ATLAS is an elementary-particle physics experiment at the LHC (Large Hadron Collider) at CERN, near Geneva. The experimental apparatus has been designed and built to measure with the highest efficiency the products of proton-proton (and also proton-ion and ion-ion) collisions at very high energies.

Various level triggers have been developed for the selection of the relevant interactions for the studies of different physical channels.

At the same time an intense software activity has been undertaken for the simulation, the reconstruction of the events and the analysis of physics channels.

The ATLAS experiment is co-financed in Italy by the National Institute of Nuclear Physics (INFN).