In September the particle physics experiment ATLAS went "end-to-end" for the first time.
When the LHC is turned on, physicists worldwide will be waiting by their computers. They'll be expecting express and non-stop delivery of massive amounts of data, streamed in a virtually seamless sequence direct to their doorstep.
And in September ATLAS used the LHC Computing Grid (LCG) to prove that this data distribution to physicists across the globe will be possible.
From cosmic ray to you
"We did the whole thing," smiles ATLAS's Kors Bos. "We mastered the entire data chain for the first time, from measurement of a real cosmic ray muon in the detector to arrival of reconstructed data at Tier-2 computers all over the world, with all the steps in between."
"We measured about 2 million muons over two weeks," says Bos. "We spent the first week setting up, but in the last week we started getting something useful. The detector measured real particles, real analysis happened at sites across Europe and the US, and it happened in quasi real time. The whole machine worked without human intervention."
Quasi real time, says Bos, will be more than sufficient to keep the chain moving.
"It's good that we could achieve this transfer in hours, but with the real data there will be a greater delay. We will need to do regular calibrations, and the processing and subsequent data transport will have to wait for that. The data chain is designed to cope with this delay and there are sufficient disk buffers at the Tier-0 stage to keep the data for as long as several days."
Domino data chain of events
The data chain is triggered only when a particle enters the ATLAS detector and produces a signal in its sensitive layers.
"We know these particles must be muons, because everything else is stopped by the metres of clay above the detector. Only muons can get through, and only muons can trigger the detector," Bos explains.
Once the particle has been detected, the domino data chain is set in motion. The raw data is sent to the Tier-0 centre, the Computer Centre at CERN, where it is recorded onto tape before being sent to a different part of the centre for reconstruction.
"The reconstructed data also goes to tape," says Bos. "It is then exported to all the Tier-1s, and when it arrives it is exported by the Tier-1s to their Tier-2s. When it arrives at the Tier-2s the physicists can pick it up and say, 'wow, look at that' ."
Bos says that while researchers from different Tier-2 centres will be analysing the data in different ways, they will all be analysing the same data.
"The data analyses are bound to be different, but if you say 'give me the data from 123, event 345 on Sunday morning', they should all be able to provide the same thing."
This data chain will be used to share ATLAS data with more than 1900 physicists from more than 160 participating institutions in 35 countries. The massive data transfer requirements of the LCG are supported in Europe and the US by the Enabling Grids for E-sciencE and Open Science Grid infrastructures.
• This article was published online in iSGTW on 19 September.