CMS during the final stages of LS2
Views on the open CMS detector to be closed up after the Long Shutdown 2 (LS2) and to get ready for the new physics run next year. (Image: CERN)

The CMS experiment

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The open CMS detector during Long Shutdown 2 (LS2). (Image: CERN)

The Compact Muon Solenoid (CMS) is a general-purpose detector at the Large Hadron Collider (LHC). It has a broad physics programme ranging from studying the Standard Model (including the Higgs boson) to searching for extra dimensions and particles that could make up dark matter. Although it has the same scientific goals as the ATLAS experiment, it uses different technical solutions and a different magnet-system design.

The complete detector is 21 metres long, 15 metres wide and 15 metres high. The CMS experiment is one of the largest international scientific collaborations in history, involving about 5500 particle physicists, engineers, technicians, students and support staff from 241 institutes in 54 countries (May 2022).

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(Image: CERN)

LS2 upgrades

1. Beam pipe

The CMS collaboration has replaced the 36-m-long beam pipe during LS2. The new beam pipe uses an aluminium alloy that reduces the induced radioactivity by a factor of five compared to the previously used stainless steel.

An important change is a new vacuum pumping group, moved away from the detector at 16 m from the Interaction Point, to facilitate maintenance. Eight new vacuum chambers of four different types have been replaced.

2. Pixel Tracker

The Pixel Tracker is the closest CMS sub-detector to the interaction point – the point of collision between the proton beams. It reconstructs the paths of high-energy charged particles and also the decay of very short-lived particles. It is composed of concentric layers and rings of 1800 small silicon modules. Each of these modules contains about 66 000 individual pixels, for a total of more than 120 million pixels.

Due to its position in the detector, the Pixel Tracker suffers a great deal of radiation damage from particle collisions. To protect it, it is kept at -20 °C, but damage still occurs. To tackle this issue, the subdetector underwent extensive repairs and upgrades in the clean room. Its design was improved and its innermost layer replaced.


Three Beam Radiation, Instrumentation and Luminosity (BRIL) instruments dedicated to the measurement of luminosity and beam conditions have been installed: the Beam Condition Monitor “Fast” (BCM1F), the Beam Condition Monitor for Losses (BCM1L), and the Pixel Luminosity Telescope (PLT). All three BRIL subsystems represent a new “generation” in their respective design history.

The BRIL instruments measure the real-time rate of collisions at CMS, improving both the trigger rates and the quality of the beams delivered by the LHC. They continuously assess the beam conditions, to protect the LHC machine and sensitive CMS sub-detectors. Lastly, their aggregated luminosity measurements help determine the expected frequency of each type of interaction (production cross sections).

4. Hadron Calorimeter

The Hadron Calorimeter (HCAL) barrel detector measures the energies of particles from LHC collisions. It detects light produced when particles interact with scintillator tiles, measuring the amount of light with a photodetector. During LS2, CMS installed new on-detector electronics, including the replacement of old hybrid photodetectors (HPDs) with new silicon photomultipliers (SiPMs), which have a three times higher photon detection efficiency and 200 times higher gain than the old HPDs.



5. Solenoid magnet

The core element of the CMS detector is the cylindrical solenoid magnet, which has a diameter of 6 metres, a length of 12.5 metres and weighs 220 tonnes. It is cooled down to a temperature of -269°C (4 K) to make it superconducting, providing a magnetic field of up to 4 Tesla.

The CMS magnet went through major interventions and repairs to assure its maximum efficiency for the long-term future. The control and the safety systems were rebuilt and part of the electronics was completely renewed.

A new powering system was implemented to control the current flow inside the magnet. Previously, the magnet needed hours to return back to full field after power disruptions, losing valuable time for detecting particle collisions. Thanks to these upgrades, it will be only need a matter of minutes, saving important time for physics research.

6. Gas electron multiplier (GEM) detectors

In the outermost layer of the CMS experiment, new GEM (gas electron multiplier) detectors have been installed to detect muons that scatter at an angle of around 10° in relation to the LHC beam axis. Measuring muons so close to the beam axis is challenging due to the high number of particles emitted at such small angles.

The GEM chambers consist of a thin, metal-clad polymer foil, chemically pierced with millions of holes, typically 50 to 100 per millimetre squared, 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 are transferred to a collection region.

In total, 72 modules, each containing two gas electron multiplier (GEM) detectors, have been inserted into the CMS experiment.

7. Cathode Strip Chambers

For the Cathode Strip Chamber (CSC) muon detectors, installed in the forward regions of the CMS Muon System, preparing for the High Luminosity LHC (HL-LHC) poses some unique challenges.

To be ready for the increase in instantaneous luminosity (frequency of proton collisions), the CSCs needed upgraded electronics. Newly developed electronics now have high speed optical links and more powerful processors, to handle the higher particle rates with no data loss.