAFT (Cosmic Rays at nearly Four Tesla) was performed to make the first detector alignment checks. Detail of the LHCb VELO sensors during maintenance. (Image: LHCb) LHCb The Vertex Locator (VELO) sensors have been moved closer to the beam, reaching their design position at 5.1 mm from the beam core. The Upstream Tracker (UT) readout electronics have been reprogrammed to improve speed and stability. A thorough cleaning campaign was carried out on the fibres of the Scintillating Fibre (SciFi) tracker and the RICH system, as well as on the windows of the RICH system photomultipliers. New magnetic field measurements were carried out to improve the knowledge of the field map and to better determine the particle momenta, and the SMOG2 gas supply system was upgraded with gas bottles at much higher pressure, allowing a longer continuous run. In general, all LHCb sub-detectors, including the calorimeter system (CALO) and muon chambers (MUON), have seen significant improvements. So the LHC machine is officially back in business once again. The energy ramp-up has begun, the beam energies will soon reach their nominal value of 6.8 TeV, and the LHC experiments will soon start collecting collision data. ________ * Run 1 took place from 2010 to 2012, Run 2 from 2015 to 2018, and Run 3 started in 2022 and will end in 2026. Long shutdowns take place between each Run for major upgrade work to be carried out.