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The LHCb Trigger and its Performance in 2011

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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.

Figures and captions

Layout of the LHCb detector.

y-LHCb[..].eps [1 MiB]
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y-LHCb-reoptimized.eps

Comparison of the $p_T$ distributions of $D^+\rightarrow K^-\pi^+\pi^+$ selected in NoBias and TIS events.

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D2HHH_pt.eps

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.

L0_Psi[..].eps [8 KiB]
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L0_Psimumu-eff.eps
L0_Had[..].eps [20 KiB]
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L0_Hadron_Eff_PT.eps

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}$ ).

L0_Psi[..].eps [8 KiB]
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L0_Psiee-eff.eps

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 $B^+$.

HLT1mu[..].eps [9 KiB]
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HLT1muon_JPsiK-PT.eps
HLT1mu[..].eps [8 KiB]
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HLT1muon_JPsiK-TAU.eps

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.

Hlt1_H[..].eps [20 KiB]
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Hlt1_Hadron_Eff_PT.eps
Hlt1_H[..].eps [20 KiB]
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Hlt1_Hadron_Eff_TAU.eps

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 $B^+$.

pt_1.eps [11 KiB]
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pt_1.eps
t_1.eps [9 KiB]
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t_1.eps

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}.

Hlt2_T[..].eps [11 KiB]
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Hlt2_TopoHadron_Eff_PT.eps
Hlt2_T[..].eps [9 KiB]
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Hlt2_TopoHadron_Eff_TAU.eps

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}.

HLT2_J[..].eps [9 KiB]
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HLT2_JPsiK-PT.eps
HLT2_J[..].eps [9 KiB]
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HLT2_JPsiK-TAU.eps

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}.

Hlt2_D[..].eps [11 KiB]
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Hlt2_DHadron_Eff_PT.eps
Hlt2_D[..].eps [9 KiB]
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Hlt2_DHadron_Eff_TAU.eps

Animated gif made out of all figures.

DP-2012-004.gif
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thumbnail_DP-2012-004.gif

Tables and captions

MC signal samples used to train the BBDT, where $K$ means $K^\pm$ and $\pi$ means $\pi^\pm$.

Table_1.pdf [52 KiB]
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Table_1.pdf

Allowed mapping points in the BBDT. The variables are explained in the text.

Table_2.pdf [55 KiB]
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Table_2.pdf

Cuts of L0 lines and their rates prior to throttling. The definition of the trigger lines is given in Section 2.

Table_3.pdf [65 KiB]
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Table_3.pdf

HLT1 muon lines and their cuts. The rate is measured on events accepted by {\tt L0Muon} or {\tt L0DiMuon}.

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Table_4.pdf

The cuts applied in {\tt Hlt1TrackAllL0} and {\tt Hlt1TrackPhoton} lines. The rate is measured on events accepted by L0.

Table_5.pdf [57 KiB]
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Table_5.pdf

HLT2 lines based on one identified muon.

Table_6.pdf [50 KiB]
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Table_6.pdf

HLT2 lines based on two identified muon.

Table_7.pdf [59 KiB]
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Table_7.pdf

HLT2 lines based on two identified muons.

Table_8.pdf [58 KiB]
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Table_8.pdf

HLT2 selection cuts applied for the exclusive lines {\tt Hlt2CharmHadD02HH$\_$D02KPi} and {\tt Hlt2CharmHadD2HHH}. The 2-track 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 30 May 2020.