overview of the high-level trigger electron and photon selection for the atlas experiment at the lhc...

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


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


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)


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


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


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


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


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


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


pT = 20 GeV convertednot conv.

effi

cien

cy

||

Converted photon reconstruction at the Event Filter could be used to reduce the rate


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


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%


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


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)


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)


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


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

overview of the high-level trigger electron and photon selection for the atlas experiment at the lhc...

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