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EPS-HEP / The LHCb Higher Level Trigger in Run II

The current LHCb trigger system consists of a hardware level, which reduces the event rate of 30 MHz of inelastic collisions to 1 MHz, at which the detector is read out. In the subsequent High Level Trigger, based on a farm of 20k parallel-processing CPUs, the event rate is reduced to what can be processed offline, about 5 kHz in Run I. In preparation for Run II, LHCb has implemented a mechanism which uses disk space in the HLT farm to buffer events while performing run-by-run detector calibrations, and which allows the HLT to exploit the time between LHC fills for processing events. We show how these changes will allow the Run II HLT to implement almost the full offline reconstruction, and the way that this approach will expand LHCb’s Charm and Kaon physics programmes in particular. We also discuss the relevance of multivariate selections in the context of such an offline-like HLT. Finally, we discuss how this offline-like HLT will allow LHCb’s output rate to be increased to 12.5 kHz in Run II, in particular by dedicating part of the bandwidth to exclusive triggers which perform the full offline selection and analysis real-time and write to disk only the few informations needed for the extraction of the physical observables. We also discuss the impact of this real-time analysis scheme on the physics programme of the LHCb upgrade, relying entirely on the HLT that will perform an offline-like reconstruction on the full 40MHz LHC bunch crossing rate in real-time.

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