This photo of part of the Large Hadron Collider shows eight coils in place. Each is 25 metres long. Credit: ATLAS experiment at CERN/Flickr.
When you press “Start” on your microwave or computer, the appliance turns on immediately. But major physics experiments like the Large Hadron Collider at the European Organization for Nuclear Research, 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 inside.
I am a physicist at CERN who has been working with my colleagues over the past few months on the process of resetting the greatest of experiments, ATLASTo collect accurate data on particle collisions and study some of the universe’s most fascinating mysteries, the collaboration must ensure that the equipment is properly calibrated.
HAS CERNTHE Large Hadron Collideror LHC, crushes protons at the highest energy ever achieved to create new particles, which physicists then capture and study through several experiments.
The LHC explores the hidden world of subatomic particlesthe fundamental building blocks of everything around us. Studying these particles helps scientists like me better understand how the universe works and changes over time.
Hibernation and awakening of the LHC
Every winter, the collider and its experiments hibernate. My team and other teams at CERN encourage them to take this winter nap for several reasons.
THE machines we use here are complex. We need time to replace parts or install new components. And, since all these machines use a lot of energyWe avoid running them in winter, when electricity costs more and the neighboring city of Geneva needs to keep its residents 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 crush protons, my colleagues and I, the experimental physicists, are preparing the experiments to quickly and accurately collect data on all the particles produced by the collider.
Tests with cosmic rays
The experimental teams are starting the first phase of the LHC’s wake-up, while the accelerator is still in standby mode. We need to start testing the particle detectors even if the collider that generates the particles is not running.
In this first phase, we use what nature constantly offers us: cosmic rays. These are subatomic particles created when energetic particles from space come into contact with atoms located high in the atmosphere.
A cosmic ray enters the LHC’s ATLAS detector on the left. Each time it hits a sensor, the ray loses some of its energy, which the detector converts into a signal and records. By tracing a line through all the sensors the cosmic particle has encountered, physicists can reconstruct its direction of arrival, its path through the experiment, and its energy. Cosmic rays help us train the sensors and check that everything is working as expected.
However, cosmic rays are random and rare, so we can’t rely on them for all of our tests. For the following tests, we use a denser and more predictable source: subatomic splashes.
Subatomic splashes to sync them all
The LHC has approximately 27 kilometers of pipelines The protons move through it. The tube is surrounded by magnets that direct the protons as they accelerate. Any particles that stray from its path are stopped by a small piece of metal called a collimator. This collimator is pushed toward the center of the accelerator tube, where the protons smash into it and interact with its atoms.
This collision creates a huge amount of particles, which then move in unison down the accelerator tube in a big splash – or, as we call them, a “splash jet.” Around mid-March, the accelerator team creates these particles for the ATLAS experiment.
The large wave of particles hits the experiment at the same time, and this wave allows us to check whether all the detectors in the experiment are responding correctly and in sync. It also allows us to test whether they can record and store data at the required speed.
Horizontal muons to calibrate them
Most of the particle detectors used in the experiments are now ready to collect new data. However, some types of LHC detectors require further testing.
One is the Tile calorimeter of the ATLAS experimenta detector that measures particle energy like neutrons and protons. It consists of rows of tile-shaped sensors, and test particles must pass through these tiles horizontally to accurately calibrate the detector.
The massive jets of particles created by the beam splash are not suitable for calibrating the tile calorimeter. The particles are not arriving at the right angle and there are too many of them at once.
To test the Tile calorimeter, we are only interested in a special type of particle – muonsMuons are similar to electrons but heavier, and they interact differently with the world around them. They can pass through multiple rows of sensors without losing much energy or being shut down – making them useful for testing particle detectors.
So towards the end of March we set up another test, again using the collimators.
This time, the LHC engineers pushed the collimator only slightly into the protons’ path, so that the particles barely brush against 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 hit the ATLAS experiment horizontally.
We use dedicated sensors to detect and report muons created by the collision with the collimator. We then track them as they travel through the Tile calorimeter.
These horizontal muons pass through all the tiles of the calorimeter in a row, allowing us to ensure that it collects data accurately.
Large Hadron Collider ready for new physics
Once the LHC is fully calibrated and ready to operate, it accelerates protons to their maximum energy – and then causes them to crash into each other.
After about 10 weeks of testing, a new season of data collection begins, bringing dreams of new discoveries.
This article was first published on The conversation. It is republished here under a Creative Commons license.