2024 was an impressive year: the most data ever collected by CMS, many important results including two landmark results (W boson mass and top entanglement), milestone pieces of the upcoming Hi-Lumi CMS detector reached, and ever more innovations pushing the frontiers of how we explore physics. But what does 2025 hold for CMS?
This will be the last full year of the LHC Run 3, and is an incredibly important one across several different areas.
Run 3
The current data taking period, Run 3, started in 2022 and will continue through to the middle of 2026, which makes 2025 the last full year of the Run! The year will consist of proton collisions and then, towards the end of the year, heavy-ion collisions.
CMS looks forward to good proton-proton production to collect good quality data so that it can integrate 100 fb-1 of proton collisions by the end of the year. CMS will use advanced trigger menus to down-select the raw data so that only interesting data are recorded. The work done last year on anomaly detection triggers is an exciting development that will be leveraged throughout the year. Hafeez Hoorani, Deputy Spokesperson specifies that “We aim to run with a trigger rate of 110 kHz, with a dead time of less than 5% with a full L1 trigger menu. We need to maximise what we select – to include most of the physics reach of CMS by selecting all possible L1 triggers.”
The heavy-ion collisions in 2025 will aim to reach the full anticipated Run 3 luminosity of 6.5 nb-1, allowing more studies of the quark-gluon plasma and the beginnings of our universe! If it is achieved, it would mean that any additional data collected in 2026 will be a huge bonus to the teams. You can find out more about the quark gluon plasma here.
However, in order to take advantage of all the potential data, the CMS detector and all of its infrastructure needs to run smoothly throughout the year, so this is a core priority for Technical Coordination teams. The CMS detector requires careful attention leading up to and through data taking to make sure it is functioning at its peak, and 2025 is the last full year that this detector will be in operation before the massive transformations starting mid-2026.
Analysis
Over the last six years, many papers have been published on the data collected during Run 2, the previous data taking period between 2015-2018.
This year will see a shift in analyses to prioritise data collected in Run 3 so far, including the newest 2024 data.
The two focal points for the analyses are precision measurements and searches. Precision measurements, such as that of the mass of the W boson announced last year, often benefit from more data. Many precision measurements study very rare processes, so by combining the vast amounts of Run 3 data already collected with the Run 2 data from the past, there are more examples of those processes to study.
For example, one analysis area that will be of great interest in 2025 is di-Higgs research: the study of two Higgs bosons produced at the same time in a single proton-proton collision. The interaction of these particles allows us to test our understanding of models within both particle physics and cosmology, and provides insights into the stability of the universe. However, in the whole of Run 2, there would have been only three to five thousand examples of this simultaneous production - and only a small proportion of those examples are picked up by the detector. By pushing the analyses to incorporate Run 3 data as well, more can be identified and studied.
Another exciting analysis area for 2025 is in top physics, particularly when looking at the production of three top quarks, which puts stringent tests on our understanding of the Standard Model of particle physics. 2024 was a big year for top physics, and so will 2025!
The use of parked data was a focus last year too, and this coming year it will allow CMS to explore areas such as vector boson fusion and some corners of bottom physics. In fact, CMS’ capabilities in b physics make it very competitive with LHCb, especially in exploring cp violation.
With over 100 analyses in the pipeline for the upcoming physics conferences, there are many more interesting results to come! These will be presented at Moriond, EPS, LHCP and other conferences throughout the year.
Above: Infographic demonstrating where we are in CMS' history. (Design by Ansar Iqbal)
Hi-Lumi
Midway through 2026 the experiments – like the whole LHC – will shut down and go into an intense period of preparation for its next incarnation. The ‘Long Shutdown 3’ is the pause in new data collection while the LHC and CMS are being transformed, ready to emerge as completely new machines for the next collision run, Run 4 – the LHC’s highest ever luminosity run.
With it comes Hi-Lumi CMS! Although many parts of the existing detector will remain, others will be drastically upgraded, and several brand new, cutting-edge systems will be built and installed. These will allow CMS to be more selective on the collisions it deems interesting, and to collect much more useful and detailed information about these collisions, despite the collisions at HL-LHC being 2-3x more complex than at present. This will be achieved by including new detectors with higher granularity than the present ones (tracker and endcap calorimeters), including cutting-edge electronics on the existing and new detectors, and significantly increasing the off-detector processing power.
Critically, the CMS magnet will remain a core part of the detector, an incredibly powerful solenoid that generates a magnetic field about 100,000 times that of the Earth, and so maintaining its functionality is essential to the longevity of the experiment. It is irreplaceable!
In order for the ambitious build to take place over the Long Shutdown 3, work for the new Hi-Lumi detector has already been underway for several years. 2025 is a pivotal year to make sure CMS hits the milestones it needs to successfully pull off the build.
The ramp up to full production starts in 2025, both for electronic components and module production of the new parts of the CMS detector. This is extremely challenging: the level of perfection and precision needed is extreme, but it is essential this work takes place over the course of this year to make sure that the teams can then start integrating bigger pieces together in within the required timelines.
The components and modules are being constructed and tested all over the world at many of the institutes collaborating on CMS. The expertise of each group is coming into play at every stage. Once these are completed, they will be sent to CERN to be put together for the next installation steps.
“It is actually a perfect moment for students and postdocs to join these efforts as they get all the experience and expertise in deeply understanding the system for the coming years. This includes all the bespoke software that is being developed and that will be key in the future,” comments Upgrade Coordinator Frank Hartmann.
In parallel, any work that can be anticipated to upgrade the experiment infrastructure at P5, the home of the CMS detector in Cessy, France, is already well underway, and will be largely completed before the start of the LS3. Legacy detector systems will be consolidated and new ones installed in anticipation of the main bulk of the upgrade activities.
For example, the New Forward Shielding (NFS) is a part of the Hi-Lumi CMS detector that is already being used, with one end already installed at the beginning of 2024 and the second to be installed in early 2025. The shielding creates a physical barrier around the beam pipe to drastically reduce unwanted signal background being picked up by parts of the CMS detector. Once both halves of the NFS are installed, CMS will see a reduction in background noise uniformly, rather than just from one side as it did in 2024 with only half the shielding in place. This means that the data in 2025 can be compared to simulations of what should be picked up – or in this case, what shouldn’t be picked up! With the comparison in place, the simulations can then be adjusted for the High Luminosity run (Run 4), and to see whether the shielding is sufficient for the experiment’s needs or whether other actions need to take place in preparation.
“We need to make sure that nothing is missing in our considerations for Run 4,” explains Technical Coordinator Paola Tropea, “we need to know if we need to change our plans to make sure everything works and use this last year to further prepare for LS3.”
As much as is possible, the surface infrastructure for Long Shutdown 3 and Hi-Lumi CMS should be built or well underway in 2025. This includes areas such as the new Radiation Protection, Handling, and Storage building, a structure purpose-built to safely store, handle, and treat parts of the detector that have been exposed to radiation. But it also includes infrastructure for key systems such as purpose-built areas for Data Acquisition purposes, the commissioning of a second hypoxic plant to provide low oxygen content to dry the air into the cold detector volumes, and CO2 cooling installations used to keep key parts of the detector very cold to mitigate the effects of radiation damage.
At a technical level, there is still work to be done to continue and complete reviews of the difference upgrade projects, some still to undergo their Engineering Design and Electronics Systems Reviews (EDRs & ESRs). Once these are in place, the manufacturing of many detector components can get underway. Finalising the plan for the vast amount of person power needed for the build of Hi-Lumi CMS is also a top priority in 2025. The intricate expertise needed will be huge for the Long Shutdown 3, and 2025 moves towards this massive call to welcome more people onto CMS sites to build its future.
People
Future-proofing the experiment for all the people involved is in fact a priority across the collaboration and not just at the technical level – and this it is not a trivial task, with over 6000 people making up the fabric of CMS.
Across all areas – be they engineering, construction, analysis, administrative, etc – there is a push to make sure CMS is ready to take on the challenge of the High-Luminosity era. The requirements for such a jump are being studied by a specialised reflection group looking at how the experiment as a whole should evolve in the coming years.
Specifically, the Diversity, Equity and Inclusion Group and the Young Scientist Committee are dedicated to making the future of the collaboration tailored and welcoming to all members, and their work will continue to be a vital part of the collaboration going forward.
The Collaboration Board, made up of representatives who take strategic decisions and discuss major directions for the experiment, will prioritise engagement in 2025:
“We believe the success of our collaboration is only possible with continuous maximal engagement in all areas. We should all remain fully engaged as members of the collaboration working towards our shared goals, but also the Collaboration Board should maintain a close working relationship with team leaders and institute representatives.” - Scientific Secretary of the CMS Collaboration Board, Didar Dobur.
Keeping the experiment running also means taking on shifts, committing to getting publications out efficiently, strong and increasing participation in the Hi-Lumi activities - especially looking forward to 2026. The work of the individual committees thus far has been greatly appreciated by all – a heartfelt thank you to all members who have made time to be part of them.
CMS looks forward to an incredibly interesting year: intense but exciting, and definitely challenging! But it is a real moment for the collaboration to come together and balance all the priorities that will govern 2025.
Throughout it all, the CMS Communications Group will ensure that all of these parallel activities are showcased to internal and external audiences alike – so make sure to stay tuned!
Links
Most Data ever collected by CMS in 2024
ECAL anomaly detection triggers
New Forward Shielding installed in 2024
Find out more about the quark gluon plasma