Here's what open-heart surgery at the LHC looks like

March 15, 2017, CERN
Preparations being made in the underground experimental cavern of CMS prior to the installation of the second-generation Pixel Tracker of CMS. Credit: CERN

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 FPIX disks were manufactured by 19 institutes in the US. They can be seen here at the CMS Tracker Integration Facility at Meyrin, Switzerland before being taken to the CMS experimental site outside Cessy, France for installation. The Pixel Tracker’s various components were stored and tested carefully on the surface in a clean room prior to installation. Credit: Maximilien Brice/CERN

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 ' 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.

To be installed within CMS, the various components of the Pixel Tracker had to be lowered by crane down the 100-metre-deep shaft into the underground experimental cavern of CMS. They were then raised by a second crane onto the installation platform for insertion. This image shows the first half of the BPIX located inside its “cassette” being placed on this platform before being inserted into the CMS detector. The BPIX, manufactured by 23 institutes from eight European countries, is only the size of a shoebox, but has a large number of electronics and cooling components that go with it. Credit: Maximilien Brice/CERN

Surgery in action! Appropriate protection during installation of the FPIX prevents contamination of the device. Credit: Maximilien Brice/CERN

Explore further: Cool running for Large Hadron Collider's CMS Tracker

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