In September, the ATLAS collaboration ran an "end to end" data chain for the first time, confirming that data distribution from the LHC to physicists across the globe will be possible. The challenge was to test the entire data chain, from the measurement of a cosmic-ray muon in the detector to the arrival of reconstructed data at computers located around the world in Tier-2 centres of the Worldwide LHC Computing Grid (WLCG).

The ATLAS detector measured about 2 million muons over a two-week period during testing, with data beginning to flow after a week of setting up. The detector measured real particles, with real analysis at sites across Europe and the US, in quasi-real time. The whole process worked with minimal human intervention.

Quasi-real time means that the transfer was achieved in hours. With real LHC collision data, however, there will be a greater delay. This is because processing and the subsequent data transport will have to wait for regular calibrations to take place. The data chain is designed to cope with this delay and there are sufficient disk buffers at the Tier-0 stage to retain the data for as long as several days.

A particle entering the ATLAS detector produces a signal and triggers the data chain. Muons are easily singled out and once detected they set the data chain in motion. The raw data are sent to the Tier-0 centre – the computer centre at CERN – where they are recorded onto tape before being sent to a different part of the centre for reconstruction.

The reconstructed data are also taped before being exported to the Tier-1 centres, and then sent to their respective Tier-2s. Physicists can then commence their analyses. While researchers will be analysing the data in different ways using different Tier-2s, they will be confident that they are all analysing the same data independent of where the data are stored. This consistency is ensured by the Grid middleware provided by the Enabling Grids for E-sciencE, NorduGrid and Open Science Grid infrastructures, which support computing for ATLAS.

Terabytes of ATLAS data travelled from the Tier-0 site at CERN to Tier-1 sites. These included seven sites across Europe, one site in the US, one site in Canada, and one site in Taiwan. Data transfer rates reached the expected maximum during the early part of September.