1. The CMS Trigger

2. Cross Sections and Rates
Cross sections for different processes vary by many orders
of magnitude
inelastic: Hz
W -> ln: Hz
tt: Hz
Higgs (100 GeV): Hz
Higgs (600 GeV): Hz
Required selectivity
1 :
Trigger -
C.-E. Wulz
C.-E. Wulz

3. Principle of Triggering
Event accepted?
YES NO
Successive steps
Depends on
Event type
Properties of the measured trigger objects
Trigger objects (candidates): e/g, m, hadronic jets,t-Jets,
missing energy, total energy
Trigger conditions: according to physics and technical priorities
C.-E. Wulz

4. CMS Detector (Compact Muon Solenoid)
C.-E. Wulz

5. Trigger Levels in CMS Level-1 Trigger
Macrogranular information from calorimters and muon system (e, m, Jets, ETmissing)
Threshold and topology conditions possible
Latency: 3.2 ms
Input rate: 40 MHz
Output rate: up to 100 kHz
Custom designed electronics system
High Level Trigger (several steps)
More precise information from calorimeters, muon system, pixel detector and tracker
Threshold, topology, mass, … criteria possible as well as matching with other detectors
Latency: between 10 ms and 1 s
Input rate: up to 100 kHz
Ouput (data acquisition) rate: approx. 100 Hz
Industral processors and switching network
C.-E. Wulz

6. Conventional Concept with 3 Steps
Investment in
specialized
processors,
control
C.-E. Wulz

7. CMS Concept with 2 Steps Advantages: Fewer components, scalable
Investment in
band width and
commercial components
Advantages:
Fewer components, scalable
C.-E. Wulz

8. Evolution of Trigger Requirements
ATLAS/CMS:
Rather high rates and large event sizes
Interaction rates:
~ Factor 1000 larger than at LEP,
~ Factor 10 larger than at Tevatron
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Split, Oct. 2002

9. Level-1 Trigger Only calorimeters and muon system involved
Reason: no complex pattern recognition as in tracker required (appr tracks at 1034 cm-2s-1 luminosity), lower data volume
Trigger is based on:
Cluster search in the calorimeters
Track search in muon system
C.-E. Wulz

10. Architecture of the Level-1 Trigger
GLOBAL TRIGGER
Global Calorimeter
Trigger
Global Muon Trigger
Regional Calorimeter
Trigger
Regional DT
Trigger
Regional CSC
Trigger
RPC
Trigger
Local Calorimeter
Trigger
Local DT
Trigger
Local CSC
Trigger
RPC Hits
Calorimeter
cell energies
DT Hits
CSC Hits
C.-E. Wulz

11. Strategy of the Level-1 Trigger
Local
Energy measurement in single calorimeter cells or groups of cells (towers)
Determination of hits or track segments in muon detectors Regional
Identification of particle signature
Measurement of pT/ET (e/g, m, hadron jets, t-jets)
Determination of location coordinates (h,f) and quality
Global
Sorting of candidates by pT/ET, quality and retaining of the
best 4 of each type together with location and quality information
Determination of SET, HT, ETmissing, Njets for different thresholds and h ranges
Algorithm logic
thresholds (pT/ET, NJets)
geometric correlations
- e.g. back-to-back events, forward tagging jets
- more detailed topological requirements optional
- location information for HLT
- diagnostics
C.-E. Wulz

12. Level-1 Calorimeter Trigger
Goals
Identify electron / photon candidates
Identify jet / t-jet candidates
Measure transverse energies (objects, sums, missing ET)
Measure location
Provide MIP/isolation information to muon trigger
C.-E. Wulz

13. Local / Regional Electron/Photon Trigger
Trigger primitive generator (local)
Flag max of 4 combinations
(“Fine Grain Bit”)
Regional calorimeter trigger
ET cut
Hit max of > ET threshold
Longitundinal cut hadr./electromagn. ET / < 0.05
Hadronic and electromagnetic isolation < 2 GeV
One of < 1 GeV h f
Jet
e/g
Electron / photon
C.-E. Wulz

14. Typical e/g Level-1 Rates and Efficiencies
Single isolated e/g rate at 25 GeV threshold: 1.9 kHz
95% efficiency at 31 GeV
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15. Jet / t Trigger
Jet ET is obtained from energy sum of 3 x 3 regions -
sliding window technique, seamless coverage up to |h| < 5
Up to |h| < 3 (HCAL barrel and endcap) the regions are 4 x 4 trigger towers
Narrow jets are tagged as
t-jets in tracker acceptance (|h| < 2.5) if ET deposit matches any of these patterns {
C.-E. Wulz

16. Typical Level-1 Jet Rates and Efficiencies
Single jet rate at 120 GeV threshold: 2.2 kHz, 95% efficiency at 143 GeV
Dijet rate at 90 GeV: 2.1 kHz 95% efficiency at 113 GeV
Single t-jet rate at 80 GeV threshold: 6.1 kHz
C.-E. Wulz
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Split, Oct. 2002

17. HT Trigger
HT, which is the sum of scalar ET of all high ET objects in the event is more useful than total scalar ET for the discovery or study of heavy particles (SUSY sparticles, top):
Total scalar ET integrates too much noise and is not easily calibrated
At Level-1 tower-by-tower ET calibration is not available
However, jet calibration is available as f(ET, h, f)
Implemented in Global Calorimeter Trigger
Rate with cutoff at 400 GeV: 0.7 kHz, 95% efficiency at 470 GeV
C.-E. Wulz

18. Muon Trigger Detectors
Drift Tube Chambers and Cathode Strip Chambers are used for precision measurements and for triggering.
Resistive Plate Chambers (RPC’s) are dedicated trigger chambers.
C.-E. Wulz

19. Local Muon Trigger Drift Tube Chambers Cathode Strip Chambers
Superlayer
Vector of 4 hit cells
Comparators allow resolution of
1/2 strip width
Station
Correlator combines vectors to track segment
6 hit strips
form track segment
C.-E. Wulz

20. Regional DT/CSC Muon Trigger (Track Finder)
Trigger relies on track segments pointing to the vertex and correlation of several detector planes
Tracks with small pT often do not point to the vertex
(multiple scattering, magnetic deflection)
Tracks from decays and punchthrough do not point to vertex in general
Punchthrough and sailthrough particles seldom transverse all muon detector
planes
C.-E. Wulz

21. Regional DT/CSC Muon Trigger (Track Finder)
Longitudinal
plane
Track Segment and direction of extrapolation
Bending
plane
Drift Tube Trigger
(CSC Trigger similar)
C.-E. Wulz

22. Regional RPC Muon Trigger
RPC-Trigger is based on strip hits matched to precalculated patterns according to pT and charge.
4/4
3/4
High pT
Low
For improved noise reduction algorithm requiring conincidence of at least 4/6 hit planes has been designed. Number of patterns is high. FPGA solution (previously ASICs) seems feasible, but currently expensive. Solutions to reduce number of patterns under study.
C.-E. Wulz

23. Global Muon Trigger DR/CSC/RPC: combined in Global Muon Trigger
Optimized algorithm (no simple AND/OR) with respect to
efficiency
rates
ghost suppression
-> Make use of geometry + quality
C.-E. Wulz

24. L1 Single & Di-Muon Trigger Rates
Trigger rates in kHz
|h| < 2.1
18, 8;8 eW =84.9 % eZ =99.7 % eBsmm = 7.2 %
20, 5;5 eW =82.3 % eZ =99.7 % eBsmm =15.1 %
14, -;- eW =89.6 % eZ =99.8 % eBsmm =27.1 %
25, 4;4 eW =74.2 % eZ =99.5 % eBsmm =18.4 %
working points compatible with current L1 pT binning
L = 2x1033cm-2s-1
L = 1034cm-2s-1
C.-E. Wulz

25. Global/Central Trigger within ATLAS and CMS
Algorithm
bits
Algorithm
bits
Thresholds already set in calorimeters and muon system. The Central Trigger Processor receives object multiplicities. It does not receive location information, therefore topological conditions are impossible. Separate RoI electronics for the Level-2 Trigger is necessary.
Thresolds set in Global Trigger. The processor receives objects with location information, therefore topological conditions are possible. Special HLT algorithms or lower thresholds can be used for selected event categories. Sorting needs some time, however.
C.-E. Wulz

26. Global Trigger
The trigger decision is taken according to similar criteria as in data analysis:
Logic combinations of trigger objects sent by the Global Calorimeter Trigger and the
Global Muon Trigger
Best 4 isolated electrons/photons ET, h, f
Best 4 non-isolated electrons/photons ET, h, f
Best 4 jets in forward regions ET, h, f
Best 4 jets in central region ET, h, f
Best 4 t-Jets ET, h, f
Total ET SET
Total ET of all good jets HT
Missing ET ETmissing, f(ETmissing)
12 jet multiplicities Njets (different ET thresholds and h-regions)
Best 4 muons pT, charge, f, h, quality, MIP, isolation
Thresholds (pT, ET, NJets)
Optional topological and other conditions (geometry, isolation, charge, quality)
C.-E. Wulz

27. Algorithm Logic in Global Trigger
Object Conditions
Logical Combinations
128 flexible parallel running algorithms implemented in FPGA’s.
Trigger decision (Level-1-Accept) is a function of the 128 trigger algorithm bits (for physics). 64 more technical algorithms possible.
C.-E. Wulz

28. Bs -> mm: Example of Topological Trigger
D- distributions
Dh similar.
Can ask for
opposite charges in addition.
C.-E. Wulz

29. Bs -> mm: Example of Topological Trigger
Topological conditions: Dh < 1.5, D < 2 rad, opposite muon charges
Gain by topological conditions and requiring no threshold on second muon with respect to working point 20; 5,5: 1 kHz rate, 7.8%efficiency (15.1% without, 22.9% with cuts).
C.-E. Wulz

30. High Level Trigger Goals
Validate Level-1 decision
Refine ET/pT thresholds
Refine measurement of position and other parameters
Reject backgrounds
Perform first physics selection
Detailed algorithms:
see talks of A. Nikitenko (e/g,jets), N. Neumeister (m) and other talks related to CMS physics and wait for DAQ/HLT TDR due Dec
C.-E. Wulz

31. High Level Trigger Challenges
Rate reduction
Design input rate: 100 kHz (50 kHz at startup with luminosity 2x1033 cm-2s-1), i.e. 1 event every 10 ms. Safety factor of 3: 33 kHz (16.5 kHz).
Output rate to tape: order of 100 Hz
Reduction factor: 1:1000
Example of allocation of input bandwidth to four categories of physics objects plus service triggers (1 or 0.5 kHz):
electrons/photons (8 or 4 kHz)
muons (8 or 4 kHz)
t-jets (8 or 4 kHz)
jets + combined channels (8 or 4 kHz)
Algorithms
The entire HLT is implemented in a processor farm. Algorithms can almost be as sophisticated as in the off-line analysis. In principle continuum of steps possible. Current practice: level-2 (calo + muons), level-2.5 (pixels), level-3 (tracker), full reconstruction.
C.-E. Wulz

32. High Level Trigger and DAQ Challenges
Processing time
Estimated processing time: up to 1 s for certain events, average 50 ms
About 1000 processors needed.
Interconnection of processors and frontend
Frontend has O(1000) modules -> necessity for large switching network.
Bandwidth
Average event size 1 MB -> For maximum L1 rate need 100 GByte/s capacity.
C.-E. Wulz

33. Conclusions
CMS has designed a Trigger capable of fulfilling physics and technical requirements at LHC.
The Trigger consists of 2 distinct levels. Level-1 is custom designed, the HLT is implemented with industrial components.
Prototypes of many Level-1 electronics boards exist.
Integration and synchronization tests are scheduled for 2003/2004.
The HLT/DAQ Technical Design Report is finalized and due to be ready at the end of 2002.
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34. Acknowledgments
my colleagues in CMS and at HEPHY Vienna, especially P. Chumney,
J. Erö, S. Dasu, N. Neumeister, H. Sakulin, W. Smith, G. Wrochna,
A. Taurok.
to the Organizing Committee of LHC Days in Split for the invitation and
the enjoyable conference.
C.-E. Wulz