1. High Level Trigger Studies for the Efstathios (Stathis) Stefanidis
ATLAS Detector
Efstathios (Stathis) Stefanidis
1st Year Transfer Report,
University College London
June, 11th 2004
Stefanidis E.

2. OUTLINE The ATLAS Detector : Overview. The Trigger System. Overview.
High Level Trigger.
Performance Studies.
IDScan.
e/γ vertical slice.
RoI size.
Future Plans.
June, 11th 2004
Stefanidis E.

3. The ATLAS Detector: Overview.
pp colliding beams TeV c.m. energy.
Design Luminosity: L=1034 cm-2s-1.
Three Parts:
Inner Detector
Calorimeter (EM – HAD)
Muon Spectrometer
Magnet System:
Solenoid: 2 T
Toroid: 0.4 T
June, 11th 2004
Stefanidis E.

4. The Trigger System: Overview.
LVL1:
40 MHz  75 kHz
< 2.5 μs
Hardware trigger
Reduced granularity information
High Level Trigger
(HLT)
LVL2:
75 kHz  2 kHz
10 ms
Full granularity information from both ID and Calorimeter
RoI mechanism
Event Filter (EF):
2 kHz  100 Hz
 2s
Sophisticated algorithms
Alignment data available
June, 11th 2004
Stefanidis E.

5. The Trigger System: HLT.
Event Selection Strategy:
Signature validation
Checks for signatures coming from interesting physics events
REJECT UNINTERESTING EVENTS VERY EARLY
Objects
Selection Signatures
Physics Coverage
Electron
e25i, 2e25i
Higgs (SM, MSSM), new gauge bosons etc
Jet
j400, 3j150
SUSY, compositeness, resonances etc.
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
June, 11th 2004
Stefanidis E.

6. The Trigger System: HLT.
Event Selection Software:
ATHENA
Written using GAUDI architecture
Provides common services (Transient Data Store, Histograms, Auditing etc)
Well-defined interface not only to developers but also to the end-users
IMPROVE COHERENCY OF THE DIFFERENT SOFTWARE DOMAINS
IDScan
LVL2 track reconstruction algorithm
SPACE POINTS as input – Track parameters as output
Runs several times per event and once per RoI
z-Finder
Hit Filter
Group Cleaner
Track Fitter
June, 11th 2004
Stefanidis E.

7. Performance Studies: IDScan.
z-Finder
σ=179.9 μm I.
II.
III.
Total Efficiency
Efficiency: |Zgen – Zrec | < 1 mm
June, 11th 2004
Stefanidis E.

8. 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 – pTrec|<15 GeV
|ηgen – ηrec|<0.01
|φgen – φrec|<0.01 rads
June, 11th 2004
Stefanidis E.

9. 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
Weνe sample
22.5 GeV
June, 11th 2004
Stefanidis E.

10. Performance Studies: RoI size.
Preliminary study
Current size of RoI: ΔηxΔφ = 0.2x0.2
COULD IT BE LESS ?  Reduce requested data + processing time
June, 11th 2004
Stefanidis E.

11. Performance Studies: RoI size.
φ resolution: 3mrad
Cut at z  2 mm
η resolution: STRONG DEPENDENCE
ON Z POSITION OF VERTEX ! ! !
Barrel: σZ =0.86 cm
EndCap: σρ =0.35 cm
June, 11th 2004
Stefanidis E.

12. Future Plans. Performance Studies IDScan : Optimize the size the RoI
e/γ analysis : Improve, validate, optimize the cuts
2004 TestBeam : Study the timing perfomance
Physics Studies
Low Mass Higgs (mH<150 GeV)
Hγγ
Hbb
Study the full production/simulation/reconstruction chain.
June, 11th 2004
Stefanidis E.