On the evening of 21 June, the ATLAS detector, now being installed in the underground experimental hall UX15 at CERN, reached an important psychological milestone: the first cosmic-ray events were recorded by the barrel hadronic tile calorimeter in situ. Although only four of the 64 calorimeter slices were included in the trigger, beautiful muon tracks were seen traversing the detector. The purpose-made trigger box selected cosmic rays passing close to the interaction region, thus giving the impression of “back-to-back” tracks.
An estimated 1 million cosmic muons enter the ATLAS cavern every 3 min, and the ATLAS team decided to use of some of them for the commissioning of the detector. For two weeks, experts of different disciplines from CERN and the experiment (cooling, high-voltage, front-end electronics, data acquisition, offline) worked underground in USA15, the counting room next to the main ATLAS cavern. Their goal was the commissioning of hardware and software systems, monitoring long-term stability and checking module uniformity and performance. The test used final components for the whole signal chain up to the counting room and provided valuable experience for the whole tile calorimeter system.
This is just the first stage of a long ATLAS commissioning programme, which will gradually see more subdetectors taking part. In autumn with a portion of the muon spectrometer already installed in the pit will begin commissioning, and will be joined in spring 2006 by the electromagnetic liquid argon calorimeter after it has been cooled. A complete “slice” of the ATLAS detector ran in a test beam during 2004, but this is the first time that events have been recorded underground.