CERN's ATLAS detector is the largest ever constructed for a particle collider

The experiment seeks to answer fundamental questions about experimental physics.
Deena Theresa
Calorimeter insertion between toroids in the ATLAS experiment detector
Calorimeter insertion between toroids in the ATLAS experiment detector

CERN 

Ten years ago, scientists announced the discovery of the Higgs boson, which helped explain why the smallest building blocks of nature have mass. For particle physicists around the world, it was a very important and long-awaited result that marked a new era of experimental physics. Physicists at the Large Hadron Collider - the world's largest and most powerful particle accelerator - located at the European Organization for Nuclear Research (CERN) in Geneva discovered the Higgs boson largely due to results from the ATLAS experiment.

What exactly is the ATLAS experiment, and why is it a big deal?

ATLAS (short for "A Toroidal LHC Apparatus") was designed as a general-purpose particle physics experiment to explore the potential of the LHC (Large Hadron Collider). According to CERN, ATLAS "is a general-purpose particle physics experiment ... [which] uses precision measurement to push the frontiers of knowledge by seeking answers to fundamental questions such as: What are the basic building blocks of matter? What are the fundamental forces of nature? What is dark matter made of?" The experiment was designed to observe phenomena that involve highly massive particles which were not observable using earlier lower-energy accelerators.

At the LHC, beams of particles swirl around a 17-mile-long (27 kilometers) underground ring of the LHC, consisting of a ring of superconducting magnets with a number of accelerating structures that boost the energy of the particles along the way before they smash into each other. As the collisions create particles at the rate of over a billion interactions per second, they are detected by the ATLAS detector — the largest detector ever constructed for a particle collider. The particle detecter captures information about them, which is later analyzed by the ATLAS scientists.

The ATLAS collaboration, the group of physicists building the detector, was formed in 1992 when the proposed EAGLE (Experiment for Accurate Gamma, Lepton and Energy Measurements) and ASCOT (Apparatus with Super Conducting Toroids) collaborations merged their efforts into building a single, general-purpose particle detector for the LHC.

What makes up the ATLAS particle detector?

Though particles normally travel in straight lines, their paths can be curved by introducing a strong magnetic field. ATLAS is made up of "six different detecting subsystems wrapped concentrically in layers around the collision point to record the trajectory, momentum, and energy of particles." A huge magnet system bends the paths of the charged particles so that their momenta can be measured as precisely as possible.

It consists of six different detecting subsystems wrapped concentrically in layers around the collision point to record the trajectory, momentum, and energy of particles, allowing them to be individually identified and measured. A huge magnet system bends the paths of the charged particles in order to make accurate measurements.

According to ATLAS, the toroidal shape of the magnets give ATLAS its name. ATLAS's" inner detector calculates the amount of curvature and the momentum of the article by tracking the precise trajectory of each particle.

CERN's ATLAS detector is the largest ever constructed for a particle collider
An image taken of the ATLAS detector during LS2.

This detector is made of three layers. First, there is an array of almost 100 million silicon pixels to detect charged particles as they shoot out from the collision point. There's a semiconductor tracker made up of millions of "micro-strips" of sensors that surrounds the pixel detector. This provides further tracking of the emitted particles. Finally, a transition radiation tracker made of 300,000 gas-filled tubes is used to detect and identify charged particles as they ionize the gas.

An array of calorimeters (devices that stop and absorb particles to measure their energy) surround the inner detector. Lastly, a three-layer, high-precision spectrometer, the outermost part of the system, is aimed at detecting a muon.

The ATLAS solenoid surrounds the inner detector at the core of the experiment. This enormously powerful magnet is 5.6m long, 2.56m in diameter, and weighs over five tonnes. By embedding more than 9km of niobium-titanium superconductor wires into strengthened, pure aluminum strips, it provides a 2 Tesla magnetic field in 4.5cm of thickness. It minimizes possible interactions between the magnet and the particles being studied.

Tracking and identifying particles: the ATLAS experiment

The particle detector can detect some of the tiniest and most energetic particles ever created on earth. It comprises "six different detecting subsystems wrapped concentrically in layers around the collision point to record the trajectory, momentum, and energy of particles," according to the ATLAS website. This allows them to be individually identified and measured.

CERN's ATLAS detector is the largest ever constructed for a particle collider
A sub-detector being assembled in a clean room for the ATLAS experiment.

These beams of particles travel at energies up to seven trillion electron volts or speed up to 99.999999 percent that of light. From the LHC, they collide at the center of the ATLAS detector, producing collision debris which then flies out multi-directionally. Every second, a billion particle interactions take place in the ATLAS detector. However, only one in a million is flagged as potentially interesting. These are then recorded for further study. Particles are tracked and identified to investigate physics theories and phenomena, including the study of the Higgs boson and the search for dark matter.

According to the U.K. Science and Technology Facilities Council, ATLAS is the largest collider detector ever constructed, with a length of 151 feet (46 meters), a diameter of 82 feet (25 m), and a weight of 7,700 tons (7,000 metric tons - similar to the weight of the Eiffel Tower). The detector is in an underground cavern that is 328 feet (100 m) below the surface, close to the village of Meyrin in Switzerland. It comprises a gigantic magnetic system, which takes the form of eight superconducting toroids that are 82 feet (25m) in length.

The ATLAS experiment website states that particle collisions take place at a rate of around a billion per second. These collisions produce data, which is recorded using more than 100 million electronic channels. These are then analyzed by scientists across the world. The collaboration is one of the largest attempted in science, boasting over 5,500 members and nearly 3,000 scientific authors.

ATLAS's most significant moment - the Higgs boson

According to CERN, there are two general-purpose detectors at the LHC - the Compact Muon Solenoid (CMS) experiment and the ATLAS. Both the detectors address similar scientific goals though they have different technical approaches and magnet designs. Their goals include answering the zillion questions scientists have about the universe, undiscovered dimensions, and dark matter.

CERN's ATLAS detector is the largest ever constructed for a particle collider
Higgs candidate event in the ATLAS detector containing 2 W bosons, in which one W decays to two jets and the other to a muon and a muon neutrino.

The discovery of the Higgs boson, the missing piece of the Standard Model, has been ATLAS's biggest moment to date. Though the particle was predicted to exist decades ago in the 1960s, it had never been observed with the previous generations of particle detectors, primarily due to its large mass and transient existence. But, on July 4, 2012, ATLAS and CMS detected the Higgs particle with a significance of "5 sigma," which means that there was less than a one in a million possibility that the detection was caused by random fluctuations (or, to put it another way, the results had a 99.99994 percent confidence). For their work, Peter Higgs and his collaborator Francois Englert won the Nobel prize "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles."

Ever since then, ATLAS has been largely occupied. CERN has reported that the ATLAS collaboration submitted its 1,000th scientific paper for publication in June 2021. Scientists are still looking for their next significant discovery after the Higgs boson. ATLAS physicists are also on the lookout for an unambiguous signal of physics beyond the Standard Model.

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