1. Status of the cross section analysis in e! e
Yelena Prok
PrimEx Collaboration Meeting
March 18, 2006

2. Outline Event Selection Kinematic and Geometric Cuts
Comparison with Simple Simulation
Binning and Acceptance
Obtaining the Yield
What remains to be done

3. Dedicated Compton Runs
40 (good) runs with the 5 % r.l. 12C target
Target density=2.193 g/cm3
Target thickness=380 £ 2.54 cm
Atomic mass= 12
Atomic number=6
2.1<Ebeam<3.1 and 4.9<Ebeam<5.5 GeV
4 (good) runs with the 5 % r.l. 9Be target
Target density=1.848 g/cm3
Target thickness=70.6 £ 2.54 cm
Atomic mass=9
Atomic number=4
4.9<Ebeam<5.5 GeV

4. Event Reconstruction Skim files were created from raw datafiles
at least 2 clusters, energy sum of 2 clusters >0.5 GeV, central hole (3.8 cm) is cut out
Standard PrimEx reconstruction program used, with the following parameters
Clustering algorithm: combination of “island” + 5x5 algorithm
Using Double Arm Compton Calibration Constants
Z=731.9 cm (survey)
Coordinate reconstruction (method 3, alignment correction, depth correction)
Non-linearity correction in energy reconstruction

5. Event Selection
To select candidates for a given event, look at the time difference distribution between the tagger and HYCAL (bit 9), then take all photons in a defined time window. Here, time window of § 6 ns is used.

6. Data Selection Elasticity cut:|(e1+e2)/ebeam-1|<0.1
4.9<Ebeam<5.5 GeV <Ebeam<3.1GeV

7. Data Selection Cluster Separation:
4.9<Ebeam<5.5 GeV <Ebeam<3.1 GeV
sep>18 cm sep > 25 cm

8. Data Selection Geometric Cuts (1)
Fiducial region clearly contaminated by pair production is cut out
Most likely, not all of the contamination is removed by this cut

9. Data Selection Geometric Cuts (2)
Particle 1
More energetic of the two
Particle 2, less energetic
Due to the potential problems with cluster reconstruction
in proximity to the central hole, a region of 6x6 cm is cut out

10. Data Selection
A simulation using GEANT 3 was performed with the incident photons in the energy ranges of 2.1 <E< 3.1 and 4.9<E<5.5 GeV and target parameters of the experiment. No radiative losses were included. The purpose of this simulation is to determine detector acceptance, check the experimental kinematic distributions and to determine a set of appropriate cuts for data selection.
For compatibility, the experimental and simulated data are arranged into pairs of particles, so that the first particle of the pair is always most energetic

11. Comparison with Simulation Cluster Separationcm cm
4.9<E< <E<3.1

12. Comparison with Simulation Geometry
4.9<E<5.5 GeV <E<3.1 GeV

13. Comparison with Simulation 1 vs 2 (deg)
4.9<E<5.5 GeV <E<3.1 GeV

14. Data Binning and Acceptance
Data is binned consistently with the photon flux binning in tagger TChannels
A simulation was done for the energy range of every bin
Same cuts were applied to the simulated and experimental data
Expected cross section and geometric acceptance determined for every energy bin

15. Bin #
tchannels
E min (GeV)
Emax(GeV)
Acceptance(%) 1
1+2
5.45
5.5
6.29 2
3+4
5.39
6.45 3
5+6
5.32
6.61 4
7+8
5.27
6.71 5
9+10
5.22
6.83 6
11+12
5.17
6.95 7
13+14
5.12
7.08 8
15+16
5.07
7.18 9
17+18
5.01
7.3 10
19+20
4.95
7.4 11 21
4.89
7.5 30
59+60
3.0
3.13
6.81 32
63+64
2.8
2.9
5.84 34
67+68
2.61
2.7 35
69+70
2.52
2.63
4.40 36
71+72
2.44
2.53
3.81 38
75+76
2.30
2.37
2.80 39
77+78
2.22
2.31
2.33

16. Counting pairs Summary of cuts: -Minimum cluster energy = 1 GeV
-Cluster separation > 18 (25) cm
-Central Area § 6 cm is cut out
-Area mostly affected by pair- production contamination is cut out
-Elasticity cut: |(e1+e2)/Ebeam-1| <0.1
All events that passed the cuts above are used to fill the following histograms:
e1+e2-Ecompton vs bin #
Ebeam-Ecompton vs bin #
1 -2 vs bin #
e1+e2-ebeam vs bin #

17. Counting pairs Ebeam-Ecomp e1+e2-Ecomp e1+e1-Ebeam 
Each type of histogram is fitted with a
gauss+gauss and gauss+polynomial functions

18. Acceptance Corrected Normalized Yield (per atom)
12C target
Acceptance corrected normalized yield NCNY:
NCNY=C/L/A
C=counts under the first gaussian function
L=*th*NA*
th=target thickness
=target density
NA=Avogadro’s Number
=atomic mass
A=geometric acceptance obtained from simulation
Detection efficiency and reconstruction efficiency are not known and set to 1

19. Acceptance Corrected Normalized Yield (per electron)
12C target

20. Acceptance Corrected Normalized Yield (per electron)
9Be, 5% r.l. target
Run 4875

21. Yield Stability vs Run Number (for 12C target)
Bin 1
Bin 6
Bin 10
Bin 2
Bin 5
Bin 9
Bin 3
Bin 4
Bin 7
Bin 8
Bin 11
Bin 30
Bin 32
Bin 36
Bin 38
Bin 35
Problem with flux
Bin 34
Bin 39

22. VETO Effect 2.1<E<3.1 4.9<E<5.5
Requiring that one of the clusters is charged and the other
one is neutral throws away ~10-20% of the data
that passed the rest of the cuts

23. VETO effect (cont) 1 neutal, 1 charged cluster No VETO cut
Background reduced: ~ 40 %
Signal reduced:~ 20 %

24. Angular Distributions DATA vs GEANT
fast
fast
slow
slow e- e-  

25. Summary and Outlook What has been done
First look at the total cross section in e! e with 12C and 9Be targets
Results are on average 5-15% below the expectation (at least, partially, this is due to the assumption that the detection and reconstruction efficiencies are 100 %.)
Results show no obvious time dependence
What needs to be done
Check of systematic effects in data analysis
Compton generator for primsim
Full simulation which will allow more realistic comparison with data, and evaluation of detection and reconstruction efficiencies
Better understanding of the backgrounds (more simulations)
Trigger Efficiency not completely understood
Radiative Corrections