ATLAS EXPERIMENT

The all-seeing eye: high-precision sensors for examining elementary particles

ATLAS is a collider experiment at the Large Hadron Collider (LHC) operated by the European Organization for Nuclear Research (CERN). It is also the name of the detector for the project to search for new particles. The ATLAS detector is one of the largest particle detectors ever built. It is 46 meters long, 25 meters in diameter, weights 7,000 tonnes, and contains sensors for 110,000,000 channels. We have discovered the Higgs boson with the data recorded by the ATLAS detector together with the CMS experiment, which is another experiment at LHC, in 2012.

The LHC is the energy frontier particle collider. The circular accelerator collides protons accelerated to almost the speed of light. The LHC was operated at the collision energy of 8 TeV (teraelectronvolts), when the Higgs boson was discovered. At this point, it had already claimed the position of the world's highest energy, but during its second operational run (Run 2), which began in June 2015, the energy reached 13 TeV, close to the design maximum. The ATLAS detector was also upgraded for Run 2, operating with high efficiency . Through the Run 2 data-taking period, the ATLAS detector recorded a large amount of collision data, which corresponded to 1.5 x 1016 proton collisions or 140 inverse femtobarn in luminosity. Using the Run 2 data, high-precision measurement of the Higgs boson was performed, resulting in discoveries of a decay and a production mode of the Higgs boson, which contributes to an understanding of the origin of elementary particle mass.

The LHC is planned to be shutdown from 2019 to 2020, and work is in progress to expand its capacity so that it can resume operation in Run 3, from 2021 to 2023, with a collision energy of 14 TeV. Upgrading is being conducted around the muon detector, and front-end and back-end trigger electronics. The ultimate objective of Run 3 will be the discovery of supersymmetry. ATLAS aims to make new discoveries that shine light on the mysteries of the origin of the universe, such as discovering particles that could potentially be dark matter.

ATLAS is an international joint research project with 183 collaborating universities and research institutions from 38 countries. It involves roughly 3,000 researchers, including roughly 1,200 graduate students. These researchers are dedicated to high-precision measurement of the Higgs boson and the discovery of new physics that go beyond the Standard Model of particle physics. Roughly 160 researchers and students from 16 Japanese universities and research institutions take part in the project. As the "ATLAS Japan Group", they are on par with the world's top researchers, carrying out cutting-edge particle physics research. ICEPP is sending about 30 researchers and students in the ATLAS Japan Group to CERN.

Since its establishment in April 1994, the ATLAS Japan Group has been playing a central role in international joint research. In addition to its involvement in the proposal and design of the ATLAS detector, it has built a superconducting solenoid, silicon tracking detectors, muon detectors, and more with the collaboration of Japanese companies. In 2007 it built the ATLAS regional computing center of the Worldwide LHC Computing Grid, the so-called Tier2 computing center, in ICEPP, where it has engaged in physics analysis for the ATLAS collaboration and physicists in Japanese institutions. The contributions of the Japanese team to the discovery of the Higgs boson have been recognized worldwide.

In parallel with Run 3, preparations are underway for the HL-LHC experiment (Run 4) scheduled to begin in 2026. The LHC's proton collision frequency is expected to increase roughly 5-fold, and the ATLAS detector's capabilities will be greatly improved. ICEPP is taking on the challenge of developing new trigger technologies that will produce high speeds, high levels of efficiency, and high levels of precision.

Why are these accelerators so enormous?

The LHC at CERN is a truly large-scale accelerator with a circumference of 27 kilometres, roughly the length of the JR Yamanote Line, which is a circular train line connecting major city centers of Tokyo.
Accelerators are devices that accelerate particles and increase their kinetic energy. In collisions between highly accelerated particles, particles might behave differently. Testing elementary particles in the macroscopic scales requires such a large-scale equipment because the smaller the object that is being examined, the greater the energy that is required. The progress made in particle physics would be inconceivable without accelerator development technologies. Accelerators have been used to study the origin of matter since the 1930s. The LHC accelerates protons, which are made up of multiple particles, in clockwise and counter-clockwise directions, making them have head-on collisions with the highest collision energies in the world.
Modern particle physics and theories of the universe believe that the high energy states created by accelerators are extremely similar to those immediately after the birth of the universe. The ultra-high levels of energy produced by the LHC, the highest in the world, are believed to replicate, for just an instant, the state of the universe one-trillionth of a second after the birth of universe.

The LHC is located in a tunnel 100 meters underground. Roughly 1,700 superconducting magnets create a powerful magnetic field that bends proton beams, narrowing them down to approximately 1/10 of the width of a human hair and making them collide. Japanese companies provided major technical contributions to the construction of the LHC. ©CERN
Conceptual image of the entire LHC. The LHC, which has a total length of 27 kilometres, contains four large detectors, ATLAS, CMS, ALICE, and LHCb, which observe the particles generated by collisions. ©CERN
This is a structural diagram of the ATLAS detector. It is placed at point of proton collisions in the LHC, and, with a high degree of accuracy, measures the types and kinetic momentum of various particles that are ejected from the collision point. ©CERN