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Heart surgery at CERN

The CMS experiment at CERN gets a new pixel detector

During the recent service pause of the CERN Large Hadron Collider (LHC), a key component of the CMS experiment was replaced at the beginning of March: the new pixel detector is even more powerful than its predecessor – raising hopes for new insights in elementary particle physics.

The pixel detector is pushed into the belly of the CMS experiment. In the middle with a flashlight: Prof. Roland Horisberger, the 'father of the pixel detector'.
Immagine: M. Brice / CERN

Heart operations are usually carried out in hospitals on patients with heart ailments. The heart operation that took place near Geneva in early March, however, did not take place in a hospital nor was it performed on a patient. It is nevertheless suited to describe the recent intervention on the Large Hadron Collider (LHC), the large particle accelerator at CERN, as a 'heart operation'. The component that was replaced during the early days of March is not only located at a central place, it is also indispensible for physical research at the LHC.

100 m Below the Ground

To understand this, you would have to hire a taxi at Geneva Airport, cross the Swiss-French border and drive to the French village of Cessy, a 15-minute drive away. On the green meadow, there is a building, which at first glance looks like a factory. But nothing is produced there; it is a place of research. The building houses the CMS experiment, one of the most refined physical experiments in the history of mankind. The CMS experiment (CMS stands for: Compact Muon Solenoid) is located 100 m below the earth's surface in a huge hall. Here one finds a colossus of several thousand tons that resembles the dimensions of a multi-family house, packed with cables, electronics and a huge magnet: the CMS detector. This research tool is able to detect elementary particles resulting from the collision of two protons that have been accelerated to a very high energy in the 27 km long, circular tunnel of the LHC.

The term 'CMS detector' is very simplistic. In fact, this device consists of five detectors, each of which can be used to detect certain types of elementary particles. The innermost of these five detectors is the pixel detector. The pixel detector is cylindrical - five meters long and 40 cm in diameter. The detector extends a few centimeters into the area where the protons collide. Thus, the pixel detector is the first to register the elementary particles produced during these collisions. The pixel detector, the heart of the CMS detector, was replaced at the beginning of March. The reason: The old pixel detector, which has been in use since 2010, was due to be replaced by an improved version in order to meet the growing number of particle collisions performed in the LHC.

Practice on a Dummy

The new pixel detector was designed and built during a period of seven years by physicists from the ETH Zurich, the University of Zurich and the Paul Scherrer Institute (PSI) as well as by other colleagues in Germany, Italy, Taiwan and CERN. During two days in early March, components of the new detector were lowered with a yellow crane into the underground hall and then slid into the CMS detector via rails, through which the old pixel detector had been removed during the weeks prior. With the aid of cameras, the new pixel detector was exactly positioned in the belly of the CMS detector.

During the installation of the pixel detector, half a dozen physicists from ETH Zurich, the University of Zurich and the PSI were involved. The team included Prof. Roland Horisberger from PSI, who had designed the first and the second pixel detectors and who is often referred to as the 'father of the pixel detector' for his significant contribution. Dr. Lea Caminada (University of Zurich / PSI) was involved in the installation of the new detector as well. "During the previous days, we have practiced the installation of the pixel detector five times on a dummy," says the physicist, who also played a major role in the construction of the pixel detector. "In order for the new pixel detector to work, we have to connect several hundred plug connections during the next days, which supply the detector on the one hand and on the other hand, transport the obtained data – converted into light pulses – from the detector," says Caminada.

Multi-Week Test Programme

Once the pixel detector is connected, it is cooled to its operating temperature of –20 °C and then subject to a multi-week test program. This must be finished by June at the latest, since the LHC is scheduled to go into operation again in June after a six-year operational break. Then the new pixel detector, the newly implanted heart of the CMS experiment, will provide the data with which the particle physicists of CERN hope to understand the elementary bricks of the world even better than they do thus far.

Author: Benedikt Vogel


  • Fisica delle Particelle Elementari
  • Particelle elementari