by When you press “start” on your microwave or computer, the device goes straight on – but big physics breakthroughs such as the Large Hadron Collider at the European Organization for Nuclear Research, known as CERN, don’t work like that. Instead, engineers and physicists must take a few weeks each year to carefully reset the collider and all the experiments.
I am a CERN physicist who has worked with my colleagues for the past few months on the reset process of the largest of the experiments, ATLAS. To collect accurate data on particle collisions and study some of the universe’s most compelling mysteries, the collaboration must ensure that the equipment is properly calibrated.
At CERN, the Large Hadron Collider, or LHC, smashes protons at the highest energy ever to create new particles, which physicists capture and study in a number of experiments.
The LHC explores the hidden world of subatomic particles, the basic building blocks of everything around us. Studying these particles helps scientists like me better understand how the universe works and evolves over time.
Hibernating and waking up the LHC
Every winter, the collider and its experiments go through the winter. I and other teams at CERN push them to take 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 those machines use a lot of power, we avoid running them in winter, when electricity costs more and nearby Geneva needs to keep its inhabitants warm.
But when spring comes, all the teams prepare the LHC and the 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, the experimental physicists, prepare the experiments to quickly and accurately collect data from all the particles that the collider produces.
Test with cosmic rays
The experiment teams begin to wake up the first stage of the LHC from hibernation while the accelerator is still asleep. We have to test the particle detectors even with the collider that creates the particles not working.
In this first step, we use what is always available, provided by nature itself – cosmic rays. These are sub-atomic particles created when energetic particles from space strike atoms high in the atmosphere.
A cosmic ray enters the ATLAS detector in the LHC on the left. Each time it hits a detector, 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 hit together, 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 thin, so we can’t rely on them for all our tests. For subsequent tests, we use a closer and more predictable source – a sub-atomic splash.
Subatomic splashes to synchronize them all
The LHC has about 17 miles (27 kilometers) of pipes that protons fly through. There are magnets around the pipe that direct the protons it accelerates. Any stray particles are stopped by a small piece of metal called a collider. This collider is pushed down into the center of the accelerator pipe, where the protons go into it and interact with its atoms.
This collision creates a huge amount of particles, which move individually along the accelerator pipe as a big splash – or, as we call them, “beam flash”. Around mid-March, the accelerator team creates these for the ATLAS experiment.
A large wave of particles hits the entire experiment at the same time, and this wave allows us to verify if all the detectors in the experiment react correctly and in sync. It also tests whether they can record and store data at the required speed.
Horizontal trusts to calibrate them
Most of the particle detectors in the experiments are now ready to receive new data. However, some types of detectors in the LHC require additional tests.
One of them is the ATLAS experiment’s Tile calorimeter, a detector that measures the energy of particles such as neutrons and protons. It is made of rows of tile-shaped sensors, and test particles must pass through these tiles horizontally to accurately calibrate the detector.
The huge sprays of particles created by beam splashes are not good for calibrating the Tile calorimeter. The particles are not coming at right angles, and there are too many at once.
To test the Tile calorimeter, we are only interested in a certain type of particle – muons. Muons are like electrons but heavier, and they interact differently with the world around them. They can pass through multiple layers of sensors without losing much energy or being stopped – making them useful for testing particle detectors.
So, towards the end of March, we set up another test, again using the allies.
This time, however, the LHC engineers push the collider just a little into the path of the protons, so the particles barely scratch the collider. The gentle friction of the protons against the metallic surface of the collider creates particles that move parallel to the accelerator pipe and hit the ATLAS experiment horizontally.
We use dedicated sensors to detect and flag muons created by the collision with the collider. Then we track them as they move through the Tile calorimeter.
These horizontal muons go through all the calorimeter tiles one after the other, so we can make sure it’s collecting data accurately.
Ready for new physics
When the LHC is all calibrated and ready to go, it accelerates protons to their maximum energy – and then pushes them to crash into each other.
After about 10 weeks of tests, a new season of data collection begins, bringing dreams of new discoveries.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of 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 ATLAS International Collaboration and co-author of the experiment results. He is a former member of CERN, and currently has a “User” affiliation with CERN,
