1. Identification of isolated photons
at high energies at H1
Bachelor Thesis
Arno Gadola
I like to welcome you to my presentation about the identification of isolated photons at high energies. Please don‘t hesitate to interrupt me, if there are questions.
Zurich, march 2008
Arno Gadola

2. Outline - Motivation - Shower Shape Variables - Analysing data
- Results
- Conclusions
Arno Gadola

3. Motivation The proton structure: - 3 valence quarks (uud)
- virtual quark-pairs ( )
- gluons
Electron - Proton scattering:
Proton probing with electron
Which processes give insight?
Neutral current, charged current
Zoom in
Next: subprocess 1/137
Direct photon interaction
Resolved photon interaction (VDM)
Q2 = -q2
Arno Gadola

4. Motivation: Prompt photons
Examples for processes that give insight to the parton structure:
Direct process in leading order:
Resolved process in leading order:
xγ = 1, xq < 1
xγ < 1, xq < 1
Prompt photons = photons from the hard interaction
Gluon <=> Photon
=> Promt photons
Prompt photon gives information about:
Perturbative QCD and QED
Structure functions F (quark and gluon density)
Momentum fraction xγ, xp carried by the interacting parton (quark or gluon) of the photon
or the proton
Arno Gadola

5. Motivation: Background
Background from neutral hadrons with photon-like clusters after standard cluster selection: 87% π0
Neutral hadrons decay in photons
Eπ [MeV]
Arno Gadola

6. Separation of signal and background!
Goal of this study
Separation of signal and background!
Energy range:
Transverse energy range of 5-10GeV  already done
Transverse energy range of 10-15GeV  this thesis
2. Generating data for the analysis with Monte Carlo
Signal: single photon events
Background: double photons events (pions)
3. Analyse the generated data (and compare to real data)
WAY to do the separation.
Transverse energy = energy projection perpendicular to the beam axis.
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7. H1 Detector: The Liquid Argon Calorimeter
Collider HERA:
Electrons: E = 27.5GeV
Protons: E = 920GeV
=> = 318GeV
Prompt photons: ET = 5-15GeV
Liquid Argon (LAr) calorimeter:
8 wheels
E.m. calo with lead absorbers
Hadron calo with steel absorbers
Covered angle 4° < θ < 154°
ONLY LAr.
Bild und Wheels beschreiben
Zoom in the em calo
Arno Gadola

8. Cluster: Shape variables
Shower shape variables:
- Longitudinal dimension RT
- Transverse dimension RL
- Energy fraction of hottest cell EHottestCell
- Energy fraction of hot core EHotCore
- Energy fraction of cells in different layers
- …..
Particle deposes energy in cells.
Cells are merged to cluster
Cluster => Shape variables
I have tried different variables.
Arno Gadola

9. Shape variables Longitudinal dimension RL [cm]
Transverse dimension RT [cm]
Example for:
Wheel 3
ET = 13-15GeV
A selection of 4 variables
RL not much separation possible
RT 2photon events yield larger RT,
especially in the forward region
Hottest FRACTION, smal values=smal E
Energy of hottest cell first in layer
fraction EHottestCellL1
Energy of hottest cell fraction EHottestCell
Arno Gadola

10. Clustering: Shape variables
Shower shape variables:
- Longitudinal dimension RT
- Transverse dimension RL
- Energy fraction of hottest cell EHottestCell
- Energy fraction of hot core EHotCore
- Energy fraction of cells in different layers
- …..
Multivariate analysis tools (root):
1. TMVAnalysis  trains methods
Likelihood, Fisher, MLP, etc.
2. TMVApplication  discriminator
Discriminator:
Put variables in analysis tool => Discriminator
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11. ET = 13-15GeV ET = 11-13GeV ET = 9-11GeV
Wheel 4
Wheel 5
Wheel 6
ET = 13-15GeV ET = 11-13GeV ET = 9-11GeV
A reason for the lack of entries for signal on right side could not be found.
Arno Gadola

12. Signal Background Wheel 4 Wheel 5 Wheel 6
ET = 10-15GeV ET = GeV ET = GeV
Good fit of real data
Peak in the middle is not physical
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Signal
Background

13. Summary Structure functions Momentum fractions xγ, xq QCD, QED test
Found good separation power
for higher transverse energies
(10-15GeV)!
Discriminator: Likelihood
Prompt photons
Get rid of evil background
- Clustertool (generating shape variables)
- Multivariate analysis tool
Energy RL RT
Shape variables
Further treatment leads to a better statement
Arno Gadola

14. Conclusions For future tasks: - Tuning of methods
- Test of other methods
- Calculation of a cross section and fit with real data
For future thesis:
- Including high energy intervall of 10 to 15 GeV
- For high energy intervalls longitudinal cluster
shape dimension RL should be included
Thanx to Katharina
And with a humorous Homer the question session is opened.
Arno Gadola

15. Questions?
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16. Shower cascade Shower stops at: Ee = Ec ≈ 610MeV/(Z+1.24)
Electormagentic shower in a calorimeter
Shower stops at: Ee = Ec ≈ 610MeV/(Z+1.24)
Arno Gadola

17. Arno Gadola

18. Shape variables Transverse energy: Central moments: Transverse radius:
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19. Shape variables Background of neutral hadrons before
standard cluster shape selection.
Background of neutral hadrons after
standard cluster shape selection.
Arno Gadola

20. Clustering: Shape variables
Cluster shapes:
- Longitudinal dimension
- Transverse dimension
- Kurtosis
- Symmetry
- Hottest cell fraction
- Hot core fraction
- First layer energy fractions
MVA:
Likelihood
Neural network
Fisher …
Discriminator
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21. ET = 9-11GeV ET = 11-13GeV ET = 13-15GeV
Wheel 1
Wheel 2
Wheel 3
Wheel 4
Wheel 5
Wheel 6
Arno Gadola