1. Simulation of physics background for luminosity calorimeter M.Pandurović I. Božović-Jelisavčić “Vinča“ Institute of Nuclear Sciences, Belgrade, SCG

2. Overview Overview - Status from Vienna - Event generators - Detector simulation -Comparison of signal to background -Conclusion - Plans FCAL Workshop Krakow 12-13 February 2006

3. Status from Vienna - Four-lepton processes are the main source of physics background for luminosity measurement - Agreement of event generators (With WHIZARD physics background has been generated with average rate of 10 -3 tracks per BX in the Luminosity Calorimeter what is in agreement with the independent study of two-photon production of electrons using Vermasseren event generator (L.Suszycki, FCAL Workshop, Tel Aviv, September 2005) - What was missing: - Detector simulation - Effect of cuts on physics background - X-angle case FCAL Workshop Krakow 12-13 February 2006

4. Event generators |  |<5.27 - Signal consists of 10 5 Bhabha events (~22 pb -1 ) generated with the cross- section of (4.58±0.02) nb, assuming that all the tracks are produced within (1.43<|  |<5.27)deg., that is between 0.7 |  | min and 2 |  | max (|  | min, |  | max defining detector acceptance) as optimized in BARBIE. Both s and t channels are included as well as vacuum polarization option and ISR. - Background are 10 5 e + e -  e + e - l + l - (l=e,  ) four-lepton events generated with WHIZARD multiparticle event generator, with the cross-section of (1.56±0.03) nb corresponding to the luminosity of app. 6 pb -1. Only tree level diagrams are included with no assumptions on beams. FCAL Workshop Krakow 12-13 February 2006 Bhabha generator: BHLUMI integrated into Detector simulation: BARBIE V4.1 Background generator: WHIZARD V1.40

5. - Detector response has been PARTIALLY simulated with BARBIE (thank you Bogdan!) in terms of hits at the front plate of the luminosity calorimeter. Generated information has been used on tracks in the LumCal acceptance region, in order to study effect of event selection criteria on signal and background. - We assumed only head-on collisions at 500 GeV cms energy. FCAL Workshop Krakow 12-13 February 2006 Detector simulation - 30 tungsten disks 1X0 thickness each, followed by sensor plane - Inner/outer disk radius 8/28 cm respectively -120 radial strips to measure azimuthal angle and 64 concentric strips to measure polar angle -E LCAL >0.8 E beam (Only for Bhaba)

6. Signal and background have been compared on the basis of well know properties of Bhabha events: acolinearity and acomplanarity and on isolation cuts based on energy balance (Ronen detector performance study) or relative energy (OPAL). FCAL Workshop Krakow 12-13 February 2006 Comparison of signal to background PHYSICS BACKGROUND (reminder) - Primary e + e - pair is emitted along a beam pipe carrying the most of energy. Secondary l + l - pair is low-energetic and distributed over a wider polar angle range. - Due to steep distribution of the produced particles (mainly electrons), the most of energy is to be deposited in the BeamCal, while the LumiCal gets mostly low- energetic particles.

7. FCAL Workshop Krakow 12-13 February 2006 BHABHA BACKGROUND

8. Isolation cuts FCAL Workshop Krakow 12-13 February 2006 1. Acolinearity cut |  |<0.06 deg (10 -3 rad) 2. Acomplanarity cut |  |<5 deg 3. Energy balance cut |E F(R) -E B(L) |<0.1 E min (relative cut) 4. Relative energy cut (OPAL) : (EF+EB)/2Ebeam>0.75 Detector performance cuts

9. FCAL Workshop Krakow 12-13 February 2006 BHABHA BACKGROUND Discriminating variables

10. FCAL Workshop Krakow 12-13 February 2006 BHABHABACKGROUND

11. FCAL Workshop Krakow 12-13 February 2006 Properties of the signal and physics background allow even a looser set of cuts then suggested from the detector performance study, that would still keep B/S ratio at the required order of 10 -4. It is important to note that estimations of efficiency should be taken with care because of the effects to be introduced by detector response (smearing of distributions) and beamstrahlung. However, signal to background ratio is such that the conclusion that physics background çan be suppressed with the Ronen’s set of cuts will hold. B/S: 5·10 -5 B/S: 4·10 -4 ‘1’,2,4 cuts’

12. FCAL Workshop Krakow 12-13 February 2006 x [cm] y [cm]

13. FCAL Workshop Krakow 12-13 February 2006 Assuming that event has been selected if there are at least two tracks accepted to be tracked in the LumiCal, the hit position (x,y) (previous figure) and energy distributions are given before and after Ronen's cut. Total energy (before cut) of hits associated with the selected tracks is of order of 17 MeV/BX for signal and 33 KeV/BX for background. E [GeV]

14. Conclusion - Study of physics background now includes detector simulation BARBIE. Still, discriminating variables (acolinearity, acomplanarity, energy balance or relative energy) are applied on generated tracks. - Due to the characteristic signature of Bhabha events, detector performance set of cuts on these variables can keep the background from four lepton processes within the required 10 -4 of background to signal ratio. FCAL Workshop Krakow 12-13 February 2006

15. PLANS -To refine this study by introducing: - Information on reconstructed tracks available in BARBIE - Beamstrahlung, polarization… - X-angle case - How precise are we in our estimations: - Comparison with other Bhabha generators (SAMBHA) - To check on overlapping Feynmann diagrams in signal and background generators -... FCAL Workshop Krakow 12-13 February 2006