Physicists and engineers at CERN use the world's largest and most complex scientific instruments to study the basic constituents of matter – fundamental particles. Subatomic particles are made to collide together at close to the speed of light. The process gives us clues about how the particles interact, and provides insights into the fundamental laws of nature. We want to advance the boundaries of human knowledge by delving into the smallest building blocks of our universe.
The instruments used at CERN are purpose-built particle accelerators and detectors. Accelerators boost beams of particles to high energies before the beams are made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions.
Founded in 1954, the CERN laboratory sits astride the Franco-Swiss border near Geneva. It was one of Europe's first joint ventures and now has 23 member states.
At an intergovernmental meeting of UNESCO in Paris in December 1951, the first resolution concerning the establishment of a European Council for Nuclear Research (in French Conseil Européen pour la Recherche Nucléaire) was adopted.
Two months later, an agreement was signed establishing the provisional Council – the acronym CERN was born.
Today, our understanding of matter goes much deeper than the nucleus, and CERN's main area of research is particle physics. Because of this, the laboratory operated by CERN is often referred to as the European Laboratory for Particle Physics.
The Large Hadron Collider (LHC) is the world’s largest and most powerful particle accelerator. It first started up on 10 September 2008, and remains the latest addition to CERN’s accelerator complex. The LHC consists of a 27-kilometre ring of superconducting magnets with a number of accelerating structures to boost the energy of the particles along the way.
Galaxies in our universe seem to be achieving an impossible feat. They are rotating with such speed that the gravity generated by their observable matter could not possibly hold them together; they should have torn themselves apart long ago. The same is true of galaxies in clusters, which leads scientists to believe that something we cannot see is at work. They think something we have yet to detect directly is giving these galaxies extra mass, generating the extra gravity they need to stay intact. This strange and unknown matter was called “dark matter” since it is not visible.
Unlike normal matter, dark matter does not interact with the electromagnetic force. This means it does not absorb, reflect or emit light, making it extremely hard to spot. In fact, researchers have been able to infer the existence of dark matter only from the gravitational effect it seems to have on visible matter. Dark matter seems to outweigh visible matter roughly six to one, making up about 27% of the universe. Here's a sobering fact: The matter we know and that makes up all stars and galaxies only accounts for 5% of the content of the universe! But what is dark matter? One idea is that it could contain "supersymmetric particles" – hypothesized particles that are partners to those already known in the Standard Model. Experiments at the Large Hadron Collider (LHC) may provide more direct clues about dark matter.
Many theories say the dark matter particles would be light enough to be produced at the LHC. If they were created at the LHC, they would escape through the detectors unnoticed. However, they would carry away energy and momentum, so physicists could infer their existence from the amount of energy and momentum “missing” after a collision. Dark matter candidates arise frequently in theories that suggest physics beyond the Standard Model, such as supersymmetry and extra dimensions. One theory suggests the existence of a “Hidden Valley”, a parallel world made of dark matter having very little in common with matter we know. If one of these theories proved to be true, it could help scientists gain a better understanding of the composition of our universe and, in particular, how galaxies hold together.
Dark energy makes up approximately 68% of the universe and appears to be associated with the vacuum in space. It is distributed evenly throughout the universe, not only in space but also in time – in other words, its effect is not diluted as the universe expands. The even distribution means that dark energy does not have any local gravitational effects, but rather a global effect on the universe as a whole. This leads to a repulsive force, which tends to accelerate the expansion of the universe. The rate of expansion and its acceleration can be measured by observations based on the Hubble law. These measurements, together with other scientific data, have confirmed the existence of dark energy and provide an estimate of just how much of this mysterious substance exists.
Invisible dark matter makes up most of the universe – but we can only detect it from its gravitational effects
For the CMS experiment, Long Shutdown 2 (LS2) is like very prolonged open heart surgery. The main goal is to improve the detector’s performance, thanks to innovative, customised components.
In the outermost layer of the CMS detector, new instruments called GEM (gas electron multiplier) detectors will be installed in order to detect muons that scatter at an angle of around 10° in relation to the beam axis. Measuring muons so close to the beam axis is very challenging due to the high number of particles coming from collisions in that area. Muons at higher angles are already covered by different detector technologies in CMS.
GEM chambers comprise a thin, metal-clad polymer foil, which is chemically pierced with millions of holes, typically 50 to 100 per square millimetre. Three of these foils combined with two electrodes make up a detector. When the muons pass through, the gas within the detector is ionised and releases electrons. These electrons drift towards the holes, where they cause an avalanche of electrons under a very strong electric field. “The electrons that we collect are not necessarily connected to the passage of a muon,” explains Michele Bianco, technical coordinator for the GEM detectors in the framework of the CMS upgrade project. “To make sure that we really are dealing with a muon, we have to locate its track in the other CMS subdetectors.” GEM detectors are, in a manner of speaking, like a piece of a puzzle. Without all the pieces, it’s impossible to know what the whole puzzle represents.
The GEM detector project for the CMS upgrade is the work of a collaboration of around 40 institutes, with by far the largest contribution coming from doctoral students and postdocs. Detector production sites located all over the world, namely in Belgium, Germany, India, Italy, Pakistan and the United States and at CERN, produced the 144 detector modules and their electronic components. Several training sessions for the external teams were held at CERN. “Kits” containing the individual pieces of the modules were then sent to the various institutes. Electronic boards, currently under production and testing at collaborating institutes, will soon arrive at CERN, where they will be integrated with the modules.
All the detectors have now been assembled and the team in charge of the project is working inside the CMS detector to prepare to install the chambers. “We need to install the chambers, but also the associated infrastructure, such as the gas, electricity and cooling distribution systems,” explains Michele Bianco. “We also plan to install the infrastructure required for the 288 future chambers that will be installed during the 2021-2022 technical stop. Then, during Long Shutdown 3 (between 2024 and 2026), 216 more modules will be added.”
Nearly 650 new detector modules will search for the muons that will be produced in CMS’s very forward region in the High-Luminosity LHC (HL-LHC) era. The “new” accelerator will produce between five and ten times more collisions than the LHC. We can expect a fruitful muon hunt.
During LS2, CMS will install 144 additional muon detector modules specially designed to detect particles produced in the very forward region
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