A jewel of particle physics, the CMS experiment is a 14 000-tonne detector that aims to solve a wide range of questions about the mysteries around the Higgs boson and dark matter. Now that the Large Hadron Collider (LHC) beam has been switched off for a two-year technical stop, Long Shutdown 2 (LS2), CMS is preparing for significant maintenance work and upgrades.

All the LHC experiments at CERN want to exploit the full benefits of the accelerator’s upgrade, the High-Luminosity LHC (HL-LHC), scheduled to start in 2026. The HL-LHC will produce between five and ten times more collisions than the LHC, allowing more precision measurements of rare phenomena that are predicted in the Standard Model to be taken, and maybe even detecting new particles that have never been seen before. To take advantage of this, some of CMS’s components need to be replaced.

In the heart of CMS

Hidden inside several layers of subdetectors, the pixel detector surrounding the beam pipe is the core of the experiment, as it is the closest to the particle-collision point. During LS2, the innermost layer of the present pixel detector will be replaced, using more high-luminosity-tolerant and radiation-tolerant components. The beam pipe will also be replaced in LS2, with one that will allow the extremities of the future pixel detectors to get even closer to the interaction point. This third-generation pixel detector will be installed during the third long shutdown (LS3) in 2024–2026.

Without missing a thing

Beyond the core, the CMS collaboration is also planning to work on the outermost part of the detector, which detects and measures muons – particles similar to electrons, but much heavier. They are preparing to install 40 large Multi-Gas Electron Multiplier (GEM) chambers to measure muons that scatter at an angle of around 10° – one of the most challenging angles for the detector to deal with. Invented in 1997 by Fabio Sauli, GEM chambers are already used in other CERN experiments, including COMPASS, TOTEM and LHCb, but the scale of CMS is far greater than the other detectors. The GEM chambers consist of a thin, metal-clad polymer foil, chemically pierced with millions of holes, typically 50 to 100 per millimetre, submerged in a gas. As muons pass through, electrons released by the gas drift into the holes, multiply in a very strong electric field and transfer to a collection region.

Fast-forward to the future

Some of the existing detectors would not perform well enough during the HL-LHC phase, as the number of proton–proton collisions produced in the HL-LHC will be ten times higher than that originally planned for the CMS experiment. Therefore, the high-granularity calorimeter (HGCAL) will replace the existing endcap electromagnetic and hadronic calorimeters during LS3, between 2024 and 2026. The new detector will comprise over 1000 m² of hexagonal silicon sensors and plastic scintillator tiles, distributed over 100 layers (50 in each endcap), providing unprecedented information about electrons, photons and hadrons. Exploiting this detector is a major challenge for software and analysis, and physicists and computer science experts are already working on advanced techniques, such as machine learning.

Building, building, building

CMS has also been involved with the HL-LHC civil-engineering work, which kick-started in June 2018 and is ongoing. The project includes five new buildings on the surface at Cessy, France, as well as modifications to the underground cavern and galleries.

CMS’s ambitious plan for the near and longer-term future is preparing the detector for more exciting undertakings. Stay tuned for more.

Read more in “CMS has high luminosity in sight” in the latest CERN Courier, as well as LS2 highlights from ALICE, ATLAS and LHCb.

More photos from CMS are available on CDS: