The CMS High-Level Trigger

The CMS High-Level Trigger

Leonard ApanasevichUniversity of Illinois at Chicago

On behalf of the CMS collaboration

Outline• Introduction to the

CMS Detector• Trigger Challenges at

the LHC• Trigger Architecture

– Level-1 Trigger– Level-1 Trigger– High Level Trigger

• HLT Processing Times• Trigger Commissioning with

Cosmic Data• Trigger Rates and Menus for

Startup29 Sept, 2009 2ICCMSE 2009

The Compact Muon Solenoid (CMS)

General purpose pp collider experiment:Searches for Higgs bosons,other new particles (SUSY,...) and new phenomena; Precision measurement of SM parameters like top and W masses, ...;Heavy ion program.

For details see e.g.:The CMS experiment at the CERN LHC,

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The CMS experiment at the CERN LHC, JINST 0803:S08004,2008;

The Large Hadron Collider

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Trigger Challenges at the LHC• At design L = 1034 cm-2s-1

– Interaction rate: ~1 GHz– ~20 pp events per 25 ns crossing– 1 kHz W events– 10 Hz top events– < 104 detectable Higgs decays/year

• Event size: ~1 MB (≈75M channels) 1000 TB/sec for 1 GHz input rate

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→ 1000 TB/sec for 1 GHz input rate– ~300 MB/sec affordable– Enormous rate reduction necessary!

• but without losing interesting physics• Select in stages:

– Level-1 Triggers• 1 GHz to 100 kHz (1/10000 reduction)

– High Level Triggers• 100 kHz to 100 Hz (1/1000 reduction)

Trigger Architecture• CMS has a two-tiered system to handle the LHC challenge

– Level-1 trigger reduces rate from 40 MHz (xing freq) to 100 kHz (max)– High-Level triggers reduce rate from 100 kHz to O(100 Hz)

Front end pipelines

Detectors

Lvl-1Lvl-1 Front end pipelines

Detectors

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Readout buffers

Processor farms

Switching network

HLT

Lvl-2

Lvl-3

Readout buffers

Processor farms

Switching network

“Traditional”: 3 physical levels CMS: 2 physical levels

• Progress in networking/switching has justified CMS choice

Level 1:Level 1:Only Calorimeter & MuonOnly Calorimeter & Muon

Simple Algorithms

Small amounts of data

ComplexAlgorithms

Hugeamounts ofdata

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Calorimeter Trigger GeometryTrigger towers:∆η = ∆φ = 0.087

HCAL

ECAL

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ECAL

HF

EB, EE, HB, HE map to 18 RCT crates

Provide e/γγγγ and jet, τ, τ, τ, τ, ET triggers

Muon Trigger Geometry

• DT: drift-tube system

• CSC: cathode strip chamber system

• RPC: resistive plate chamber

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plate chamber system

Initial coverage of RPC is staged to η<1.6

Initial coverage of CSC 1st station is staged to η<2.1

Level-1 Trigger Menu

• Determined by the physics priorities of the experiment

• L1 Menu optimized to fit within the L1 bandwidth

• Designed with a safety factor of 3 in mind– underestimation of input cross

sections, poor beam conditions,

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sections, poor beam conditions, detector performance, etc.

• Realistic menu including double and mixed triggers for specific physics channels

• Two L1 menus currently available– L = 8e29 cm-2s-1 (day-1 menu)– L = 1e31 cm-2s-1 (MC studies)

High Level Trigger Strategy

100 kHz

40 MHz

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~150 Hz

High Level Trigger (HLT)

• High-Level triggers reduce rate from 100 kHz to O(150 Hz)

• HLT does event reconstruction “on demand” seeded by the L1 objects found, using

1 PB/s

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L1 objects found, using full detector resolution

• Algorithms are essentially offline quality but optimized for fast performance

300 MB/s

Farm of ProcessorsONE event, ONE processor

•High Latency•Simpler I/O•Sequential programming

• Each HLT trigger path is a sequence of modules

• Processing of the trigger path stops once a module returns false

• Reconstruction time is significantly improved by doing regional data-

HLT Algorithm DesignL1 seeds

L2 unpacking (MUON/ECAL/HCAL)

Local Reco (RecHit)

L2 Algorithm

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improved by doing regional data-unpacking and local reconstruction across HLT

• All algorithms regional– Seeded by previous levels (L1, L2, L2.5)

Algorithm

Filter

L2.5 unpacking (Pixels)

L2.5 Algorithm

Local Reco (RecHit) “Local”: using one sub-detector only

“Regional”: using small (η, φ) region

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CMS Trigger/DAQ Evolution• CMS DAQ is a number of

functionally identical, parallel, small DAQ systems– Build up 512×512 switch

from 8 64×64 switches (DAQ slices)

• HLT filter farm is

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• HLT filter farm is currently comprised of 720 dual-quad core, 2.6 GHz, 16 GB memory Dell PE 1950 PC’s– 7 out of 8 cores (~5000)

dedicated to HLT processing

• Remaining processor does event building, acts a resource broker, and other services.

µµµµ triggers: performance

• Signal efficiency (εHLT/[εL1*A])

• Background rates at L = 1032 cm-2 s-1

Threshold: double µ non-isolatedThreshold: single

µ isolatedThreshold: single

µ non-isolated

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Single muon Di-muon

e/γγγγ triggers: performance

• Efficiency on benchmark channels (estimated from MC simulation)– Electrons:

Background rates at L = 1032 cm-2 s-1

Electrons

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– Photons:

Photons

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Jets and Missing ET• Jets:

– At HLT level, using an iterative cone algorithm with cone radius:

out of calorimeter “towers” (HCAL + projection on ECAL - deposits above certain thresholds)

• ETmiss:

2 2( ) ( ) 0.5R φ η= ∆ + ∆ =

Rates for L = 1032 cm-2 s-1

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• ET :– Algebraic sum of “transverse energies”

(ET = E sinθ) of calorimeter objects• Objects can be individual calorimeter cells

or jets

– Can be used in combination with 1 or more jets


ττττ- and b-tagging at HLT

• Hadronic τ decays:– Level 1: seed from calorimeter towers in a narrow region– Level 2: HLT jet reconstructions and ECAL isolation– Level 3: Track isolation using pixel tracks in a smaller

rectangular region or full tracks in a larger region

• b-tagging triggers:

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• b-tagging triggers:– b-lifetime:

• Jet requirements + global pixel track reconstruction to determine primary vertex at HLT level ß requirement of at least 2 tracks with transverse impact parameter significance above a certain threshold

– b → µ :• Jet requirements + L2/L3 muon reconstruction ß requirements

on ∆R(µ-jet) and pT(µ) relative to the jet direction


HLT Timing Considerations

HLT Timing is influenced by:• Trigger menu (L1T & HLT)

– Determined by physics priorities• Input L1Trigger rate

– Limited by bandwidth – Parameters of various L1T algorithms, e.g., H/E

• HLT algorithms and configuration– Parameters of various L1T algorithms, e.g., H/E

• HLT algorithms and configuration– Standard trigger paths at HLT seeded by L1 trigger bits– Order of modules and filters in a path– Parameters of the modules and filters

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Allowable CPU performance:• L1 input rate / number of processors

• @ 50 kHz: 100 ms/evt• @ 100 kHz: 50 ms/ evt

HLT Processing Times

• Average time needed to run full Trigger Menu on L1-accepted events: 43 ms/event– Core 2 5160 Xeon processor

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– Core 2 5160 Xeon processor running at 3.0 GHz

• CPU times strongly dependent on HLT input

• “Tails” have a significant impact on the average time– Will eliminate with time-out

mechanism


Trigger Commissioning

• CMS has collected over 300 million cosmic ray events

• Can use these data for: – compare data and L1 emulation– check synchronization of signals

from the various sub-detectorsfrom the various sub-detectors– Measure trigger rates from

cosmic rays and from noise– Evaluate trigger efficiencies– Compare resolutions between

trigger and reconstructed objects

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Trigger Commissioning (II)Synchronization of signals from different detector sub-systems demonstrated

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L1 emulator: bit by bit software emulation of the L1 trigger

Excellent agreement between emulation and data

L1 trigger efficiencyas a function of offline reconstructed pT

Trigger Menu for Startup• Muons:

– L1/L2/L3 muons run unprescaled at pT=20/9/3 GeV

• Electrons:– No isolation requirements at L1– Unprescaled at pT=10 GeV with large pixel-

matching window (LW)• Photons:

– No isolation requirements at L1

• Muon: 33 Hz• Electron: 30 Hz• Photon: 9 Hz

Trigger Rates by Object

– No isolation requirements at L1– Unprescaled at pT=15 GeV with no isolation

requirement• Jets:

– No L1/HLT jet corrections applied at startup

– Unprescaled at pT=30 GeV (~60 GeV corr.)• MinBias:

– Several algorithms installed based upon HF-tower ET over threshold, HF ET ring sums, Ecal ET, and pixel triplets

• Photon: 9 Hz• Jet: 36 Hz• MET & HT: 5 Hz• B-Tau: 4 Hz• MinBias: 14 Hz• Cosmics/Halo: 7 Hz• Total: 138 Hz

24ICCMSE 2009Rates for L=8e29 cm-2s-1

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Summary

• A new crossing every 25 ns with ~ 20 events at design luminosity → 1 GHz of events input

• All data stored 3 µs then all but 50-100 kHz rejected– Rejection of 99.99% of data without losing discovery physics!

• Remaining event filtering done on a farm of CPUs running offline quality reconstruction algorithms

• Rate of storage to archive is ~ 150 Hz

LHC Trigger is Challenging

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• Rate of storage to archive is ~ 150 Hz– Rejection of 99.99997% of data without losing discovery physics!!

• Fast selection and high efficiency is obtained for the physics objects and processes of interest

• Commissioning work already going on using Cosmic data• HLT and L1 have been developed for early luminosities• The overall CPU requirement is within the system capabilities

CMS trigger system is ready for collisions!!!

The CMS High-Level Trigger

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