The Atlas Muon Calibration Stream: Design and Performance

In the Atlas experiment, the calibration of the precision chambers of the muon detector is very demanding, since the rate of muon tracks required to get a complete calibration in homogeneous conditions and to feed prompt reconstruction with fresh calibration constants is very high (several hundreds Hz for 8-10 hours runs). The best place to get muon tracks suitable for muon detector calibration is the second level trigger, where the pre-selection of data sitting in a limited region by the first level trigger allows to select all (and only) the hits from a single track and to add some useful information to speed up the calibration process. Furthermore, online data extractions allows calibration data collection without performing special runs that would require special tuning of parameters of the Atlas TDAQ system. A complex system, involving a specific data collection path in the Atlas TDAQ, the quasi-online distribution of data through the grid to three calibration farms sitting in Tier-2 computing centers and the storage and replication of calibration parameters into local and central databases, is described, and its current performance and possible evolution are discussed.