The Software Architecture of the LHCb High Level Trigger

Abstract

The LHCb experiment is a spectrometer dedicated to the study of heavy flavor at the LHC. The trigger system plays a key role in selecting signal events and rejecting background at the data acquisition phase. The bulk of the LHCb trigger is implemented in software and deployed on a farm running about 20000 processes to analyse incoming events parallel. This system, called the High Level Trigger (HLT), is responsible for reducing the rate from the maximum at which the detector can be read out, 1 MHz, to the 3 kHz which can be processed off-line. Because of the specific LHC running conditions, the HLT had to cope with three times higher event multiplicities than it was designed for in 2010 and 2011. This contribution describes the software architecture of the HLT, focusing on the code optimization and commissioning effort which took place during 2010 and 2011 in order to enable LHCb to trigger efficiently in these unexpected running conditions, and the flexibility and robustness of the LHCb software framework which allowed this reoptimization to be performed in a timely manner.