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 bunchbunch 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 datadriven method is used to evaluate the performance of the trigger on several charm and beauty decay modes.
Layout of the LHCb detector. 
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Comparison of the $p_T$ distributions of $D^+\rightarrow K^\pi^+\pi^+$ selected in NoBias and TIS events. 
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Efficiency $\epsilon^{\rm TOS}$ of $B^+\rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} (\mu^+\mu^)K^+$ as a function of $p_T$ ( $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ ) for {\tt L0Muon} and {\tt L0DiMuon} lines. 
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The efficiency $\epsilon^{\rm TOS}$ of {\tt L0Electron} is shown for ${B}^0\rightarrow J/\psi{(\rm e^+e^)}K^{*0}$ as a function of $p_T$ ( $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ ). 
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Efficiency $\epsilon^{\rm TOS}$ of {\tt Hlt1TrackMuon}, {\tt Hlt1DiMuonHighMass} and {\tt Hlt1DiMuonLowMass} for $B^+ \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} (\mu^+\mu^) K^+$ as a function of the $p_T$ and lifetime of the $B^+$. 
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Efficiency $\epsilon^{\rm TOS}$ of {\tt Hlt1TrackAllL0} is shown for $ B ^ \rightarrow D ^0 \pi ^ $ , $ B ^0 \rightarrow D ^ \pi ^+ $ , $D^0\rightarrow K^\pi^+$ and $D^+\rightarrow K^\pi^+\pi^+$ as a function of $p_T$ and $\tau$ of the $ B $ meson and prompt $ D $ meson respectively. 
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Efficiencies $\epsilon^{\rm TOS}$ of {\tt Hlt2DiMuonJPsiHighPT} and {\tt Hlt2DiMuonDetachedJPsi} for $ B ^+ \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} K ^+ $ as a function of $p_T$ and $\tau$ of the $B^+$. 
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Efficiency $\epsilon^{\rm TOS}$ if at least one of the lines {\tt Hlt2Topo$n$Body}, with $n=2,3$, selected the event for $ B ^ \rightarrow D ^0 \pi ^ $ and one of the lines with $n=2,3,4$ for $ B ^0 \rightarrow D ^ \pi ^+ $ as a function of $p_T$ and $\tau$ of the $ B $ meson. The efficiency is measured relative to events that are TOS in {\tt Hlt1TrackAllL0}. 
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Efficiency $\epsilon^{\rm TOS}$ if at least one of the lines {\tt Hlt2Topo$n$Body} or {\tt Hlt2TopoMu$n$Body}, with $n=2,3$, selected events for $ B ^+ \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} K ^+ $ , as a function of $p_T$ and $\tau$ of the $ B $ meson. Also shown is $\epsilon^{\rm TOS}$ if the line {\tt Hlt2Topo$n$Body}, with $n=2,3$, selected the events. {\tt Hlt2Topo2Body} shows the inclusive performance of the topological lines. The efficiency is measured relative to events that are TOS in either {\tt Hlt1TrackAllL0} or {\tt Hlt1TrackMuon}. 
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Efficiency $\epsilon^{\rm TOS}$ of the lines {\tt Hlt2CharmHadD2HHH} and {\tt Hlt2CharmHadD02HH$\_$D02KPi} for $D^+\rightarrow K^\pi^+\pi^+$ and $D^0\rightarrow K^\pi^+$ respectively as a function of $p_T$ and $\tau$ of the $ D $ meson. The efficiency is measured relative to events that are TOS in {\tt Hlt1TrackAllL0}. 
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Animated gif made out of all figures. 
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MC signal samples used to train the BBDT, where $K$ means $K^\pm$ and $\pi$ means $\pi^\pm$. 
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Allowed mapping points in the BBDT. The variables are explained in the text. 
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Cuts of L0 lines and their rates prior to throttling. The definition of the trigger lines is given in Section 2. 
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HLT1 muon lines and their cuts. The rate is measured on events accepted by {\tt L0Muon} or {\tt L0DiMuon}. 
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The cuts applied in {\tt Hlt1TrackAllL0} and {\tt Hlt1TrackPhoton} lines. The rate is measured on events accepted by L0. 
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HLT2 lines based on one identified muon. 
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HLT2 lines based on two identified muon. 
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HLT2 lines based on two identified muons. 
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HLT2 selection cuts applied for the exclusive lines {\tt Hlt2CharmHadD02HH$\_$D02KPi} and {\tt Hlt2CharmHadD2HHH}. The 2track cuts refer to a candidate constructed of two tracks, and $m_{\rm corr}$ is defined in equation 2. The angle $\alpha$ is the angle between the momentum of the $ D $ and the vector connecting the PV with the $ D $ vertex. Some selections require that at least one or two tracks pass a cut, indicated with "$\geqq $". 
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Created on 12 October 2019.