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

Chapter 2: The Accelerator and the Experiment 63
from the calorimeters, the L1 trigger looks for signatures that may indicate physics of
interest, such as high-energy electrons, photons, jets, τ leptons, large missing energy
and/or total energy, and high-pT muons. Another function of the L1 trigger is to
define Regions of Interest (RoI’s), i.e. , regions of the detector in which interesting
activity has been found. RoI’s are used for seeding the Level-2 trigger. The L1 trigger
takes about 2.5 µs to make an accept/reject decision, which is equivalent to 100 bunch
crossings at the 25 ns crossing interval. This duration is known as the trigger latency.
The maximum L1 accept rate which the readout systems can accommodate is 75 kHz,
upgradeable to 100 kHz.
The front-end readout electronics of each subdetector contains data buffers known
as pipelines. Pipelines in the muon system and the calorimetry have enough depth
to hold data for the L1 latency of 2.5 µs. Upon an L1 accept decision, data from the
relevant cells is transferred to Readout drivers (RODs).
The Level-2 (L2) trigger looks only at the RoI’s defined by L1. It has access to
full-granularity information from all subdetectors within the RoI’s. The information,
extracted from the relevant RODs, amounts to ≈ 2% of the total event data for a
typical event [98]. The L2 trigger has a latency of about 40 ms, and decreases the
event rate to 3.5 kHz.
Events selected by the L2 trigger are transferred to the event builder, which is the
first stage of the Level-3 (L3) trigger. The event builder uses elaborate algorithms to
fully reconstruct events using information from the entire detector. The reconstructed
events are then moved to the Event Filter (EF), which performs the final selection
of events to be written to mass storage. The EF typically takes 4 seconds to process