The LHCb Trigger

The LHCb TriggerNiko Neufeld

CERN, PH

Niko NeufeldLHCb Trigger, RICH-2004

Outline

•The 3 levels of the LHCb Trigger– Level-0 hardware trigger

•Fully synchronous and pipe-lined (deadtime < 0.5%)

•Pile-up System

•Calorimeter and Muon

•Flexible L0 Decision unit

– Level-1 software trigger•Partial read-out: Vertex Detector (VeLo), Trigger

Tracker (TT) and L0 summary

– High Level software trigger (HLT)•Full read-out: all detector data

•Using the RICH in the High Level Trigger

Com

mon

h

ard

ware


The LHCb trigger at a single glance

~ O(1) kHz


Trigger Rates OverviewLevel-0 30 MHz pp x-ings 10 MHz “visible” @2.1032

Multiplicity/Pile-Up: 7 MHz

ET1,h,e,0): 1 MHz

Level-1 VELO: impact parameter VELO+TT: momentum

VELO+L0 M

HLT L1(VELO+TT)+T: 10 kHz VELO+TT+T: dp/p<1%

exclusive channels, full reconstruction for 200 Hz

Level-0 Level-1HLT

L1-confirmationHLT

Full reconstruction


Level-1 Decision Algorithm

Bandwidth division:

Overlaps are absorbed in this direction

PT1,PT2 µµ γ e

OR(L1) ⇒ L1 yes/no

µ

PT1,PT2?

PT1,PT2?

L1PTL1J/ψ L1µµ L1γL1µ L1e

Parallel (overlapping) trigger lines

DC 04, L1Decision v4r0

GenericSingle-muonDimuon, generalDimuon, J/PsiElectronPhoton

Bandwidth (kHz)

Photon Electron

Dimuon, J/Psi

Dimuon, general

Single-muon

Adjusted for

overlap Generic 30.0

(75.2%)30.0

(75.2%) 8.8 (22.1%) 1.7 ( 4.1%) 1.8 ( 4.6%) 3.9 ( 9.9%) 4.0 (10.0%)

3.0 ( 7.4%) 1.2 ( 3.0%) 1.1 ( 2.7%) 2.3 ( 5.9%) 2.3 ( 5.8%)


Efficiency for generic L1 trigger


Key features of DAQ hardware

• All detectors use standardized read-out boards (two variants next slide)

• Use commercial (mostly even commodity) components wherever possible: PCs, (copper) Gigabit Ethernet, Ethernet routers

• Use the same infrastructure (network and computer farm) for L1 and HLT

• Accommodate a soft real-time requirement: Level 1 latency no larger than 58 ms

• Large system: 3000 Gigabit Ethernet links, 1800 PCs, several 100 Ethernet switches


The RICH readout board • Standardised read-

out boards: 9U x 400 mmm

• Gets data from detectors, from up to 48 optical links de-serialises, zero-suppresses, etc…

• All boards are controlled by commercial Microcontroller (Creditcard-PC)

• Data sent out by standard Gigabit Ethernet Mezzanine card via up to 4 Gigabit Ethernet links (over copper)


Multiplexing Layer

FE FE FE FE FE FE FE FE FE FE FE FE

Switch Switch

Level-1Traffic

HLTTraffic

1000 kHz5.5 GB/s

40 kHz1.6 GB/s

94 SFCs

Front-end Electronics

7.1 GB/s

TRM

Sorter

TFCSystem

L1-Decision

StorageSystem

Readout Network

Switch Switch Switch

SFC

Switch

CPU

CPU

CPU

SFC

Switch

CPU

CPU

CPU

SFC

Switch

CPU

CPU

CPU

SFC

Switch

CPU

CPU

CPU

SFC

Switch

CPU

CPU

CPU

CPUFarm

~1800 CPUs

Software trigger Hardware (DAQ)

~ 250 MB/stotal

TIER0

Scalable in depth: more CPUsScalable in width: more detectors in Level-1

Using the RICH in the High Level Trigger


• HLT involves exclusive selection of (many) B decay modesby checking the invariant-mass of combinations of 2–8 tracks

eg Bs DsDs

KKKK

• Long tracks (tracks having info from VeLo and T-stations) are used ~ 30 per event

If only selection is on charge, then for Bs DsDs

~ 156 combinations 107

• If K/ identification were available, the number of combinations to be checked would be substantially reduced

~ 24 ×102 103 combinations for Bs DsDs

example

HLT could take less time per channel, if K/ ID is fast

• 10 ms/event to run HLT on a CPU in 200720 ms/event for exclusive selections

RICH in HLT: What does it get us & what does it cost us?


Hit pixels shown on HPD detector planes, for a typical eventCrosses mark impact point of tracks (as if they were reflected)

RICH-2

First step: parameterise ring distortions so that problem can be solved on HPD planes avoiding the need to determine the Cherenkov angle for each pixel-track combination:


Same event: now ray trace “fake” photons to the detector plane, emitted from each track at fixed C = 30 mrad,uniformly distributed around azimuthal angle :


• Plot radius r vs to calibrate the distortion of the rings

• If plotted relative to the average radius r, all rings show the same distortion: r = A cos 2(A 2.5 mm)


• Reconstructed Cherenkov angle for all long tracks passing through RICH-2, vs momentum (on log scale)

e

p

K


• Same plot selecting out the true kaons onlyNote that below threshold, peak search finds ~ random C


• Determine the average ring radius for each track by ray tracing a few photons (currently 6) — fast

• Then apply correction: C = 30 r / (r A cos 2) mrad

Ray traced photons Pixel hits from full simulation

= 0.1 mrad = 0.7 mrad

~ as good as offline resolution on C


• Plot reconstructed C for all photons relative to a track

• “Local” algorithm can be made by searching for peak,

treating hits from other tracks as background

• Scan over C to find value that maximizes significance

Single track All tracks


Cut on (rS/B max-rtrue pion)

Cutting the pion band : Selecting the kaon band :

Cut on (rS/B max-rtrue kaon)

RICH 2 RICH 2


Local HLT algorithms: performance

Kaon-ID efficiency (purple) / pion misid (blue) using gas info

Cut pion band Select kaon band

Good performance & fast


Global HLT algorithm: principle

• Cherenkov angle calculated on HPD plane (discussed)• Do not use aerogel (for the moment)• Do not calculate contribution of every pixel to every track; rather assign pixels to ring image closest to track• Only consider pion vs kaon hypotheses in LL

Implement global LogLikelihood with some simplifications withrespect to full glory of “offline” reconstruction:

Main advantages of this w.r.t. local approach:

• Simultaneous treatment of signal and background• Method much better suited to looking for below threshold kaons, which is a v. challenging problem for local method


Global HLT algorithm: performance

(K) = 88%

(K) = 15%

(K) = 92%

(K) = 11%

Online Offline

Good (but not exact) correlation with offline. Speed 11 ms (1 GHz PIII) per event when only “long” tracks are used. Promising!


Summary

•LHCb uses a 3 level trigger system

•Two levels of software trigger provide maximum flexibility at high rate

•RICH information is available in the trigger and potentially very useful

•Fast algorithms are being developed and look promising

•We are currently implementing, deploying and eagerly awaiting 2007


Acknowledgements

•The work of many people has been presented in this talk

•I would like thank in particular the LHCb RICH, Online and Electronics groups

Backup Slides


Data Flow

Level 0 trigger

1MHz

MultiplexingLayer

FE FE FE FE FE FE FE FE FE FE FE FE

Switch Switch

Level-1Traffic

HLTTraffic

126Links

44 kHz5.5 GB/s

323Links4 kHz

1.6 GB/s29 Switches

32 Links

94 SFCs

Front-end Electronics

Gb EthernetLevel-1 Traffic

Mixed TrafficHLT Traffic

94 Links7.1 GB/s

TRM

Sorter

TFCSystem

L1-Decision

StorageSystem

Readout Network

Switch Switch Switch

SFC

Switch

CPU

CPU

CPU

SFC

Switch

CPU

CPU

CPU

SFC

Switch

CPU

CPU

CPU

SFC

Switch

CPU

CPU

CPU

SFC

Switch

CPU

CPU

CPU

CPUFarm

62 Switches

64 Links88 kHz

~1800 CPUs


L1 efficiencies overviewOffline selected events:

Reconstructible events:

(1.55 +/- 0.18)% - Why so low?

4-prong give best generic efficiency!

Hadrons trigger

e and !


LHCb Software triggers

Level-1 HLT

Input rate 1 MHz Stage 1) 40 kHzStage 2) 10 kHz

Data used VeLo, TT, L0 summary

Stage 1) like L1Stage 2) all

Output rate 40 kHz O(2 kHz) out of which 200 Hz fully reconstructed

Mean time on 2007 CPU

1 ms 10 ms (total)10 ms (Stage 2)

Maximum allowed time

58 ms (data transport included)

not applicable (limited only by available CPU power)


Algorithm: at leastone cluster/>threshold and“Global”<threshold

orDi-muon>threshold

Level-0: B-signatures

HCAL clusters

>90% K decay

Nominal threshold

Global event variables:reject “complicated” events

Nominal threshold

Trigger on B’s:thresholds on highest ET:

• h-cluster• e-cluster•-cluster• 0-cluster

Or highest pT

•’s


Dedicated HLT RICH algorithms

First step: parameterise ring distortions so that problemcan be solved on HPD planes, without having to solvequartic equation for each pixel-track combination:

Study distortion by plotting local radius - mean radiusvs photon azimuth. Works well – but not so good for aerogel

RICH 2


Local HLT algorithms: principle

Plot Cherenkov angle for nearby hits to track of interest andlook for peak – below shown integrated over many tracks

Method OK for gas rings, but is not so good (yet) for aerogel

Gas, R2 Gas R1 Aerogel


Cutting the pion band : Selecting the kaon band :

RICH 1 RICH 1

Cut on (rS/B max-rtrue pion) Cut on (rS/B max-rtrue kaon)

Date post:	09-Jan-2016
Category:	Documents
Upload:	carr
View:	44 times
Download:	0 times

The LHCb Trigger

Documents