When you press “start” on your microwave or computer, the appliance turns on immediately—but large physics experiments like the Large Hadron Collider at the European Organization for Nuclear Research, better known as CERN, don’t work that way. Instead, engineers and physicists must take a few weeks each year to carefully reset the collider and all the experiments on it.
I am a physicist at CERN who has been working with my colleagues for the past few months on the reset process of the largest experiment, ATLAS. To collect accurate data on particle collisions and study some of the most fascinating mysteries of the universe, the collaboration must ensure that the equipment is properly calibrated.
At CERN, the Large Hadron Collider (LHC) smashes protons at the highest energy ever, creating new particles. These particles are then captured by physicists and studied in various experiments.
The LHC explores the hidden world of subatomic particles, the fundamental building blocks of everything around us. By studying these particles, scientists like me can better understand how the universe works and evolves over time.
Hibernation and waking up the LHC
Every winter, the collider and its experiments hibernate. My team and other teams at CERN push for this hibernation for a few reasons.
The machines we use here are complex. We need some time to replace parts or install new components. And since all these machines consume a lot of power, we avoid running them in the winter, when electricity is more expensive and nearby Geneva needs to keep its residents warm.
But when spring arrives, all teams prepare the LHC and the experiments for a new season of data collection.
While engineers and technicians are busy resetting the accelerator and preparing it to smash protons, my colleagues and I, the experimental physicists, are preparing the experiments to quickly and accurately collect data from all the particles produced by the particle accelerator.
Testing with cosmic rays
The experiment teams are starting the first phase of waking the LHC from hibernation while the accelerator is still asleep. We need to start testing the particle detectors even if the collider that creates the particles is not working.
In this first phase we use what is always available, provided by nature itself: cosmic rays. These are subatomic particles that are created when energetic particles from space hit atoms high in the atmosphere.
A cosmic ray enters the ATLAS detector in the LHC on the left. Every time it hits a sensor, the ray loses some of its energy, which the detector converts into a signal and records. By drawing a line through all the sensors the cosmic particle encountered, physicists can reconstruct its direction of arrival, its path through the experiment, and its energy. Cosmic rays help us train the sensors and verify that everything works as expected.
However, cosmic rays are random and rare, so we can’t rely on them for all our tests. For subsequent tests, we’ll use a denser and more predictable source: subatomic spatter.
Subatomic splashes to synchronize them all
The LHC has about 27 kilometers of pipes through which protons fly. The pipe has magnets around it that guide the protons it accelerates. Any particles that stray from their path are stopped by a small piece of metal called a collimator. This collimator is pushed into the center of the accelerator pipe, where the protons collide with it and interact with the atoms.
This collision creates a huge amount of particles, which then move in unison along the accelerator tube as a big splash – or, as we call it, a “beam splash.” Around mid-March, the accelerator team will create this for the ATLAS experiment.
The large wave of particles hits the experiment all at once, and this wave allows us to verify that all the detectors in the experiment are responding correctly and in sync. It also tests whether they can record and store data at the required speed.
Horizontal muons to calibrate them
Most of the particle detectors in the experiments are now ready to acquire new data. However, some types of detectors in the LHC require additional testing.
One of these is the ATLAS experiment’s Tile calorimeter, a detector that measures the energy of particles such as neutrons and protons. It consists of rows of tile-shaped sensors, and test particles must pass horizontally through these tiles to accurately calibrate the detector.
The huge sprays of particles created by jet spatter are not good for calibrating the Tile calorimeter. The particles do not come at the right angle and there are too many at once.
To test the Tile calorimeter, we are only interested in one specific type of particle: muons. Muons are similar to electrons, but are heavier, and they interact differently with the surrounding world. They can pass through multiple rows of sensors without losing much energy or being stopped, which makes them useful for testing particle detectors.
That’s why we set up a new test at the end of March, using the collimators again.
This time, however, the LHC engineers push the collimator just a little bit into the path of the protons, so that the particles just barely touch the collimator. The gentle friction of the protons against the metal surface of the collimator creates particles that travel parallel to the accelerator tube and land horizontally on the ATLAS experiment.
We use special sensors to reveal muons created by the collision with the collimator and mark them. We then track them as they move through the Tile calorimeter.
These horizontal muons pass through all the tiles of the calorimeter in a row, so we can be sure that the data is collected accurately.
Ready for new physics
Once the LHC is fully calibrated and ready to use, it accelerates the protons to their maximum energy – and then pushes them so that they collide with each other.
After about 10 weeks of testing, a new season of data collection begins, bringing with it dreams of new discoveries.
This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and reliable analysis to help you understand our complex world. It was written by: Riccardo Maria Bianchi, University of Pittsburgh
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Riccardo Maria Bianchi is a member of the international ATLAS Collaboration and co-author of the experiment results. As a former CERN Fellow, he currently holds a “User” affiliation at CERN,