When you press “start” on your microwave or computer, it immediately powers on—but that’s not how big physics experiments like the Large Hadron Collider at the European Organization for Nuclear Research, known as CERN. Instead, engineers and physicists must set aside a few weeks each year to carefully reset the collider and all the experiments on it.
I am a CERN physicist who has been working with colleagues over the past few months to reset the largest of the experiments, ATLAS. To collect accurate data on particle collisions and study some of the most intriguing mysteries of the universe, the collaboration needs to make sure the equipment is properly calibrated.
The Large Hadron Collider (LHC) at CERN smashes protons together to the highest energy ever achieved, creating new particles that physicists then capture and study in a variety of experiments.
The LHC investigates the hidden world of subatomic particles, the fundamental building blocks of everything around us. Studying these particles helps scientists like me better understand how the universe works and how it evolves over time.
Putting the LHC to sleep and waking it up
The collider and its experiments hibernate every winter. Myself and other teams at CERN force them into this hibernation for a number of reasons.
The machines we use here are complex. We need some time to change parts or install new components. And considering that all these machines use a lot of power, we avoid running them in the winter months when electricity is more expensive and nearby Geneva needs to keep its residents warm.
But when spring arrives, all teams are preparing the LHC and experiments for a new season of data collection.
While engineers and technicians work to reset the accelerator and prepare it to smash protons, my colleagues and I, experimental physicists, are preparing experiments to promptly and accurately collect data from all particles produced by the collider.
Testing with cosmic rays
The experimental teams begin the first phase of waking the LHC from sleep mode while the accelerator is still asleep. We need to start testing particle detectors even when the collider that creates the particles is not working.
In this first stage, we use what is always available, provided by nature itself – cosmic rays. These are subatomic particles created when energetic particles from space hit atoms high in the atmosphere.
A cosmic ray enters the ATLAS detector at the LHC, left. Each time it hits a sensor, the beam 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 the particle’s direction of arrival, its path through the experiment, and its energy. The cosmic rays help us train the sensors and verify that everything works as expected.
But cosmic rays are random and infrequent, so we can’t rely on them for all our tests. For subsequent tests, we use a more intense and predictable source – subatomic spikes.
Subatomic jumps to keep it all in sync
The LHC has about 17 miles (27 kilometers) of tubes through which protons fly. There are magnets around the tube that guide the protons as they accelerate. Any particles that stray from the path are stopped by a small piece of metal called a collimator. This collimator pushes the protons toward the center of the accelerator tube, where they hit it and interact with its atoms.
This collision creates a huge amount of particles, which then travel together as a big splash down the accelerator pipe—or, as we call it, a “beam splash.” In mid-March, the accelerator team creates these for the ATLAS experiment.
The large particle wave hits the experiment simultaneously, allowing us to verify that all detectors in the experiment are responding correctly and in sync. It also tests whether they are recording data at the required speed.
To calibrate horizontal muons
Most particle detectors in the experiments are now ready to take new data, but some types of detector at 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 large particle sprays created by the beam bounces are not good for calibrating the Tile calorimeter. The particles are not coming at the right angle and there are too many of them at once.
To test the Tile calorimeter, we’re only interested in one particular type of particle – muons. Muons are similar to electrons but heavier and interact with the world around them differently. They can pass through multiple rows of sensors without losing much energy or being stopped – making them useful for testing particle detectors.
So towards the end of March, we did another test, again using collimators.
But this time, LHC engineers nudge the collimator just slightly into the path of the protons, so the particles barely scrape the collimator. The protons’ slight friction against the metallic surface of the collimator creates particles that travel parallel to the accelerator tube and strike the ATLAS experiment horizontally.
We use special sensors to detect and label muons that collide with the collimator, then track them as they move through the Tile calorimeter.
These horizontal muons pass through all tiles of the calorimeter in order, so we can be sure that the data is collected correctly.
We are ready for new physics
Once the LHC is fully calibrated and ready to operate, it accelerates the protons to their maximum energy and then forces them to collide with each other.
After about 10 weeks of testing, a new data collection season begins, dreaming of new discoveries.
This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and trusted analysis to help you understand our complex world. 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’s results. A former CERN Fellow, Bianchi currently has a “User” relationship with CERN.