This is not a simulation!

The ATLAS barrel tile calorimeter has recorded its first events underground using a cosmic ray trigger, as part of the detector commissioning programme.


A cosmic ray muon recorded by the ATLAS barrel tile calorimeter at 18:30, on 21 June 2005. The calorimeter has three layers and a pointing geometry. The light trapezoids represent the energy deposited in the tiles of the calorimeter depicted as a thick disk.

ATLAS has recorded its first events underground. On the evening of 21 June, the ATLAS detector, now being installed in the underground experimental hall UX15, reached an important psychological milestone: the barrel hadronic tile calorimeter recorded the first cosmic ray events in the underground cavern. An estimated million cosmic muons enter the ATLAS cavern every 3 minutes, and the ATLAS team decided to make good use of some of them for the commissioning of the detector.

The purpose-built trigger system selected cosmic rays - mainly muons - passing 'back-to-back' through the top and bottom of the calorimeter. Although only 8 of the 128 calorimeter slices ('superdrawers') were included in the trigger, beautiful muon tracks were seen traversing the detector.

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.

For two weeks, experts of different disciplines from CERN and the experiment (cooling, high-voltage, front-end electronics, data acquisition, offline), led by Richard Teuscher, worked underground in USA15, the counting room next to the main ATLAS cavern. The goal of the exercise 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 valuable experience was gained for the whole tile calorimeter system.

The project leader for the tile calorimeter, Robert Stanek, said: "The first picture of cosmic rays from the ATLAS pit is like a snapshot - it captures many years of hard work by a team from dozens of countries around the world."

This is just the first stage of a long ATLAS commissioning programme. After the completion of the installation of the barrel toroid magnet system, the barrel calorimeter will move to its final position in autumn 2005 (from the temporary 'parking' position it occupies now) and the final services will be connected. This autumn, a fraction of the muon spectrometer already installed in the pit will join the programme. The electromagnetic calorimeter will then be integrated after being cooled down in spring 2006.


View of the ATLAS cavern when the sixth toroidal coil was lowered at the beginning of July. The hadronic calorimeter can be seen at the end of the cavern.