Who is a CERN employee

Particle accelerator of the CERN : The largest machine in the world is getting an update

Physics is in a tight spot: in the past few decades it has perfected its theory of matter: the so-called standard model. Researchers have repeatedly confirmed this model with high precision. They even found the Higgs boson predicted by the theory after a long search. Six years ago it was detected in particle collisions in the Large Hadron Collider near Geneva. These collisions in a ring tunnel of the world's largest particle accelerator a good 100 meters below ground produce all sorts of particles, the traces of which are followed in detectors as high as a house.

What holds the world together ... and nobody knows

And yet physicists know that their theory is not perfect. The standard model needs to be revised. Some even say: replaced. Because there are also forms of matter and energy that have no place in this model. They are currently still called "dark matter" and "dark energy" because so little is known about them.

Nobody doubts that they exist, because without them galaxies would fly apart and the universe could not expand, which it does. But nobody knows what they are made of. They were never observed in any experiment. So physicists look for some flaw in their theory to have a clue as to which direction physics must take.

“To make it easier: think of a cube,” says physicist Oliver Brüning. It should be someone who you know must be pricked. But up until now, every number appeared the same number of times when rolling the dice. What do you do? You keep rolling the dice - and hope that it will soon become clear which numbers come up more often and which less often. It is similar with the standard model, explains Brüning: You have to analyze even more particle collisions in the LHC accelerator and continue to search for particles that behave differently than the standard model predicts.

The LHC currently manages around a billion collisions per second. That sounds like throwing the dice a lot - and yet it is surprisingly little when you consider how many particles the machine hurls around in a circle: after all, around 300 trillion protons encounter around 300 trillion other protons coming from the opposite direction more than 10,000 times per second. But these particles are all positively charged and repel each other. So you keep your distance. This is a lot of space for the oncoming particles to slip through. That should change soon.

The ground-breaking ceremony for an upgrade of the LHC will take place on Friday: after a few modifications, it should be able to handle around three times as many collisions from 2025 and collect ten times as much data as before. Brüning is the deputy head of this expansion, which is called "HiLumi", an abbreviation for "High Luminosity". It is so called because it will simply shine more intensely when more particles collide.

Claustrophobic protons under peer pressure

In 1995, the physicist Brüning moved from the German electron synchrotron Desy in Hamburg to the European research center Cern, which operates the LHC. He is generally responsible for future projects there, so he thinks about which instruments his colleagues will need in a few years. “If at some point we know in which direction physics is developing, we should be prepared and know the technical and financial framework conditions for the required instruments,” says Brüning. The lead times are long in particle physics: the physicists have been working on some components for HiLumi for more than 15 years.

After the groundbreaking ceremony, two of the four detectors will each be dug 600 meter long additional tunnels. Additional cooling units and transformers are to be accommodated there. “The ring tunnel is actually already full,” explains Brüning. The new magnets are not only around 50 percent stronger than the current ones, they also generate more heat and therefore require additional cooling systems. In addition, their power supply must be controlled very precisely. Therefore, these components should also be housed in the new tunnels - shielded from the radiation in the main tunnel.

The magnets should focus the proton beams more strongly. The particles fly in packets of around 120 billion. These are shaped like a ten centimeter long, hair-thin needle. There is a gap of a few meters between the packages. Before these elongated particle clouds fly into the detectors, they should be compressed by the new magnets to half their current diameter. The space between the protons should therefore shrink in order to increase the collision rate. When that is done, ten proton needles next to each other will be as thick as a human hair.

Pool crabs

In a further step, devices called “crab cavities” are installed and each proton packet rotates a little with magnetic force. When a parcel flies from left to right, the tip is pushed up and the end down, making it look like the parcel is moving sideways - like a crab on the beach. With the oncoming parcel, you do it the other way around. This trick is also supposed to make more protons collide. Currently, the needles do not meet head-on because they are only deflected at the last moment to collide. You pierce your side. In the future, the two particle clouds will penetrate each other more strongly, giving more protons the chance to collide.

The third improvement is a new injector called “Linac 4”, which will feed the proton packets into the accelerator ring from 2020 onwards. Then the packets will no longer consist of 120, but of 240 billion protons each. In addition, the reliability of the system is to be increased in order to spend less time on maintenance and to allow more particles to collide.

Cern is the engine of innovation - says the Cern employee

The alternative would have been to accelerate the protons even more and let them collide with one another with even more energy. But there is no plan to make it affordable, says Brüning. The upgrade to tenfold the amount of data will cost around 800 million euros. This also includes expanding the computer, because the detectors already deliver around 50 million gigabytes of measurement data a year. The data is distributed to research centers around the world for analysis. Brüning therefore sees CERN as a pioneer in computer technology: "Cloud computing was largely driven by our physical experiments," he says. For “HiLumi”, both networks and computers have to become faster - and of course larger data memories are required.

The LHC has been running for around ten years, and another ten years are planned after the renovation. If you cannot find anything during this time that can serve as a point of reference for a new physics, it will be tight - not for the particles, but for the physicists. It would be like rolling a thousand times and still not being able to prove that the dice was marked. "If you don't find the key to the new physics in ten more years," says Brüning, "you have to think fundamentally."
