CMS Trigger Strategy

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CMS Trigger Strategy

• Sridhara Dasu, University of Wisconsin, CMS
  Collaboration

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The Large Hadron Collider
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Physics in LHC Era
Trigger Challenge

• Electroweak Symmetry Breaking Scale
• Higgs discovery and higgs sector characterization
• Quark, lepton Yukawa couplings to higgs
• New physics at TeV scale to stabilize higgs
  sector
• Spectroscopy of new resonances (SUSY or
  otherwise)
• Find dark matter candidate
• Multi-TeV scale physics (loop effects)
• Indirect effects on flavor physics (mixing, FCNC,
  etc.)
• Bs mixing and rare B decays
• Lepton flavor violation
• Rare Z and higgs decays
• Planck scale physics
• Large extra dimensions to bring it closer to
  experiment
• New heavy bosons
• Blackhole production

Low ? 40 GeV
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The LHC Trigger Challenge

• Physics at EWSB scale
• 115 lt Mhiggs lt 250 GeV
• Decays to gg, WW, ZZ
• 2-g PT20 GeV, Lepton PT 40 GeV
• TeV scale supersymmetry
• Multiple leptons, jets and LSPs (missing PT), HT
  300 GeV
• QCD Background
• Jet ET 250 GeV, rate 1 kHz
• Jet fluctuations ? electron BG
• Decays of p, k, B ? muon BG
• Technical challenges
• 40 MHz input ? fast processing
• 100 Hz output ? physics selection
• 109 events per year ? 102 higgs events

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Multi Level Trigger Strategy

• Level 1
• Coarse object identification
• Limited isolation

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L1T Algorithms ? tracking

• Link local track segments (in CSC) into distinct
  3D tracks (FPGA logic)
• Reconstruction in ? suppresses accelerator muons
• Measure pT, ?, and ? of the muon candidates in
  the non-uniform fringe field in the endcap iron
  (SRAM LUTs)
• Require 25 pT resolution for sufficient rate
  reduction
• Send highest quality candidates for combination
  with other ? detectors (similar algorithm for DT
  in barrel) and make final L1 trigger decision
  with pT cut

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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
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L1T Algorithms e/?
Triggers are mostly due to energetic ?0s in
em-rich jets
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L1T t / Jet Algorithm
Jets are real - however, difficult to get low pT
jets ?-jets are mostly fake
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Missing / Total ET Algorithm
360o
LUT
Strip ET sum over all h
L1 MET is easily spoiled by instrumental problems
For sums ET scale LSB (quantization) 1 GeV is
used Df 20o used instead of HCAL tower size
Df 5o
f

40o
20o
0o
-5
ET
5
0
h
MET
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L1 Trigger System Production
RCT Receiver card
RCT Jet/Summary card
RCT Electron isolation card
Optical links
CSC Track-Finder

• Custom ASICs
• Large FPGAs
• SRAM
• Gbit/s Optical links
• Dense boards

SRAM
FPGA
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Example Level-1 Trigger Table (L2 x 1033)
? 3 safety factor ? 50 kHz (expected start-up DAQ
bandwidth)
Only muon trigger has low enough threshold for
B-physics (aka Bs?mm)
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Level-1 Trigger Rates
250 GeV jets80 GeV tt
30-40 GeV for m or e20 GeV each for gg

• Trigger cuts determine physics reach!
• Efficiency for H?gg and H?4 leptons gt90 (in
  fiducial volume of detector)
• Efficiency for WH and ttH production with W?ln
  85
• Efficiency for qqH with H?tt (t?1/3 prong
  hadronic) 75
• Efficiency for qqH with H?invisible or H?bb
  40-50

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Ground Reality at L1 Trigger

• No tracker in level-1 trigger
• Electron, photon and p0 looks the same
• Trigger level muon PT is poorly measured
• Jet trigger limitations
• Limited capability to get low PT jets
• Limited calorimeter resolution
• Calibration to take out h,f variation in response
• Poor measurement of missing ET
• At best the resolution can be
• Calibration not possible
• Underlying event and pileup contribution
• Additional limitations due to trigger
  calculations

Most of L1 output are mistags - Another factor of
1000 rejection at HLT
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The High-Level Triggers
In CMS all trigger decisions beyond Level-1 are
performed in a Filter Farm running normal CMS
reconstruction software on PCs The filter
algorithms are setup in several steps HLT does
partial event reconstruction on demand seeded
by the L1 objects found, using full detector
resolution Algorithms are essentially offline
quality but optimized for fast performance
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HLT Executable Structure
eJet
MET
HT
2 e/?
1-Jet
2-Jet
e/?
?
?
??
??
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0
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0
0
1
1
0
0
1
0
0
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128-bit L1 Word
Step 1
Step 1
Step 1
Step 1
Step 2
Step 2
Step 2
Step 2
HLT Algorithm StepsPath stops when a selection
step fails
Step 3
Step 3
Step 3
Step 3
Step 4
Step 4
Step 4
Step 4
Step 5
Step 5
Step 5
Step 6
Step 6

0
1
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HLT Bits
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HLT e/? Selection

• Initial steps using calorimeter (Level-2 e/?)
• Search for matching Level-1 e/? object
• Use 1-tower margin around 4x4-tower trigger
  region
• Bremsstrahlung recovery super-clustering
• Select highest ET cluster
• Bremsstrahlung recovery
• Road along f in narrow ?-window around seed
• Collect all sub-clusters in road ? super-cluster

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HLT t-jet tagging
Final steps in HLT paths involve tracking which
is more time consuming Reconstruct tracks only in
the region of interest around Level-2 tagged
objects
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Example HLT Trigger Menu (L2x1033)
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Standard Model Higgs Decay
H?WW()?ln ln H?ZZ()?4 leptons
H?gg
qqH?tt
qqH?WW?ln jj qqH?ZZ?ll nn qqH?ZZ?ll jj
ttH?Wb Wb bb WH?ln bb

• 115 lt MH lt 130 GeV
• H?bb dominates - however, to avoid backgrounds,
  use H?gg
• MH gt 130 GeV
• H?WW, H?ZZ, H?WW, H?ZZ (at least one lepton to
  avoid backgrounds)

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MSSM Higgs

• Higgs sector h0, H0, A0, H
• Light higgs h0 - standard model like
• At high tanb, H?bb, H?tt and H?cc dominate

Challenges Trigger, b and t tagging
Measure decay to bb and tt
qq?bbH large and well understood
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qqH - Low mass H challenges

• Higgs production in weak boson fusion
• Accompanied by jets in the forward direction
• Forward jet characteristics
• Lower jet ET (underlying event pileup problem)
• Challenge for trigger (level-1 and higher levels)
• Trigger on higgs decay products
• Decays to Z (ee, mm) or photons (OK)
• Low threshold electron/photon algorithm
• Decays to t
• Narrow jet tag - dedicated t algorithm (OK)
• Tag jets with location information (Can do this
  at Level-1)
• Additional reduction in background
• Require two forward tag jets
• Require Dh between the jets
• Decays to b jets (dominant but dirty mode)
• Four jet trigger including forward tag jets
• Require Dh between jets
• Require two jets to be central (b tagging)

D. Zeppenfeld et al.
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Diffractive Higgs Trigger

• Rewarding if there is sufficient production
• Tagged protons give good MH measurement
• Since expected s is small, need all H decays
• Central detector
• Two Low PT jets from low mass Higgs decay
• Must tag as b-jets at HLT
• hlt2.5
• Essentially nothing else in the detector
• Require small HT - ETjet1 - ETjet2
• Pileup persists
• Proton taggers
• Too far to be part of trigger
• Trying to get around speed of light problem

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SUSY Efficiencies
MSUGRA provides a benchmark
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Summary

• LHC Trigger is Challenging
• The choice of physics studied is already made at
  level-1 trigger
• Choices made with calorimeter and muon systems
  only
• Complete object reconstruction at higher level
  trigger
• Optimum resolution online with calibration and
  alignment
• Includes b/t tagging in high level trigger farms
• Both CMS and ATLAS have designed trigger systems
  for golden discovery modes (lepton, diphoton,
  muti-jets)
• Exploit qqH, WH, ttH production to cover
  difficult regions
• Definitive exploration of higgs sector is assured
• Pickup searches for new particles where left off
  by Tevatron
• Innovative designs may allow more measurements
• Topological selection starting from level-1
• Low mass higgs some MSSM higgs decays to tt
• Measurement of Yukawa couplings
• Invisible higgs decays