Measurement of the Z boson cross-section in - Harvard University ...

Chapter 2: The Accelerator and the Experiment 62
barrel, for example, must be aligned using straight tracks that pass through overlap
regions between these chambers and others that are connected to the optical grid 9 .
Track-based alignment must also be used to monitor the relative alignment of the
barrel and the endcap muon systems, and of the muon system and the inner detector.
Finally, track-based alignment provides a cross-check on the optical alignment system.
2.2.5 Trigger and Data Acquisition System (TDAQ)
The goal of the trigger system is to select physically interesting events for perma-
nent storage. It also reduces the high event rate to an acceptably small value that the
storage hardware can handle. At the design LHC luminosity of 10 34 cm −2 s −1 , about
20 hard interactions are expected to occur per bunch-crossing. At a bunch-crossing
rate of 40 MHz, this implies 10 9 events per second. On the other hand, available
resources limit output to the storage to about 300 Hz. Hence, the trigger system
must decrease the event rate by up to seven orders of magnitude. To accomplish this,
ATLAS has three levels of trigger: Level-1, a hardware trigger, and Levels -2 and -3,
both software triggers.
The DAQ system is responsible for data movement, starting from the front-end
readout electronics to the transfer of raw events to the permanent storage. Figure 2.13
shows a block diagram of the ATLAS trigger and data acquisition system. The various
parts of the system are summarized below.
The Level-1 (L1) trigger is implemented in the calorimetry and the muon system.
Using information from muon trigger chambers and reduced-granularity information
9 A large number of straight tracks were obtained during runs with the magnetic fields turned off.