ATLAS experiment

ATLAS experiment
source
Name	ATLAS
Caption	A cutaway diagram of the ATLAS detector.
Collaboration	ATLAS Collaboration
Accelerator	Large Hadron Collider
Location	CERN
Energy	13.6 TeV (center-of-mass)
Luminosity	~2×10³⁴ cm⁻²s⁻¹
Website	https://atlas.cern

ATLAS experiment. It is one of the two general-purpose particle detectors at the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research. The experiment is designed to explore fundamental questions in particle physics, including the origin of mass, the nature of dark matter, and the differences between matter and antimatter. Its monumental scale and complexity make it a flagship project in modern high-energy physics.

Overview

The ATLAS detector is situated approximately 100 meters underground at Point 1 of the LHC ring near the main CERN site in Meyrin, Switzerland. It was conceived and constructed by a global collaboration of scientists and engineers, with its initial design phase beginning in the early 1990s. The detector recorded its first cosmic ray events in 2006 and began colliding proton beams at unprecedented energies with the start of the LHC in 2009. As a general-purpose detector, it is instrumented to measure a wide range of particles produced in proton–proton collisions and heavy-ion collisions, providing a broad window into phenomena predicted by the Standard Model and beyond.

Detector design

The ATLAS detector is a cylindrical structure, 44 meters long and 25 meters in diameter, weighing about 7,000 tonnes. It employs a layered, onion-like design with several concentric sub-detector systems, each specialized for measuring different particle properties. Innermost is the Inner Detector, which tracks charged particles within a 2 tesla (unit) magnetic field provided by a central solenoid. This system includes silicon-based technologies like the Pixel Detector and the Semiconductor Tracker, surrounded by a Transition Radiation Tracker. Surrounding this are the calorimeter systems: the liquid argon electromagnetic calorimeter measures electrons and photons, while the steel-scintillator tile calorimeter measures hadrons. The outermost Muon spectrometer uses large toroid magnets and monitored drift tubes to precisely measure muon trajectories. This comprehensive design allows for the accurate reconstruction of collision events.

Physics goals and discoveries

The primary physics objectives include rigorous testing of the Standard Model, such as precise measurements of the W boson and top quark, and the search for new particles and forces. A landmark achievement was the joint discovery with the CMS experiment of a Higgs boson-like particle in July 2012, a breakthrough confirming the Brout–Englert–Higgs mechanism. Subsequent measurements have focused on determining the new particle's properties, including its spin, parity, and couplings to other particles. The experiment also conducts extensive searches for evidence of supersymmetry, extra dimensions, and particles that could constitute dark matter, such as weakly interacting massive particles. Analyses of lead-ion collisions probe the properties of the quark–gluon plasma, a state of matter believed to have existed just after the Big Bang.

Collaboration and operation

The project is operated by the ATLAS Collaboration, one of the largest collaborative efforts in the physical sciences, involving over 5,500 scientists and engineers from more than 180 institutions in 42 countries. Major contributing nations and regions include those from the United States, Germany, the United Kingdom, Japan, and Italy. The detector is maintained and upgraded during extended shutdowns of the LHC, such as the recent installation of the Inner Tracker for the High-Luminosity LHC project. Day-to-day operation is managed by a rotating team of shift crews and experts based at the CERN Control Centre and remote operations centers worldwide, ensuring continuous data-taking during collision runs.

Data analysis and computing

The experiment generates enormous volumes of data, with raw data rates exceeding 1 petabyte per second during collisions. A sophisticated multi-tiered trigger system reduces this to a manageable rate for permanent storage. The recorded data is processed and distributed for analysis via the Worldwide LHC Computing Grid, a global computing infrastructure connecting hundreds of data centers. Physicists use advanced statistical methods and software frameworks like ROOT to compare collision data against simulations from event generators such as Pythia and Sherpa. This global computing model enables thousands of researchers to perform independent analyses, leading to hundreds of scientific publications each year in journals like Physical Review Letters and the European Physical Journal C.

Category:Particle physics experiments Category:CERN experiments Category:Large Hadron Collider