Dress Rehearsal for ATLAS debut

15 January 2008

“By doing this test, we’re trying to practice everything – from data coming off the experiment  all the way to shipping the data around and analysing it– under conditions just as they will be when real data-taking begins.”

Dave Charlton, and indeed the whole ATLAS collaboration, have a mammoth job on their hands; Dave has been tasked with coordinating the Full Dress Rehearsal (FDR) for the ATLAS experiment, a run-through which he describes as “essential, almost as much as ensuring the detector itself actually works”.

A huge challenge facing all ATLAS collaborators, the Dress Rehearsal will involve entering a prepared set of mixed simulated Monte Carlo data into the system as if it were originating from the detector itself.

Until now, reconstruction of data has not been time-critical. The FDR will, for the first time, allow us to see whether or not the system can deliver these within the short time-scales required.

Once the LHC and ATLAS are up and running, data will be fed out to the grid and spread all over the world for permanent storage and analysis. Initially, CERN will farm out the data to ten Tier 1 centres  across the globe, which will subsequently distribute it amongst their associated local ‘clouds’ of Tier 2 centres. The FDR exercise will discover whether or not the whole chain will work, under LHC-running conditions, prior to LHC switch-on.

“When the LHC starts we want to analyse the data within a few days. We want a very quick feedback to see what’s good, what the problems were, if there’s any new physics, if there’s anything really exciting in there,” said Dave. “To do that we must make sure this whole system works; that we can get the data out to people where they can look at it very quickly.”

Muriel is ready for the Full Dress Rehearsal... What about you?

A team of around ten people is preparing  the huge data samples needed to emulate LHC rates. Given the size of this test, only two one-week runs – in February and May – are planned between now and the LHC switch-on date.

In addition to the dedicated team, there is a whole network of GRID computing experts toiling away to ensure that data distribution over the GRID will work as expected The FDR exercise is intended to pull together the work of the various groups, a particularly tricky endeavour according to Dave:

“It’s difficult to coordinate people because the different disciplines are used to working in different ways. There’s more hierarchy in computing, but less in physics analysis – if people see something interesting they have to have the freedom to be able to go off and investigate those things, because occasionally they can turn out be very important.”

Dave hopes that the FDR process will show that any problems with the detector can be highlighted quickly, and shifts in its performance can be tracked, before real data-taking begins.

“Although we know how to do that already, by taking data away and working on it for months, we don’t know how to do it in a day,” he said. “This is one of the things that, by doing the FDR, we should improve a lot. Either that or find that the model is broken. But if that is the case – better that we know about it!”

Ceri Perkins


Ceri Perkins