The LHCb trigger system: performance and outlook

The LHCb experiment is dedicated to the study of heavy flavour at the LHC. The LHCb trigger system plays a key role in selecting signal events and rejecting background. The bulk of the LHCb trigger is implemented in software and deployed on a farm of 29,000 processor cores. This system, called the high level trigger, is responsible for reducing the event rate from 1 MHz, at which the LHCb detector can be read out, to 5 kHz, which can be written to disk. With its flexible design, the LHCb trigger can quickly adapt to changing running conditions and has performed far beyond its design in terms of signal efficiencies. The trigger system showcases a number of pioneering concepts, among them the use of multivariate classifiers to identify b -hadrons and the buffering of events to local disks and their processing at a later time, when the LHC is not producing collisions. The design of the trigger system, its performance during 2011 and 2012 and planned improvements for data taking in 2015 and beyond are discussed