After more than two years of maintenance and upgrades, the Pixel Tracker has been installed at the center of the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC) and is now ready for commissioning.
Of all the CMS subdetectors, the Pixel Tracker is the closest to the interaction point (IP) – the point of collision between the proton beams. In the core of the detector, it reconstructs the paths of high-energy electrons, muons, and electrically charged hadrons, but also the decay of very short-lived particles such as those containing beauty or “b” quarks. Those decays are used, among other things, to study the differences between matter and antimatter.
The Pixel Tracker 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 120 million pixels. The pixels’ tiny size (100×150 μm2) allows the trajectory of a particle passing through the detector to be precisely measured and its origin determined with a precision of about 10 μm.
Due to its location very close to the IP, the Pixel Tracker suffers a great deal of radiation damage from particle collisions. In the innermost layer, a mere 2.9 cm away from the beam pipe, around 600 million particles pass through one square centimeter of the detector every second. Low temperatures help to protect the Pixel Tracker from this high radiation (it is kept at -20 °C), but some damage still occurs.
To tackle this issue, the subdetector underwent extensive repairs and upgrades in the clean room where it was stored after its extraction from the cavern at the beginning of Long Shutdown 2. Its design was improved and its innermost layer replaced. The pixel detector was then reinstalled at the center of the CMS detector and is now ready for commissioning.
The final installation was the latest of the many achievements of the CMS Tracker group, one of the largest sub-groups of the CMS collaboration with about 600 people from over 70 institutions in 19 countries.