Abstract

This paper presents the design of the LHCb trigger and its performance on data taken at the LHC in 2011. A principal goal of LHCb is to perform flavour physics measurements, and the trigger is designed to distinguish charm and beauty decays from the light quark background. Using a combination of lepton identification and measurements of the particles' transverse momenta the trigger selects particles originating from charm and beauty hadrons, which typically fly a finite distance before decaying. The trigger reduces the roughly 11 MHz of bunch-bunch crossings that contain at least one inelastic pp interaction to 3 kHz. This reduction takes place in two stages; the first stage is implemented in hardware and the second stage is a software application that runs on a large computer farm. A data-driven method is used to evaluate the performance of the trigger on several charm and beauty decay modes.

Highlights

  • 20m z a software application that runs on a farm of Personal Computers (PCs)

  • It has evolved significantly compared to reference [1], in which it is assumed that the LHC machine would operate with a 25 ns bunch separation [2], and that LHCb would limit the number of visible pp interactions1 such that the average number of visible interactions per bunch crossing μ ≃ 0.4

  • The HLT runs on the Event Filter Farm (EFF) that is a farm of multiprocessor PCs

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Summary

First level trigger

L0 is divided into three independent triggers; the L0-Calorimeter trigger, the L0-Muon trigger and the L0-PileUp trigger. The RS emulates the state of the FE buffers to protect against their overflow. It has information on the state of the buffers in the readout boards of all sub-detectors and the availability of the PCs in the farm. Based on this information it can retain or throttle a bunch crossing

L0-Calorimeter trigger implementation
L0-Muon trigger implementation
High level trigger
Topological trigger lines
Topological multivariate lines
Exclusive lines
Data-driven trigger performance determination
Trigger performance
Bandwidth division procedure
L0 performance
HLT1 performance
HLT2 performance
Findings
Summary

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