1. Workshop on Forward Calorimetry Prague, April 16 Impact of Bhabha scattering on the BeamCal performances
Vladimir Drugakov NC PHEP, Minsk/DESY Zeuthen
Andrey Elagin JINR, Dubna

2. Motivation Benchmark geometry of the BeamCal:
5 mrad ≤ θ ≤ 28 mrad, where 5 mrad ≤ θBeamPipe ≤ 5.45 mrad
Bhabha scattering e- e+ -> e- e+ (nγ)
σ ~ 1/θ3 => very high probability at very small angles
When one or several particle from such event are found but event is not identified like Bhabha, it becomes severe backgroud for other physic processes.
Why some events cannot be fully reconstructed and identified:
- some particles are lost
- pure energy resolution
Why particles can be lost:
- reconstruction problem due to beamstrahlung background;
- kinematics - γ-radiation deflect particles.

3. Beamstrahlung Background
Energy flux through particular cell. Train dynamics.
R-φ distribution of energy flux for particular BX .

4. Single Electron Reconstruction Efficiency
a) and b) represents different φ angles

5. Total Probability = 0.85 BX-1
Bhabha kinematics
Examples
BHLUMI generator
Θmin = 0.3 mrad γ e- e+
Total Probability = 0.85 BX-1 e- e+ γ
Only 1 electron hits detector
How correct it works
at such small angles?

6. TEEGG generator Generate process e- e+ -> e- e+ γ(γ) Options:
- only 1 e- or e+ in detector
- only γ in detector

7. Reconstruction
First step is to simulate and reconstruct e- e+ -> e- e+ process
BHLUMI generator
Total Probability = 0.11 BX-1
Hit BP 11.5%
1000 event were tracked by BRAHMS and then reconstructed:
- 10.5% - 0 e± - leakage
- 6.7% - 1 e± - leakage + reconstruction imperfection
- 82.2% - 2 e±

8. Reconstruction. Angular resolution.

9. Reconstruction. Energy resolution and cuts

10. Outlook A lot of work is to be done:
Verify BHLUMI reliability at submrad angles
(Analysing full Bhabha event zoo)
Find out identification criteria
Find out minimal angle
Finally: Find out reconstruction rate
Find out veto rate