VUB Physics Students Visit
CERN
Visit of the Physics and Astronomy students from the Vrije Universiteit Brussel (VUB) to CERN.
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The CERN site of Prevessin is located next to the French village Prevessin-Moens. This is the second large experimental CERN site, apart from the main CERN site of Meyrin. Unlike the Meyrin site, which is located on the French-Swiss border but in practise is as if in Switzerland, the Prevessin site is fully in French territory.
The site was built on the north end of the 6.9km SPS accelerator ring
https://home.cern/science/accelerators/super-proton-synchrotron
https://en.wikipedia.org/wiki/Super_Proton_Synchrotronwhich was the accelerator at which, among other things, the W and Z bosons were discovered 40 years ago. Nowadays the SPS is a pre-accelerator for the LHC, but at the same time also several secondary beams are extracted that feed quite a few experiments, like NA62 (NA stands for “North Area”).
Given this concentration of activities, it is not a surprise to find the CERN Control Centre (CCC) there, the main hub from where the complex CERN accelerator system is steered. The site also hosts the control centre of the AMS experiment (an anti-matter experiment on the ISS), a large are for CMS activities including muon labs; (and a very enjoyable barbecue area :) ).
Also the VUB has had recent activities on this site: a few of the silicon tracking modules that were recently constructed in our labs were tested for an extended time in a high-energy muon beam. A picture with one of our postdoctoral researchers working on this is below:
In the map below, you can see the Meyrin and Prevessin sites, and the circular SPS accelerator between them. The small circle in the Meyrin site is the PS accelerator, an even smaller (and older) accelerator that is the step before protons enter the SPS. The biggest circle which we only partially see on this picture is the LHC tunnel.
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The access points to the LHC tunnel (the previous LEP tunnel) are numbered from 1 to 8. Point 1 (next to the main CERN site in Meyrin) houses the ATLAS experiment, Point 2 the Alice experiment, Point 8 the LHCb experiment, and Point 5 (in the far north of the ring near the village of Cessy) the CMS experiment. The other points house other services for the LHC: for instance Point 4 is where the beams receive a little accelerating push at every of their 11'000 passages per second, while Point 6 is where the LHC proton beams, which carry an enormous amount of energy, can be safely disposed of in the “beam dump”.
Point 5 is visible from far in the countryside because of its main high building in which the CMS detector was constructed, with a diameter of 15m and a length of about 25m. After a lot of testing of the CMS detector in the surface hall, the detector was lowered underground in pieces by a huge crane through a large access shaft. The central piece with the undivisible magnet was the heaviest, about 2000 tons, to lower 100m down.
On the site at Point 5 you will also find many other buildings housing services for the CMS experiment. Of note are the control room and the on-site computing resources for the so-called “High-Level Trigger”, a system that makes sure the dataflow is reduced to a managable level for the subsequent computing services – only 0.003% of the collisions is selected to be kept.
Below you find an overview map with the full LHC ring and the surrounding region, from Geneva with the Rhone river in the south, to the village of Gex at the feet of the Jura mountains in the north. The gray line with red hatching is the border between France and Switzerland.
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Speakers: David Vannerom (Vrije Universiteit Brussel (BE)), Steven Lowette (Vrije Universiteit Brussel (BE))
Collisions in CMS produce gazillions of particles, but most of these (except muons) are readily absorbed by the heavy material of the CMS detector. One of the hypotheses for new physics beyond the particles that we know could be the presence of particles with a small electric charge, which would easily fly out of the detector. A few years ago, a new small detector was proposed to capture such particles, to be placed in an unused gallery above the CMS experiment. In 2019, VUB joined in on this experiment, and at this very moment we are finalizing construction of a first version of this detector that will take data during the LHC Run3 years 2023-2025.
A sketch of the detector and its positions with respect to CMS can be seen below. Unfortunately, it is not possible to visit the detector itself, but we will pass by the platform in the main CMS construction hall where the milliQan detector was constructed and tested before it was brought downstairs.
More info in a news item from Oct '22: https://cms.cern/news/milliqan-getting-ready-have-look-dark
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Speaker: Maarten Litmaath (CERN)
The ATLAS detector is them big sibling of CMS, neatly placed in Point 1 across the LHC ring.
It has very similar physics program, but has chosen often very complementary detector design with respect to CMS choices. Characteric for ATLAS is the very large outer magnet system generating a toroidal field that bends particles along z; a much lighter outer structure (ATLAS would float on water, CMS would sink), and an electromagnetic calorimeter with a characteristic accordian structure. The downside of being so big is that one cannot see the whole detector when downstairs (so we stay on the surface); the most famous picture where you can still see all was taken during construction (see below).
The ATLAS Collaboration is also somewhat larger than CMS, but in many respects they are similar in purpose and structure. In the early nineties, particle physicists in Belgium wisely decided to put all eggs in one basket and chose to stick to one big future project at the LHC. That's why we now have no participation in ATLAS. The Netherlands did the opposite: they joined ATLAS instead of CMS.
The choice to build 2 experiments with similar purpose serves two purposes. On the one side, a discovery is a big claim to make, and – to quote Carl Sagan – extrordinary claims require extraordinary evidence. And this is exactly what happened with the Higgs boson discovery in 2012. In a spirit of friendly though fierce competition, the two expimerents presented the discovery together, after independently, and with very complementary techniques, the same underlying physics had been unearthed.On the other side, it also is a more mundane insurance for things going really wrong. Since all these machines are their own prototypes, developing a lot of needed technology on the way, it requires half a miracle (no kidding, just amazing scientists and engineers) to make everything come together and actually work. Accidents do happen (and have happened, also with CMS!), so better to be prepared.
ATLAS has a neat website where you can explore more:
https://atlas.cern/Discover/Detector
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