The "Standard Model": our vision of the elementary constituents and their interactions as described by the relativistic quantum field theory and by very deep principles of symmetry.
Since this model is not capable of encompassing all known physical phenomena (gravitation, matter and dark energy, matter-antimatter asymmetry), our aim is to seek experimental clues that can put us on the path of a more complete theory.
Principles of the experiment:
Atlas observes spectacular collisions of protons with a total energy of 13 TeV in the centre of mass (14 TeV from 2021). These protons are accelerated by the LHC, an underground accelerating ring of 27 km in circumference. The LHC tunnel is fully equipped with superconducting magnets that direct and focus protons on circular trajectories. The successive layers of the Atlas detector are used to determine the trajectories of charged particles and to measure the energy of most charged and neutral particles. The curvature of trajectories in the magnetic field makes it possible to determine their momentum and their electrical charge. Of the thousand million collisions produced per second, only a few possess particular characteristics that can lead to discoveries. The trigger system selects these events to avoid recording an enormous - and unnecessary - amount of information.
The analysis of particles from proton frontal collisions has already revealed the existence of a new particle, the Higgs boson, which is essential for the validation of the "Standard Model". Current and future data taking may help to shed some light on unknown processes at the core of matter.
All magnets are superconducting. Electromagnetic and hadronic calorimeters complete this apparatus. A central solenoid magnet provides a field of 2 Teslas for precise trajectography with the inner detector.
The Atlas detector, measuring 46 m long and 25 m high, is the largest and, together with the CMS detector, the most complex detector ever built for a particle physics experiment. It is also remarkable for its magnetic configuration. The large size of the magnetic volume (without iron, unlike the case of a conventional device) allows an accurate measurement of high energy muons.
The magnetic fields used for the outer muon spectrometer are very different:
Atlas: the eight coils, enclosed in a cryostat, are assembled in a large structure, in which a very large electric current creates a toroidal magnetic field around the axis of the beam and not along the axis of the cylinder as in CMS. The Atlas muon spectrometer is a stand-alone system that requires little monitoring after start-up.
CMS: Most of the experiment is a cylinder in which a very powerful solenoidal magnetic field acts along the proton beam. The return field in the iron yoke is also used to complete the measurement of the muon momentum in the inner detector.
Electromagnetic calorimeters that measure the energies and positions of photons and electrons use very different techniques: A lead-liquid argon sandwich for ATLAS, which favors radiation resistance and accuracy of position measurement, scintillating crystals for CMS which favors the resolution of the energy measurement, in particular for low values.