ATLAS detector records its first curved muon

The barrel muon spectrometer of the ATLAS detector has acquired its first cosmic event in a magnetic field produced by the barrel toroid magnet. This was an important test of the chambers in their final configurations, and marked the first triggering and measurement of curved cosmic ray muons in ATLAS.

A 3-D event display of a cosmic muon event, showing the path of a muon travelling through three layers of the barrel muon spectrometer. Three of the eight coils of the barrel toroid magnet can be seen in the top half of the drawing.

On the screen of the event display of the ATLAS barrel muon spectrometer appeared a handful of small dots. They were sparsely spaced, but organised in such a way that enough information was there for one to make out a curve. This may be where the meaning ends to the untrained eye; but for those in the know, this was a moment to behold.

During the weekend of 18 to 19 November 2006, the barrel toroid magnet of the ATLAS detector underwent a stability test. The magnet was powered up to its nominal current of 20500 amperes and maintained at that level for more than a day. This presented the perfect occasion to measure some curved cosmic muons in the barrel muon spectrometer. The 7-metre-thick barrel spectrometer makes up the outer layer that surrounds the length of the ATLAS detector. Divided into 16 sectors, and further sub-divided into hundreds of stations, it will be used to measure the momenta of muons produced in the collisions of the LHC.

When a muon passes through the spectrometer, the 'resistive plate chambers' provide a trigger mechanism, instructing the detector to begin recording. The 'monitor drift tubes' then accurately measure the curvature of the path of the muon passing through 3 successive layers of the spectrometer (see diagrams). These layers - inner, middle, and outer - are distributed over a distance of 7 metres. The time and location of passage is individually recorded for each layer; when combined, they form an overall picture of the muon track. Each precision chamber layer is able to locate the muon coordinate with an accuracy of at least 50 micrometers. The barrel trigger chambers can measure the crossing time of the muon with an accuracy of the order of 3 billionths of a second. The precise alignment of the layers of the spectrometer is essential for calculating the curvature of the track, hence the momentum.

Visual display from the ATLAS barrel muon spectrometer showing the detection of a curved cosmic ray muon through 3 layers of the spectrometer.

During the magnet stability test, 13 stations in the 'feet sectors' of the barrel muon spectrometer (located at the base of the ATLAS detector), and the chain of trigger mechanisms, including the central trigger processor, were switched on. More than a million 'muon events' were recorded at rates of 18-30 Hz. These data are currently being analysed; they will be used to optimise the trigger system, calibrate and align the chambers, and to provide the first measurements of the momenta of cosmic ray muons in the ATLAS experiment. Data obtained from magnetic field probes mounted on the muon stations will help to complete a three-dimensional map of the magnetic field in different places within the detector.

Tests on the remaining sectors of the barrel muon spectrometer will be carried out in March 2007, mainly in the absence of a magnetic field. These are expected to be completed by November, when ATLAS expects the first particle beam in the LHC.