The CMS High-Level Trigger

Leonard ApanasevichUniversity of Illinois at

Chicago

On behalf of the CMS collaboration

ICCMSE 2009 2

Outline• Introduction to the CMS Detector• Trigger Challenges at the LHC• Trigger Architecture

– Level-1 Trigger– High Level Trigger

• HLT Processing Times• Trigger Commissioning

with Cosmic Data• Trigger Rates and Menus

for Startup29 Sept, 2009

ICCMSE 2009 3

The Compact Muon Solenoid (CMS)

29 Sept, 2009

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, JINST 0803:S08004,2008;

ICCMSE 2009 4

The Large Hadron Collider

29 Sept, 2009

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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– ~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)

ICCMSE 2009 629 Sept, 2009

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

Readout buffers

Processor farms

Switching network

Detectors

Lvl-1

HLT

Lvl-1

Lvl-2

Lvl-3

Front end pipelines

Readout buffers

Processor farms

Switching network

Detectors

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

• Progress in networking/switching has justified CMS choice

ICCMSE 2009 7

Level 1: Only Calorimeter & Muon

Simple Algorithms

Small amounts of data

ComplexAlgorithms

Hugeamounts ofdata

29 Sept, 2009

ICCMSE 2009 829 Sept, 2009

The Level-1 Trigger

• Reduce data rate from 40 MHz to 100 kHz while keeping the interesting physics events– Custom electronic boards

and chips• Selects muons, electrons,

photons, jets– ET and location in detector

• Also Missing ET, Total ET, HT, and jet counts

• Total decision latency: 3.2 s• 128 Level-1 trigger bits. Bits

can be set using logical combinations of Level-1 objects.

Electrons, Photons, Jets, MET Muons

3<||<5 ||<3 ||<3 ||<2.1 0.9<||<2.4 ||<1.2

ICCMSE 2009 929 Sept, 2009

Calorimeter Trigger Geometry

Trigger towers: = = 0.087

HCAL

ECAL

HF

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

Provide e/g and jet, , t ET triggers

ICCMSE 2009 1029 Sept, 2009

Muon Trigger Geometry

• DT: drift-tube system

• CSC: cathode strip chamber system

• RPC: resistive plate chamber system

Initial coverage of RPC is staged to <1.6

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

ICCMSE 2009 1129 Sept, 2009

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, 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)

ICCMSE 2009 12

High Level Trigger Strategy

29 Sept, 2009

100 kHz

40 MHz

~150 Hz

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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 full detector resolution

• Algorithms are essentially offline quality but optimized for fast performance

300 MB/s

1 PB/s

Farm of ProcessorsONE event, ONE processor

• High Latency• Simpler I/O• Sequential programming

ICCMSE 2009 14

• 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-unpacking and local reconstruction across HLT

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

HLT Algorithm Design

L1 seeds

L2 unpacking (MUON/ECAL/HCAL)

Local Reco (RecHit)

L2 Algorithm

Filter

L2.5 unpacking (Pixels)

L2.5 Algorithm

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

“Regional”: using small (η, φ) region

29 Sept, 2009

ICCMSE 2009 1529 Sept, 2009

CMS Trigger/DAQ Evolution

• CMS DAQ is a number of functionally identical, parallel, small DAQ systems– Build up 512512 switch

from 8 6464 switches (DAQ slices)

• 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.

ICCMSE 2009 16

m triggers: performance

• Signal efficiency (eHLT/[eL1*A])

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

Threshold: double m non-isolatedThreshold: single

m isolatedThreshold: single m non-isolated

29 Sept, 2009

Single muon Di-muon

ICCMSE 2009 17

e/g triggers: performance

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

– Photons:

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

Electrons

Photons

29 Sept, 2009

ICCMSE 2009 18

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:

– Algebraic sum of “transverse energies” (ET = E sinq) of calorimeter objects• Objects can be individual calorimeter cells or jets

– Can be used in combination with 1 or more jets

2 2( ) ( ) 0.5R

Rates for L = 1032 cm-2 s-1

29 Sept, 2009

ICCMSE 2009 19

t- and b-tagging at HLT

• Hadronic t 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:

– b-lifetime:• Jet requirements + global pixel track reconstruction to

determine primary vertex at HLT level requirement of at least 2 tracks with transverse impact parameter significance above a certain threshold

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

requirements on DR(m-jet) and pT(m) relative to the jet direction

29 Sept, 2009

ICCMSE 2009 20

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

29 Sept, 2009

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

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

ICCMSE 2009 21

HLT Processing Times

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

29 Sept, 2009

ICCMSE 2009 22

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-detectors

– Measure trigger rates from cosmic rays and from noise

– Evaluate trigger efficiencies– Compare resolutions between

trigger and reconstructed objects

29 Sept, 2009

ICCMSE 2009 23

Trigger Commissioning (II)

29 Sept, 2009

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

Excellent agreement between emulation and data

Synchronization of signals from different detector sub-systems demonstrated

L1 trigger efficiencyas a function of offline reconstructed pT

ICCMSE 2009 24

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– 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

• Muon: 33 Hz• Electron: 30 Hz• Photon: 9 Hz• Jet: 36 Hz• MET & HT: 5 Hz• B-Tau: 4 Hz• MinBias: 14 Hz• Cosmics/Halo: 7 Hz

• Total: 138 Hz

Trigger Rates by Object

Rates for L=8e29 cm-2s-1

29 Sept, 2009

ICCMSE 2009 2529 Sept, 2009

Summary

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

• All data stored 3 s 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– 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

LHC Trigger is Challenging

CMS trigger system is ready for collisions!!!