J. Leonard, U. Wisconsin 1

Commissioning the Trigger of the CMS Experiment at the CERN Large Hadron Collider

Jessica L. Leonard

Real-Time Conference

Lisbon, May 2010

J. Leonard, U. Wisconsin 2

CMS Commissioning: 2009 and 2010 Collisions

Collision data taken at 900 GeV, 2.36 TeV, and 7 TeV

Currently ~9.8 nb-1 of 7 TeV collision data recorded at CMS

– Increased by factor of 3 in last 5 days

– 93% efficient at data-taking

– 99% of detector channels operational

J. Leonard, U. Wisconsin 3

CMS Trigger Principles LHC collides two beams of proton bunches at 40 MHz

Event: two protons interact → produce end-product particles → end up in detector

Don't have resources to record all events We want to keep “interesting” events: new physics But most events are types we've seen many, many times

What are “interesting” events? Events with: High-energy particles Isolated particles “Missing energy”

Trigger: system to quickly decide which events are potentially interesting based on signatures

J. Leonard, U. Wisconsin 4

Particle Signatures

Reduces event rate from 40 MHz (collision rate) to 100 kHz

Reconstructs e/g, jet, energy sum, and muon objects using custom electronics

Muon systems

Hadronic calorimeterElectromagnetic

calorimeter

Tracker

J. Leonard, U. Wisconsin 5

Data Flow Through Trigger

40 MHz

~100 kHz

~200 Hz

Level-1 Trigger (L1) Custom electronics (ASIC,

FPGA) Uses simplified detector

information Electromagnetic and

hadronic calorimeters Three muon systems

Quick! Few s

High-Level Trigger (HLT) Computer farm Uses more detailed event data

Regional unpacking of detector readout

Can afford to be slower (lower rate) 10's of ms

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Trigger Menus Single criterion for “interesting event”: trigger path

When an object or combination of objects fulfills requirements specified by trigger path, that path “fires”: event information passed along for further processing

Example: HLT_Mu5: requires a muon object with energy greater than 5 GeV

Trigger menu Set of criteria for trigger-worthy events Prescales: “pass” rates reduced by some factor if rate is

too high L1, HLT each has own trigger menu

HLT criteria more complex than L1 An L1 “pass” for a given criterion causes related HLT paths

to be run on that event

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BCM1

BCM2

HF

BSC HF

Trigger Menus for Startup

Zero-bias (any proton event)

Beam pickup (BPTX)

Minimum-bias (event with detector activity > noise)

Beam scintillator counter (BSC)

Detector activity triggers

Make best use of low luminosity (low event frequency)

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Trigger Commissioning: Trigger Menu Evolution

Gradually enabled physics-object triggers electron/photon, muon, jet triggers L1 triggers “unmasked” after comparison to trigger simulation, studying

rates (“accept event” signal enabled) HLT algorithms enabled after running offline on data to study time

performance

Minbias triggers will start getting prescaled (higher luminosity = higher event rate)

Run 132440 Run 135993

J. Leonard, U. Wisconsin 9

L1 Commissioning: Unexpected Effects

Physics-related signals in calorimeter Particles interacting with photo-diode electronics cause extraneous

signals Developing algorithms to deal with and correct for it at trigger level

Periodic spikes in trigger rate from resistive plate chamber (RPC) muon system

Traced to specific condition: CMS magnet and cavern lights on at the same time

Solution: turn off cavern lights!

J. Leonard, U. Wisconsin 10

L1 Commissioning: Timing

Particles take longer to get to outer parts of detector Cables between detector parts and trigger have

different lengths All event information needs to get combined correctly!

Bunch crossing every 25 ns

Particles travel 7.5 m in 25 ns

Time-of-flight of order of bunch crossing interval

J. Leonard, U. Wisconsin 11

L1 Commissioning: Timing Experience

Timing scan Scan a range of

timing delays Find best alignment

between subsystem trigger signal and min-bias trigger signal

Cathode strip chamber (CSC) muon system successfully timed in triggers

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L1 Bunch Crossing Identification

Sample of min-bias events

Trigger by BSC coincidence

Fraction of candidates that are in time with bunch crossing (BPTX trigger)

Plotted as function of L1-assigned E

T

Denominator is number of L1 candidates with +/- 2 bunch crossings of BPTX trigger

Noise pollutes efficiency at low ET

J. Leonard, U. Wisconsin 13

L1 Commissioning: e/ Object Trigger Efficiency

How many offline reconstructed electrons/photons are matched to electron/photon trigger objects?

Trigger efficiency Number of objects found

in L1 >= 2 GeV divided by number found in event reconstruction

Use all reconstructed objects

Includes inefficiency from masked channels, out-of-time triggers

Efficiency at plateau very good

J. Leonard, U. Wisconsin 14

HLT Commissioning: Event Rates

Rates well-understood Predicted by running

algorithms on simulated data

Actual rates agree well with prediction

Preparing for higher luminosity

Current menu: 1e28 cm-2s-1

Menus already developed for 1e29, 2e29, 4e29 . . .

Reoptimization of 1e31 menu ongoing

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Conclusions

Useful trigger commissioning experience gained Level-1 Trigger performing well in collision data-

taking, based on timing studies and efficiency curves

High-Level Trigger running smoothly, moving from minimum-bias triggers to physics object triggers

Trigger configuration will continue to evolve with the changing luminosity

We look forward to more data!

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Backup Slides

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Level-1 Trigger

40 MHz → 100 kHz

ASIC/FPGA algorithms use simplified detector information to reconstruct physics objects

electron/photon

jet energy sum muon

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High-Level Trigger

100 kHz → 100 Hz Computer farm

combines detailed detector information

Reconstructs more complex event information than L1

Algorithms optimized for fast performance

– Regional unpacking of data

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L1 Subdetector Synchronization

CSC trigger timing– 99.3% of triggers on

time (0.2% early, 0.5% late)

– Will improve with more statistics and analysis

RPC trigger timing– 27.3% of triggers

late before corrections, improves to 1.2% after

– [What are these corrections?]

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L1 Trigger Synchronization

L1 trigger bit timing alignment stable– Triggers fire on time with respect to bunch crossing

J. Leonard, U. Wisconsin 21

L1 Commissioning: e/ Object Trigger Efficiency

How many offline reconstructed objects are matched to 2-GeV electron/photon trigger object?

Trigger efficiency Number of objects found

in L1 divided by number found in event reconstruction

Exclude out-of-time triggers

Exclude masked channels

Require simple (small) reconstructed objects