High Level Trigger Studies for the ATLAS Detector Efstathios (Stathis) Stefanidis

RAL Summer SchoolSeptember, 2004

Efstathios (Stathis) [email protected] http://www.hep.ucl.ac.uk/~sstef

High Level Trigger Studies for the

ATLAS Detector

Efstathios (Stathis) Stefanidis

University College London



OUTLINE The ATLAS Detector : Overview.

The Trigger System.

i. Overview.

ii. High Level Trigger.

Performance Studies.

i. IDScan.

ii. e/γ vertical slice.

iii. RoI size.

Future Plans.



The ATLAS Detector: Overview.1. pp colliding beams

14 TeV c.m. energy.

2. Design Luminosity: L=1034 cm-2s-1.

3. Three Parts:

a) Inner Detector

b) Calorimeter (EM – HAD)

c) Muon Spectrometer

4. Magnet System:

a) Solenoid: 2 T

b) Toroid: 0.4 T



The Trigger System: Overview.1. LVL1:

• 40 MHz 75 kHz • < 2.5 μs • Hardware trigger• Reduced granularity information

2. LVL2: • 75 kHz 2 kHz • 10 ms • Full granularity information

from both ID and Calorimeter• RoI mechanism

3. Event Filter (EF): • 2 kHz 100 Hz 2s • Sophisticated algorithms• Alignment data available

High Level Trigger (HLT)



The Trigger System: HLT.

• RoI mechanism

(i) Defines the area where HLT

will start from.

(ii) Seeded by LVL1

(iii) Only the data needed are

transferred

MINIMISE THE PROCESSING TIME AND NETWORK TRAFFIC

Objects Selection Signatures

Physics Coverage

Electron e25i, 2e25i Higgs (SM, MSSM), new gauge bosons etc

Jet j400, 3j150 SUSY, compositeness, resonances etc.

1. Event Selection Strategy:

• Signature validation

Checks for signatures coming from interesting physics events

REJECT UNINTERESTING EVENTS VERY EARLY



The Trigger System: HLT.2. Event Selection

Software:

• ATHENA

(i) Written using GAUDI architecture

(ii) Provides common services (Transient Data Store, Histograms, Auditing etc)

(iii) Well-defined interface not only to developers but also to the end-users

IMPROVE COHERENCY OF THE DIFFERENT SOFTWARE DOMAINS

• IDScan

(i) LVL2 track reconstruction algorithm

(ii) SPACE POINTS as input – Track parameters as output

(iii) Runs several times per event and once per RoI



Performance Studies: IDScan.Pattern Recognition

Algorithms Low Luminosity

High Luminosity

IDScan (89.0 ± 0.4) %

(82.9 ± 0.5) %

xKalman (88.9 ± 0.4) %

(84.0 ± 0.5) %

iPatRec (87.2 ± 0.4) %

(82.2 ± 0.5) %

|pTgen – pT

rec|<15 GeV

|ηgen – ηrec|<0.01

|φgen – φrec|<0.01 rads



Performance Studies: e/γ slice.•Determine selection efficiencies and rates at each Trigger Level.•Apply isolation, energy, tracking matching etc cuts.

Trigger Level

% Efficiency w.r.t LVL1

Rate

LVL1 100 7.9 kHz

LVL2Calo 95.7 1.9 kHz

LVL2ID 88.4 359.1 Hz

LV2IDCalo 86.3 136.1 Hz

EFCalo 84.8 92.3 Hz

EFID 80.0 64.1 Hz

EFIDCalo 73.4 31.1 Hz

•Electron selection •Weνe sample

22.5 GeV



Performance Studies: RoI size.How is the RoI constructed?

• Take the information from LVL1 Calo : (η,φ) of the active region. The η coordinate is calculated w.r.t to z=0

• Construct the shape of the RoI:

z=0 ± 168 mm (3 of the beam spread)

η ± 0.1

φ ± 0.1 radMotivation?

Optimize the size of the RoI.

Less Space Points.

Less Combinatories.

Quicker Algorithms.

Improve efficiency.Implementation?

Use the shower position at the 1st and 2nd EM sampling.



Performance Studies: RoI size.

Z vertex resolution Phi vertex resolution Theta vertex resolution

σ_low = 3.626 cm

σ_high = 3.593 cm

σ_low = 1.719 mrad

σ_high = 1.692 mrad

σ_low = 14.26 mrad

σ_high = 14.11 mrad



Future Plans.

•Performance Studies

IDScan : Optimize the size the RoI

e/γ analysis : Improve, validate, optimize the cuts

•Physics Studies

Low Mass Higgs (mH<150 GeV)

Hγγ

Hbb

Study the full production/simulation/reconstruction chain.

Documents

High Level Trigger Studies for the ATLAS Detector Efstathios (Stathis) Stefanidis