Scientists at CERN have now completed "open-heart surgery" on one of the detectors at the Large Hadron Collider (LHC). In a complex operation that ran from 27 February to 9 March, the giant Compact Muon Solenoid (CMS) detector received a new "heart" – it's Pixel Tracker.
Detectors at the LHC, such as CMS, record the signatures of particles produced when beams of protons (or, occasionally, lead nuclei) are smashed together. The detectors are built around the LHC's beam pipe, within which the collisions take place. As the particles fly through the detectors, they traverse several layers of equipment that are tasked with making specific measurements about their properties. But, when these collisions occur, it isn't a single proton hitting another proton: several dozen simultaneous collisions take place within CMS. This phenomenon is known as "pile-up" and can be thought of as exposing a film camera to multiple images and recording all the multiple exposures in a single photograph.
The tracking system determines the trajectories of charged particles flying through it, and identifies the charge and momenta of the particles, helping to determine the origins of the various particles seen by CMS. Physicists can thus separate the overlapping collisions into individual interactions.
The CMS tracking system is made of silicon sensors and has two components that perform a complementary roles: the inner of the two is called the Pixel Tracker and the outer one is the Strip Tracker. The Pixel Tracker sees the greatest onslaught of particles flying through CMS and, unavoidably, it will lose its ability to measure the particles' properties accurately. In addition, the LHC continues to improve its performance and is expected to provide CMS with an even greater number of simultaneous interactions: even more exposures on each photograph. It had therefore been planned around five years ago to replace the original Pixel Tracker of CMS, removed earlier this year, with an entirely new one.
The new Pixel Tracker has four layers instead of the previous three in the central region (called BPIX for Barrel PIXel) and has three disks instead of the previous two capping each end (called FPIX for Forward PIXel). These additional layers raise the number of silicon pixels in CMS from 66 million to 124 million, increasing the "resolution" of the "photographs" CMS takes, so to speak.
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