LHC provides data again

Upgrade of the Pixel Detector of the CMS Experiment

After a half-year break, the Large Hadron Collider (LHC) at CERN will go into operation again in June. During the last few months, intensive maintenance work on the particle accelerator has been done. The physicist community is hoping to gain new insight into the building bricks that make up our world.

Prof. Florencia Canelli (University of Zurich) in front of the CMS detector at CERN.
Image: B. Vogel

From spring 2010 to the end of 2012, the LHC was in operation. After a maintenance break, it went into operation again from spring 2015 to late 2016. LHC researchers’ most spectacular finding was the discovery of the Higgs boson in July 2012. Since then, the public has had to wait for new, spectacular results. But scientific knowledge requires patience, especially in the field of particle physics, which studies unimaginably small dimensions and requires huge effort to push back the limits of ignorance.

Three Swiss Partners

The effort required to conduct experiments in particle physics is made clear by the new pixel detector of the Compact Muon Solenoid (CMS) experiment – one of four large experiments being carried out in the CERN particle accelerator LHC. The pixel detector, a central component of the CMS experiment, had been in operation since the start of the LHC and was replaced in spring 2017 by a new, more powerful successor.

The pixel detector was designed and built by Swiss physicists from the ETH Zurich (group of Prof. Rainer Wallny), the Paul Scherrer Institute (group of Prof. Roland Horisberger) and the University of Zurich (groups of Prof. Florencia Canelli and Prof. Ben Kilminster). The upgrade of the pixel detector - from conception to construction and installation - has taken for seven years, employed about 40 students, researchers and professors, and required an investment of CHF 4 million from the Swiss side - a huge scientific, technical and organizational effort.

“I came to the University of Zurich as a physics professor in 2012, when the design of the new pixel detector was already established,” says Florencia Canelli, an Argentinian particle physicist who researched and taught in Chicago before moving to the University of Zurich. “For me the pixel detector was new at the time, but I had the support of the people who had the specialized know-how.”

Camera with 120 million Pixels

The new detector consists of over 1800 silicon sensors with a total of 120 million pixels, each one only one tenth of a millimetre in size. The sensors are arranged in the detector in four superimposed layers. If a pixel is traversed by an elementary particle, it generates an electrical impulse. From the signals, the physicists can determine the trajectory and then the impulse of the elementary particles that arise during a proton-proton collision.

In addition to other parts, ETH and the Paul Scherrer Institute (PSI) built the inner two layers of sensor modules (layer 3 was built in Italy and at CERN, layer 4 in Germany). The sensor modules sustain a higher hit rate, and the additional fourth layer increases precision and provides redundancy.

The University of Zurich took over the challenging task of developing and manufacturing the complex system for the control and readout of the detector, the power supply and cooling of the electronic components. “In contrast to the first pixel detector, we have built the mechanics lighter and placed the readout electronics away from the detector. This has allowed us to further improve the resolution of the detector,” says Florencia Canelli. The layers of sensor modules and readout electronics were assembled and tested at the PSI in 2016/17, before the pixel detector was transported to CERN at the end of January.

An Almost Eerie Precision

Thus, a device of unbelievable precision has emerged, as Günther Dissertori, physics professor at ETH Zurich and deputy spokesman of the CMS experiment, explains: “The protons collide almost at light speed in the LHC. The elementary particles that are produced in this way have a lifetime of only about one picosecond (billionth of a second), before they decay into other particles. That is, when it has moved only millimetres from the point of the proton-proton collision, it has already vanished. The pixel detector is so accurate that it is able to observe such processes adequately.”

The new pixel detector will be in service in the LHC for at least the next five years. If the scientists involved succeed in understanding the world of elementary particles in the coming years, the pixel detector will have made an important contribution.

Enrichment for Students

Just as important as the pixel detector is for research, it is also critical for academic teaching at the university. “The students are thrilled when they can work on the construction of such a detector. This work is new and the students learn practical things,” says Florencia Canelli, adding: “There are only a few academic institutions that have the opportunity to build research tools of this calibre. This was a one-time opportunity – it will be a few years before the next detector is due.”

Author: Benedikt Vogel


  • Elementary particles
  • Particle Physics