Il Trigger di Alto Livello di CMS N. Amapane – CERN Workshop su Monte Carlo, la Fisica e le...
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![Page 1: Il Trigger di Alto Livello di CMS N. Amapane – CERN Workshop su Monte Carlo, la Fisica e le simulazioni a LHC Frascati, 25 Ottobre 2006.](https://reader035.fdocuments.net/reader035/viewer/2022070401/56649f1b5503460f94c3165b/html5/thumbnails/1.jpg)
Il Trigger di Alto Livello di CMSIl Trigger di Alto Livello di CMS
N. Amapane – CERN
Workshop su Monte Carlo, la Fisica e le simulazioni a LHC
Frascati, 25 Ottobre 2006
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The CMS High Level TriggerNicola Amapane 2
MUON BARREL
CALORIMETERS
Silicon MicrostripsPixels
ECAL Scintillating PbWO4
Crystals
Cathode Strip Chambers (CSC)Resistive Plate Chambers (RPC)
Drift TubeChambers (DT)
Resistive PlateChambers (RPC)
SUPERCONDUCTINGCOIL
IRON YOKE
TRACKER
MUON
ENDCAPS
Total weight : 12,500 tOverall diameter : 15 mOverall length : 21.6 mMagnetic field : 4 Tesla
HCAL Plastic scintillator brass
sandwich
The Compact Muon Solenoid
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The CMS High Level TriggerNicola Amapane 3
LHC Event Rates
Acceptable storage rate: 100 Hz
Max DAQ 100 kHz
Machine Rate: 40 MHz
pp interactions
Particle mass (GeV/c2)
rate @ nominal LHC luminosity
Pile-up
On-line trigger selectionSelect 1:4x105
Decide every 25 ns!
Off-line analysis
Signals
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The CMS High Level TriggerNicola Amapane 4
Trigger Architecture
• CMS choice: All further selection in a single phisical step (HLT)– Build full events and analyze them “as in offline”
– Invest in networking (rather than in dedicated L2 hardware)
100 kHz
100 Hz
40 MHz
100 GB/s!!
• Start from 40 MHz → Decision every 25 ns– Too small even to read raw data
– Selection in multiple levels, each taking a decision using only part of the available data
• The first level (L1) is only feasible with dedicated, synchronous (clock driven) hardware
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The CMS High Level TriggerNicola Amapane 5
Level-1 Trigger• Custom programmable processors
– To minimise latency
• Synchronous decision every 25 ns– delayed by 3.2 s = 128 BX
(Max depth of pipeline memories)
• Max output max DAQ input – Design: 100 kHz; at startup: 50 kHz
• Only detectors and calorimeters– e/, , jets, jets, ET
miss, ET
• Selection by the “Global Trigger”– 128 simultaneous, programmable algorithms, each allowing:
• Thresholds on single and multiple objects of different type• Correlations, topological conditions• Prescaling
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The CMS High Level TriggerNicola Amapane 6
Trigger detectors
• ECAL up to ||<3• HCAL: |h|< 3 (HB, HE); 3<|h|<5.191 (HF)• Muon (DT, CSC, RPC): |h|<2.4
– But trigger electronics only up |n|<2.1
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L1 Trigger Table
For L= 2x1033 cm-2s-1
(CMS Physics TDR v.2)
Assume 50 KHz DAQ available at low luminosity + factor 3 safety
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The CMS High Level TriggerNicola Amapane 8
DAQ
Event building
HLT farm (O(2000 CPU)
L1
Modular, 8 “slices”
4 to be installed at startup
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The CMS High Level TriggerNicola Amapane 9
CMS HLT
• Run on farm of commercial CPUs: a single processor analyzes one event at a time and comes up with a decision
• Has access to full granularity information• Freedom to implement sophisticated reconstruction
algorithms, complex selection requirements, exclusive triggers…
Constraints:– CPU time (Cost of filter farm)
• Reject events ASAP: set up internal “logical” selection steps– L2: muon+ calorimeter only– L3: use full information including tracking
– Must be able to measure efficiency from data• Use inclusive selction whenever possible
– Single/double object above pT/ET, etc.• Define HLT selection paths from the L1
– Keep output rate limited (obvious…)
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The CMS High Level TriggerNicola Amapane 10
Example: Muon HLT
• Key is to achieve the best pT resolution (and suppress non-prompt muons and b,c decays)
threshold [GeV/c]T
p0 10 20 30 40 50 60 70 80
Rat
e [H
z]
10-2
10-1
1
10
102
103
104
105
106
/K
L0K
cb
*/0Z
Integral rate (ℒ = 1034 cm-2s-1)
KL
/K
c,b
WZ/*
Threshold on generated pT (GeV/c)
100 Hz
Rat
e (H
z)
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The CMS High Level TriggerNicola Amapane 11
HLT Muon Reconstruction
• Level-2: “confirm” L1 refitting hits in the muon chambers with full granularity – Regional reconstruction seeded by L1 muons– Kalman filtering iterative technique
– pT resolution: 10% to 16% depending on (muons from W decays)
• Level-3: Inclusion of Tracker Hits– Regional tracker reconstruction seeded by L2 muons
– pT resolution: achieve full CMS resolution of 1% to 1.7% depending on (muons from W decays)
• Isolation in calorimeters (at L2) and tracker (L3) to suppress b,c decays and non-prompt muons
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The CMS High Level TriggerNicola Amapane 12
1/pT Resolutionbarrel overlap endcaps
= 0.12 = 0.14 = 0.17
= 0.013 = 0.015 = 0.018
Level-2:
Improve L1barr. ovr. end.0.17 0.22 0.20
Level-3:Full resolution
10x scale
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The CMS High Level TriggerNicola Amapane 13
Single Muon Rates
ℒ = 1034 cm-2s-1
100 Hz
L2,L3 reduce the rate by improving the pT resolution
L2 is justified as it reduces the rate to allow more time for processing data from the tracker
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The CMS High Level TriggerNicola Amapane 14
HLT Reconstruction
• – L2: cluster ECAL deposits into “superclusters” and apply ET threshold– L3: isolation in HCAL and tracker
• e– L2 common with – L2.5: match the supercluster with a track in the pixel detector– L3: isolation in HCAL and tracker, cut on E/p
• Jets– Iterative cone algorithm in calorimeters + energy corrections (non-linearity)
• MET– Vector sum of transverse energy deposit in calorimeters, incl. muons
• Tau– Look for isolated “narrow” jet, either:– Isolation in ECAL+pixel– Isolation in the tracker
• B-tagging– L2.5: impact parameter with pixel track stubs– L3: with regional track reconstruction
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The CMS High Level TriggerNicola Amapane 15
Setting trigger tables
• HLT trigger paths start from corresponding L1 paths• Tresholds are set distributing bandwidth to the various
paths in order to maximize efficiencies– There can be significant overlaps– Iterative process
• Thresholds (and streams) will change with luminosity– And according to the physics of interest at the time of operation– Reference: 2x1033 cm-2 s-1
– Evolution of selection with luminosity is a delicate issue, up to now studied in detail only for jet (with prescales)
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The CMS High Level TriggerNicola Amapane 16
HLT Trigger Table
L= 2x1033 cm-2s-1
(CMS Physics TDR v.2)
contd…
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The CMS High Level TriggerNicola Amapane 17
HLT Trigger Table (cont).
120 Hz
L= 2x1033 cm-2s-1
(CMS Physics TDR v.2)
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The CMS High Level TriggerNicola Amapane 18
Some HLT Efficiencies
At low luminosity, relative to events in detector acceptance:
W e 68%W 69%Z 92%Z ee 90%tt +X 72%H(115 GeV)77%H(150) ZZ498%H(120) ZZ4e 90%A/H(200 GeV)2 45%H+(200-400)58%
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Triggers and offline analysis
• The HLT selection can have an impact on analysis– May reduce signal efficiency and phase-space
• Unless off-line selection is tighter than HLT– Simulation of the HLT selection is a part of analysis!
• Specific exclusive triggers can be implemented for channels where the default trigger tables are not enough, but:– How much the selection costs in term of rate and CPU?– Is it possible to understand the selection efficiency from the data?
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Conclusions
• Trigger at LHC is an integral part of the event selection
• CMS uses a single physical step after L1, to achieve a rejection factor of ~1000
• HLT algorithms have the full event data available and no limitation on complexity, except for CPU time
• Inclusive triggers based on the presence on one or more objects above pT/ET thresholds are normally sufficient to get good efficiency on most signal
• More sophisticated selections are possible if necessary
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The CMS High Level TriggerNicola Amapane 21
References
• CMS DAQ/HLT TDR, 2002, CERN-LHCC-2002-026– Full study of HLT rates, timing, benchmark signal efficiencies
• CMS Physics TDR Volume 1 (2006), CERN-LHCC-2006-001– Detector performance, reconstruction
• CMS Physics TDR Volume 2 (2006), CERN-LHCC-2006-021, – Update of HLT rates and trigger tables (Appendix E)