1. Event Classification & Background Rejection
Bill Atwood & Friends

2. Recap Since DC1 DC1 Closeout February 2004
Begin Recon-Rewrite, May 2004
First up: TkrRecon
Participants: Tracy Usher, Leon Rochester, Johann Cohen-Tanugi, WBA
Next up: CalRecon
Participants: Tracy Usher, Philippe Breul, Pol d'Avezac, Fred Piron, Eric Nuss,
David Chamont, WBA
Next up: AcdRecon
Participants: Heather Kelly, Eric Charles, WBA
Development of Alternative High Level Analysis
Toby Burnett & Riccardo Rando et al
Begin DC2 Analysis, May 2005
First Energy Analysis

3. Energy Methods Method % Computed % Best Est. Parametric 100 63.6
Profile
49.9
24.7
Last Layer
23.4
5.5
Tracker
16.5
6.3
4 Methods*
3 Only cover a part of Glast Phase Space
*Pol d'Avezac has combined Tracker & Last Layer
Parametric
Tracker
Last Layer
Profile
Only Parametric Available: 37.7%
This tends to be the Local Land Fill (City Dump!)
Unfortunately there are too many events here
to simply throw out.
Compare each Method
against a "standard"
defining:
Energy Resolution
Model
Select Method giving
the Highest CT Prob.
Data Set: All Gamma (GR-HEAD1.615)
sModel = /(McLogEnergy) *(McLogEnergy-2.)2

4. E4: Unbalanced CTs - 2s on
s= /(log(E))3
Data Set: AG1617-mod

5. How to Tell What You Have:
In an analysis you may wish to check which energy method your "signal"
is relying on. In the IM environment this is captured by a set of
CATAGORICAL Variables and life is easy... however this was not
captured in the translation to GLEAM.
Here's how to tell:
Compare:
CTBParamProb
CTBProfileProb
CTBLastLayerProb
CTBTrackerProb
CTBBestEnergyProb
The one which matches is the method used for the event in question.
CTBBestEnergy, CTBLogEnergy, and CTBBestEnergyProb summarize
the final results

6. PSF Analysis IM "glue" to put events bact together and produce
Split according to
Thick and Thin Layers:
Tkr1FirstLayer (Thick) or 6-17 (Thin)
IM "glue" to put events
bact together and produce
CTBBestXDir,YDir,ZDir and
CTBCORE (the "good" PSF
probability variable)
CTs: Of Events with Valid VTX
choose to use 1Tkr or VTX
Solution: CTBVTX
Split according to VTX
Solution
(VtxAngle > 0 && CTBVTX > .5
CTs: PSF Image Sharpening
Produces CTBCORE Variable

7. PSF Analysis Results What CTBCOR Does On Axis can vary PSF by ~ 30%
(at the expense of Aeff)
The 95/68 Ratio improves significantly
cos(q) < -.95
cos(q) < -.2

8. How to Tell What You Have:
To tell which analysis branch you have
- THICK or THIN? THICK == Tkr1FirstLayer < 5
THIN == !THICK
Note: Tracks can not originate in Layers 0 and 1
- VTX or 1TKR ? VTX == VtxAngle > 0. && CTBVTX > .5
1TKR == !VTX
Note: We could have used VtxStatus bits instead of VtxAngle
CTBBestXDir,YDir,ZDir, CTBCORE summarize the results
THIN Layers
THICK Layers
Cuts: CTBGAM > .5 && CTBBestZDir < -.95 && CTBBestEnergyProb > .35

9. Back Ground Rejection Evolution
- Use of only Classification Trees not adequate
(Typ. Rejection Power )
- Improve Input to CTs via "typical" HEP Style cuts
(Implemented as PreFilters)
- Divide sample according type
1) Topology (VTX – 1Tkr)
2) Obs. Cal. Energy (use 6 ~log(E) bins)
3) Location (Lowest energy only)
(Gives 14 Separate catagories)
What's left is what CTs are poor at finding or Irreducible (start here)
Results in a g Bkg. Rejection Prob.
CTBGAM
Class 3

10. First: Irreducible Back Grounds
Correlated
Uncorrelated
e+ Blanket Conversion
Proton - Blanket Interaction
> 60% Remaining background are from gs produced locally
They are Irreducible

11. Second: Weakness in CT Analysis
Subsequent Search for "Hot Spots" in GLAST Phase space revealed -
Correlated Events in the Top
Elimination of Events
which project back
to Ribbons
Cut Summary
Bkg: %
All Gam: %
Top Ribbons
More Ribbons
Holes at the Top of
Side Ribbons

12. Correlations among Variables
Heavy Ion Filter
CTBBestEnergy > 1000 & ((CalTransRms - 1.5*Tkr1ToTTrAve) < 5)
& CTBGAM > .5
What's
Eliminated
Cut Summary
Bkg: %
All Gam: %
Scrambled Tracks Filter
(Tkr1FirstLayer - Tkr2FirstLayer) <= 0 & Tkr2FirstLayer > 2 & Tkr2TkrHDoca > 10 &
(CTBGAM+.16*CTBBestLogEnergy) < 1.32
Cut Summary
Bkg: %
All Gam: %

13. These Post-Processing Cuts can be found at:
This leaves an Aeff vs Energy with features (not good...). Smooth Response by
"boosting" low energy CTBGAM :
CTBGAM = min(1., CTBGAM*min(2.0, (3.5/min(CTBBestLogEnergy, 3.5))^2))
Base Class 3 – No Post-Processing Filters
CTBGAM > .5
Base Class 3 – Post-Processing Filters & CTBGAM Boost at Low Energy
CTBGAM > .5

14. Class 3 Class 3 Post Process Cuts +
Careful: Order of Plots different Top to Bottom

15. Demonstrations with DC2 Data from Galactic Anti Center Region (thanks to Julie)
Ooppsss - Almost forgot – The Limb g issue....
Rocked Backgrounds
What's cut out:
CTBGAM > .5
2216 evts
Evts. in
Common
277 evts
Evts. Missed by
BestZDir > -.3
127 evts
Julie's Cut
Bill's Original Cut
Cut Used +
ZDir < -.3
Evts. Missed by
ZenithTheta > 100
32 evts.
Residual Bkg. Evts.
436 Total Evts.

16. On with the show...
Galactic Anti-Center: Class A – No Post Proc. Cuts

17. Class A Events + Post-Proc. Cuts

18. What was killed by Post-Proc. Cuts
The Fog of Back Ground...

19. Image Resolution
The Geminga Pulsar
Low Res.
Good Res.
Best Res.

20. Estimating Residual Backgrounds
The ~20k Background events remaining after PreFilters
As with previous experiments – look for orbit associated modulation in signal.
Use this as a measure of remaining contamination.
Seems to be ~ 10%
Need to "fold" all the orbits on top of each other – Hard due to multiple
components and SAA (holes).
Steve's suggestion: Profile residuals events vs the Trigger Rate

21. Summary DC2 Background Rejection gets us close... however
There is no factor of 2 left – the Irreducible Component Dominates
Background Rejections is a "work-in-progress"
We just ran out of time. We have made 3 complete passes so far
and a Pass 4 is in the works.
Most Analysis are not that sensitive to Background Contamination
Thankfully residual backgrounds are essentially spread ~ uniformally
across the sky. As such contamination in a few sq. degrees is small (part in 10-4)
Work will continue and perhaps a "reprocessed" DC2 Dataset
will appear prior to the DC2 Closeout (similar to real life)
The instrument "hand-off" is approaching and we need to
meet the Science Requirements by then.