The CMS Trigger System

Wesley Smith
University of Wisconsin

At the LHC design luminosity of 10^34 cm-2 s-1, a beam crossing every 
25 ns contains on average 17.3 events. These 10^9 interactions per 
second must be reduced by a factor of 10^7 to about 100 Hz, the 
maximum rate that can be archived by the on-line computer farm.  
The CMS Trigger System does this in two steps. The level-1 trigger 
first reduces the rate to 100 kHz using coarsely segmented data from 
calorimeter and muon detectors to produce a trigger decision in 
3 microseconds while holding all the high resolution data in pipeline 
memories in the front-end electronics. All triggered events are then 
read out and further processed by the High Level Trigger which uses 
an on-line computer farm running a hierarchy of algorithms to achieve 
the remaining rate reduction.