Status of the cross section analysis in e  ! e 

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 SelectionElasticity cut:|(e1+e2)/ebeam-1|<0.1 4.9<Ebeam<5.5 GeV 2.1<Ebeam<3.1GeV

7. Data SelectionCluster Separation: 4.9<Ebeam<5.5 GeV 2.1<Ebeam<3.1 GeV sep>18 cm sep > 25 cm

8. Data SelectionGeometric Cuts (1) 4.9<E<5.5 2.1<E<3.1 Fiducial region clearly contaminated by pair production is cut out Most likely, not all of the contamination is removed by this cut

9. Data SelectionGeometric 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 SimulationCluster Separation cm cm 4.9<E<5.5 2.1<E<3.1

12. Comparison with SimulationGeometry 4.9<E<5.5 GeV 2.1<E<3.1 GeV

13. Comparison with Simulation1 vs 2 (deg) 4.9<E<5.5 GeV 2.1<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

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 DistributionsDATA vs GEANT 4.9<E<5.5 2.1<E<3.1 fast fast slow slow e- e-  

25. 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 Summary and Outlook