The ATLAS Experiment observes the production of top quark pairs – a major milestone of the early LHC physics programme. As the heaviest known elementary particle with a strong coupling to the Higgs boson, the top quark is key to understanding physics at the energy frontier. As the LHC generates hundreds of millions of top quarks, the ATLAS Experiment is able to study the particle’s properties in great detail.

Only 3 weeks into its first LHC heavy-ion run, ATLAS observed an unexpectedly large imbalance of energy in pairs of jets created in lead-ion collisions at the LHC. This striking effect, which is not seen in proton–proton collisions, could be a sign of strong interactions between jets and a hot, dense medium (quark-gluon plasma) formed by the colliding ions. Details studies of the effect followed this first observation.

The start of a fantastic era of physics! ATLAS records first collisions at 0.9 TeV.

To celebrate the end of the ATLAS construction phase, the Collaboration hosts a party at CERN's magnet testing facility (SM18). This was the largest ATLAS celebration yet, with collaboration members travelling from around the world to mark this milestone.

Team of engineers begin excavating a series of underground caverns in Meyrin, Switzerland. These caverns will be home to the ATLAS Experiment and its supporting infrastructure.

The LHC Experiments Committee and CERN Director-General, Chris Llewellyn Smith, approve the construction of the ATLAS detector. As the first Technical Design Reports are approved, ATLAS teams all over the world begin building detector components and worked on final technical developments.

The LHC will be upgraded to 14 TeV collision energy. The first major upgrade is Phase I, scheduled for 2018, and Phase 2 in 2022. The experiments will continue taking data until 2035. By then ATLAS expects to have collected 100 times more data than they had at the beginning of Long Shutdown 1.

In February 2013, the LHC and the ATLAS Experiment begin the first Long Shutdown. This maintenance period would see the first upgrades to the ATLAS Experiment installed, in preparation for higher luminosity operations.

On 4 July 2012, as a curtain raiser to the year’s major particle physics conference, ICHEP 2012 in Melbourne, the ATLAS and CMS experiments present their latest preliminary results in the search for the long-sought Higgs particle. Both experiments have observed a new particle in the mass region around 125-126 GeV. 

The next step is to determine the precise nature of the particle and its significance for our understanding of the universe. Are its properties as expected for the long-sought Higgs boson, the final missing ingredient in the Standard Model of particle physics? Or is it something more exotic? The Standard Model describes the fundamental particles from which we, and every visible thing in the universe, are made, and the forces acting between them. All the matter that we can see, however, appears to be no more than about 4% of the total. A more exotic version of the Higgs particle could be a bridge to understanding the 96% of the universe that remains obscure.

Explore the resources prepared for press.

ATLAS records collisions at 7 TeV centre-of-mass energy for the first time.

Particle physicists around the world anticipate a rich harvest of new physics as the LHC begins its first long run at an energy three and a half times higher than previously achieved at a particle accelerator.

Explore the resources prepared for press.