overview of the high-level trigger electron and photon selection for the atlas experiment at the lhc...
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Overview of the High-Level Trigger Electron and Photon Selection for the ATLAS Experiment at the
LHC
Ricardo Gonçalo, Royal Holloway University of London
On behalf of the ATLAS HLT group
NSS 2005 – Puerto Rico, 23-29 October 2005
Ricardo Goncalo, Royal Holloway University of London2ATLAS HLT e/gamma selection
Outline
ATLAS and the Large Hadron Collider The ATLAS High-Level Trigger Electron and Photon selection Performance studies Summary and outlook
ATLAS and the LHC
The Large Hadron Collider
The ATLAS experiment
Trigger requirements
Ricardo Goncalo, Royal Holloway University of London4ATLAS HLT e/gamma selection
The LHC
The LHC will start operation in 2007 and will represent the high-energy frontier in collider physics
Much is expected of the LHC and its experiments: Study the origin of the
electroweak symmetry breaking
Test models of physics beyond the Standard Model
Perform precision Standard Model measurements
… and still be able to detect unexpected new physics
CM energy 14 TeV
Time between collisions 25 ns
Interactions/bunch crossing ~5-25
Initial luminosity 1033 cm-2s-1
Design luminosity 1034 cm-2s-1
Ricardo Goncalo, Royal Holloway University of London5ATLAS HLT e/gamma selection
ATLAS
Large angular coverage (||<5; tracking coverage up to ~2.5)
Liquid Argon electromagnetic calorimeter with accordion geometry
Iron-scintillating tile hadronic calorimeter; tiles placed radially and staggered in depth
Toroidal magnetic field in muon spectrometer (supercondutor air-core toroids)
Ricardo Goncalo, Royal Holloway University of London6ATLAS HLT e/gamma selection
Challenges faced by the ATLAS trigger
“Interesting” cross sections at least ~108 times smaller than total cross section
25ns bunch crossing interval (40 MHz)
Up to 25 proton-proton interactions per bunch crossing (depending on luminosity)
Offline processing capability: ~200 Hz
~5 events selected per million bunch crossings
High-pT events smeared by soft pile-up events
The ATLAS High-Level Trigger (HLT)
The ATLAS trigger
HLT e/ selection
Selection method
Ricardo Goncalo, Royal Holloway University of London8ATLAS HLT e/gamma selection
The ATLAS trigger
Level 1: Hardware based (FPGA/ASIC) Coarse granularity detector data Average execution time 2.5 s Output rate ~75 kHz
Level 2: Software based Only detector sub-regions
processed (Regions of Interest - RoI) seeded by level 1
Full detector granularity in RoIs Fast-rejection steering Average execution time ~10 ms Output rate ~1 kHz
Event Filter: Seeded by level 2 Full detector granularity Potential full event access Offline-like algorithms Average execution time ~1 s Output rate ~200 Hz
Hig
h-L
evel
Tri
gg
er
Ricardo Goncalo, Royal Holloway University of London9ATLAS HLT e/gamma selection
HLT e/ selection
High transverse momentum electrons and photons are an important part of several physics signatures
Fake signals produced by narrow jets and by 0
Physics coverage Signatures (initial lumi) Rate
ElectronsHiggs, susy, W, top, heavy gauge bosons, extra dimensions
e25i, 2e15i, e60 ~40 Hz
Photons Higgs, susy, extra dimensions 60, 220i ~40 Hz
Muons (high-pT)Higgs, susy, W, top, heavy gauge bosons, extra dimensions, B physics
20i, 210, 26 ~40 Hz
Jets Susy, resonances, compositeness j400, 3j165, 4j110 ~20 Hz
Jets+ETmiss Susy, leptoquarks j70 + xE70 ~5 Hz
Tau+ETmiss MSSM Higgs, susy 35 + xE45 ~10 Hz
Others Prescaled + calibration + monitoring ~20 Hz
Ricardo Goncalo, Royal Holloway University of London10ATLAS HLT e/gamma selection
Match?
Selection method EMROI
L2CALO
Pass?
L2Tracking
cluster?
EFCALO
Track?
EFTracking
Cluster?
EF Pass?
L2 seeded by Level 1Full detector granularityFast calorimeter cluster reconstruction(only cluster for triggers)Fast tracking algorithms
L1 region of interest: , , ET threshold, isolation in EM calorimeter Coarse granularity
EF seeded by Level 2Full detector granularityOffline-type reconstruction algorithms for calorimeter clusters and inner detector tracksRefined alignment and caibration
L1
L2
EF
Event rejection possible at each step
HLT Performance studies
Single electron signaturesPhoton signatures
Trigger optimizationPhysics applications
Trigger efficiency from dataTiming studies
Test beam studies
Ricardo Goncalo, Royal Holloway University of London12ATLAS HLT e/gamma selection
Signature example: single e
e25i Efficiency RateLevel 1 % kHzL2 Calo % kHzL2 Trk % HzL2 match % HzEF Calo % HzEF Trk % HzEF match % Hz
Barrel-endcap crack excluded
Passed level 2 background (approx)
We 20%
Zee 6%
e from b and c decays 8%
(quark brem & prompt) 14%
Other (0, jets, etc) 52%
Monte-Carlo samples: Single electrons QCD di-jet sample with ET>17 GeV
Pileup and noise included Using fully simulated data for the initial
luminosity scenario we get: Note: uncertainty on QCD jet cross
section is a factor of 2-3 Trigger cuts optimized as function of
e25ielectronpT>25 GeVisolated
Ricardo Goncalo, Royal Holloway University of London13ATLAS HLT e/gamma selection
Photon menus
Using fully-simulated Monte-Carlo data we get: For 220i and 60
220iphotonpT>20 GeVisolated
double trigger
Efficiency 220i 60 220i 60
Level 1 94% 85% 98%
Level 2 84% 81% 94%
Event filter 78% 69% 89%
Rate 2 Hz 10Hz
Barrel-endcap crack excluded
pT = 20 GeV convertednot conv.
effi
cien
cy
||
Converted photon reconstruction at the Event Filter could be used to reduce the rate
Ricardo Goncalo, Royal Holloway University of London14ATLAS HLT e/gamma selection
Efficiency optimization
Efficiency must be balanced against the trigger output rate to optimize available bandwidth
Tools in place to do automatic optimization by scanning selection cuts parameter space
Efficiency vs. rate/jet rejection curve provides continuous set of working points
L2 Tracking
Every point in the plot corresponds to a set of selection cut values
Envelope is optimum rejection for each efficiency value
Ricardo Goncalo, Royal Holloway University of London15ATLAS HLT e/gamma selection
Physics applications
Ze+e- and We important channels for: Precision SM physics Detector commissioning Detector calibration Luminosity measurement
Efficiency numbers wrt the following kinematical cuts: Ze+e-:
2 electrons with ET>15 GeV, ||<2.5 We:
1 electron with ET>25 GeV, ||<2.5 H (mH=120 GeV)
1 photon with ET>20 GeV, ||<2.5 1 photon with ET>40 GeV, ||<2.5
Efficiency Ze+e- We
2e15i 67.2%
e25i 92.9% 79.6%
e60 20.4% 6.9%
all 94.8% 80.3%
Barrel-endcap crack excluded
Efficiency 220i 60 220i 60
Level 1 94% 85% 98%
Level 2 84% 81% 94%
Event filter 78% 69% 89%
Rate 2 Hz 10HzH (mH=120 GeV) Barrel-endcap crack excluded
Ricardo Goncalo, Royal Holloway University of London16ATLAS HLT e/gamma selection
Trigger efficiency from data
Electron trigger efficiency from real Ze+e- data
1. Tag Z events with single electron trigger (e.g. e25i): N1
2. Count events with a second electron (2e25i): N2
3. Fit Z mass peak + linear fit to background (B)
4. Efficiency is function of N1, N2, B1 and B2
No dependence found on background level (5%, 20%, 50% tried)
Estimated systematic uncertainty small
~3% statistical uncertainty after 30 mins at initial luminosity
Method Ze+e- counting
L 2 efficiency (%) 87.00.2 87.00.6
MZ (GeV)
Ricardo Goncalo, Royal Holloway University of London17ATLAS HLT e/gamma selection
Timing studies
Timing of the trigger algorithms essential for performance
Times estimated for a 8 GHz CPU and 1 RoI/event
Level 2 latency is 10 ms; still work to do here but much progress made recently
Most of the Level 2 time taken by unpacking of data
(transit from detector/buffer included???)
Event Filter time small wrt allowed latency (~1s)
Ricardo Goncalo, Royal Holloway University of London18ATLAS HLT e/gamma selection
Test beam studies
Objective was to study e/ separation and electron efficiency in realistic detector
A good opportunity to test the tools
Tracking algorithms used without modifications
Tracking efficiency measured always above 95%
TRTLAr
Tilecal
MDT-RPC BOS
TRTLAr
Tilecal
MDT-RPC BOS
Data sample
Electron eff. (%)
fake rate (Hz)
20 GeV 95.30.4 1.60.2
50 GeV 94.90.3 0.70.2
Conclusions and outlook
Ricardo Goncalo, Royal Holloway University of London20ATLAS HLT e/gamma selection
Conclusions
The LHC will turn on in 2 years time (not such a long time to go) The short available time and high pileup rate in the LHC pose serious
challenges that the trigger must ovecome The e/ trigger signatures cover a wide range of physics channels
essential to the ATLAS programme Much work still needed to guarantee we’ll be ready for data taking But: much ground already covered, e.g. timing of data preparation,
bremstrahlung recovery in offline tracking HLT e/ signatures are well developed and seem able to cope with the
harsh LHC environment Signatures exercised on fully simulated physics channels, both
relevant for physics measurements and for detector calibration Efficiency measurements also done in realistic environment of
testbeam Many tools in place to assist trigger development, tuning and study
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