Il Trigger di Alto Livello di CMS N. Amapane – CERN Workshop su Monte Carlo, la Fisica e le...

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

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


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


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


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


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


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


DAQ

Event building

HLT farm (O(2000 CPU)

L1

Modular, 8 “slices”

4 to be installed at startup


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…)


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)


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


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


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


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


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)


HLT Trigger Table

L= 2x1033 cm-2s-1


contd…


HLT Trigger Table (cont).

120 Hz

L= 2x1033 cm-2s-1



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%


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?


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


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)

Il Trigger di Alto Livello di CMS N. Amapane – CERN Workshop su Monte Carlo, la Fisica e le...

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