What tips for a visit to CERN
A ring to find them: a visit to CERN
With our red helmets we walk through corridors with concrete walls and feel like we are in a bunker, only everything is brighter and newer here. After a few steps, however, it is over again and we are standing in front of a metal door. Nothing here prepares us for what is waiting behind it, but at least nobody feels the urge to increase the tension. The door opens and there is a huge cavern in front of us, crammed with a lot of state-of-the-art technology. We have reached the destination of our trip. ATLAS is one of the two largest experiments on the Large Hadron Collider, the largest scientific instrument in the world.
No protons are currently circling in the LHC, as the system has been upgraded for the next expansion stage since February 2013. The world's largest particle accelerator had previously worked better than hoped for over three years. The most important discovery was undoubtedly the Higgs boson to complete the Standard Model of particle physics. But the research is not over and in April the LHC is to go back into operation, but then with a significantly higher performance: The center of gravity energy achieved - the most important unit of measurement of the particle accelerator - should almost double. On this occasion, we made our way to Geneva to pay a visit to CERN before the end of the renovation.
Famous at last
It is not far from Geneva Airport to the European Organization for Nuclear Research (Conseil Européen pour la Recherche Nucléaire, CERN). A few minutes by bus and tram and we're there. The days when only a dusty dirt road led to the site on the French border are almost forgotten. But many buildings are still reminiscent of the early days. Even if the research center only really got into public awareness with Dan Brown's "Illuminati", the giant accelerator LHC (Large Hadron Collider) and the discovery of the Higgs boson, it is already celebrating its 60th birthday. And time has left its mark. The yellowed buildings and the linoleum in the corridors take visitors back thirty years, only the flat screens do not fit into the picture. We are told later that fresh money is being invested in new technology, new experiments and new buildings.
Our first station is the CMS control center on the CERN site, where we meet the DESY physicist Christoph Wissing, who is responsible for CMS computing. The compact muon solenoid experiment is one of the four large experiments connected to the LHC. Like the other experiments, the CMS is located in an underground cavern through which the pipes of the LHC run. It was closed again after the renovation of the past months and is currently being calibrated. Since the LHC does not yet deliver any particles, cosmic radiation has to be used, mainly muons, which race straight through the system when the magnet is switched off. It is still quiet in the control center, the great stress is still ahead. It is then distributed over this room in the CERN premises, another control center directly at the CMS, one at the Fermilab (Fermi National Accelerator Laboratory) near Chicago and a slightly smaller one at DESY in Hamburg. All four are permanently linked by video transmissions.
The time between the runs, as the months with the LHC working are called internally, are used by researchers and experts to reevaluate the data that has already been collected, for example with improved algorithms and taking into account further decay options. At ATLAS (A.Toroidal L.HC A.pparatuS.), the second large LHC experiment, the accuracy of the measurement of the Higgs mass could be increased again by a factor of 3 to 125.36 + -0.37 GeV / c². To a large extent unnoticed by the public, the research continues even after it has been switched off. Nonetheless, there is of course a great anticipation for the second run. When the LHC is up and running again, it will shoot protons into the experiments from two sides at almost the speed of light almost around the clock. The beam quality lasts for up to 15 hours, then the so-called luminosity is too low and the beam is rebuilt, which takes around two to three hours.
Wissing then leads us to the ATLAS colleagues and to the undoubted highlight of the visit, the way below the surface of the earth. Markus Elsing and Rolf Seuster are off to their ATLAS experiment, a few days before it is also closed in preparation for the second run. Here we have to put our helmets on and then we take the elevator 100 meters down. Passing two server rooms, full of special electronics for processing the raw data, it goes into the cavern. The detector alone is more than 25 meters in diameter and 45 meters long, and the entire room is a bit larger at 35 meters wide, 40 meters high and 55 meters long. With this huge instrument the smallest building blocks of the universe are explored.
We see the end caps of the experiment, which have been brought into a parking position for the conversion and have not yet been pushed back together. The onion skin principle, in which the detectors are built around the central steel tube, becomes visible. From 2015, proton beams will again be shot at each other over a length of a few centimeters in the heart of the facility. During the collisions, the protons are destroyed and their enormous (natural and motion) energies create new and sometimes unknown particles that fly in all directions. The detectors should then prove this. It was exactly the same with the Higgs boson, which had only been predicted theoretically before its discovery. Which particles now follow it is an exciting question. The experiment is also intended, for example, to search for supersymmetric particles, hypothetical particles postulated by one of the most plausible extensions of the accepted theory.
For the researchers, the second run is like a journey into unknown waters and everyone can take something with them.
The part of ATLAS that is visible to us consists of various measuring devices that have been manufactured at universities and research institutions all over the world. There are also huge magnets that generate a magnetic field of up to 4 Tesla in order to deflect the produced particles. From the radii of the trajectories, conclusions can be drawn about their impulses and properties. When the protons collide in a very small space and at a frequency of 40 megahertz, more than 100 terabytes of raw data are generated every second. In the first stage, special electronics are used to pre-select interesting collisions from the raw data. After this level-1 trigger, around 200 gigabytes per second remain, which are processed further in the small data center in front of the door in the second trigger. After this stage, there are about 2.5 gigabytes or about 1000 collisions per second (1 kilohertz) left. This data flows into the computer center of CERN for further processing, the next destination of our visit.
The whole world in Switzerland
But before that there is lunch and in the canteen not only the international character of CERN becomes clear - dozens of languages buzz through the air - but also its youth. The canteen is full of young people, students and doctoral students. Many of the approximately 10,000 guests on the premises are only here for weeks or months to do research directly at CERN. The institution cooperates with hundreds of universities and research institutions, not only in the 21 European CERN member states but worldwide. That is why around one hundred countries are represented on the site.
We are now on the way to the CERN computer center, the place where all the data comes in that were considered interesting enough by the algorithms. Here, where you can now look around using Google Streetview, they are saved twice - on hard drives and tapes - and also shoveled into the grid. This computer network connects all partners and distributes the computing power required for the researchers to dozens of countries. CERN is - now with an addition in Budapest, Hungary - its head, as Tier 0. This is followed by thirteen Tier 1 locations, which in turn are above the hundreds of Tier 2 and even lower participants. This once hierarchical structure is now being broken down, as the animals can also communicate with one another.
The grid not only offers the scientists the necessary backup storage space, it is also where the calculations that make up particle physics are carried out. If scientists create a calculation task, the network allocates the capacities. Data centers all over the world are connected and available (this can even be observed live). There are also political reasons for the fact that the work is distributed in such a way. It would certainly be more efficient to set up the infrastructure directly at CERN, but the member states wanted to keep the researchers and parts of the technology in the country. In return, they provide the capacities there at their own expense. So CERN saves money and at the same time the system is driving the development of distributed computer systems.
Stefan Lueders is responsible for IT security in this structure. A task that is of course different from that of his colleagues in business. CERN is not the real target for data theft, he explains: construction plans and measurement results are or will be public. More important is the protection of personal data, just like in every university, the importing of updates against Heartbleed & Co and reacting to the hackers who hit again and again, be it white hats, gray hats or black hats. His IT infrastructure not only has to cope with the immense amounts of LHC raw data (20-30 petabytes per year), but also with everyday things such as e-mail, web servers. As a little extra, the providers have their own at CERN Internet node set up, the CIXP.
For us it is now time to say goodbye. We are driven back to the entrance, past those laboratories where antimatter is produced and further, much smaller rings for particle accelerators. We haven't seen everything from CERN by a long way. And CERN has not come to the end either. The planning for an even bigger ring has already begun. The larger the circumference, the less energy the particles lose on their rounds. Since this has to be added back in to keep the beam energy constant, larger accelerators achieve higher collision energies. There is still a lot to discover in the unspoilt waters.
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