Beam-Beam interaction SIMulation: GUINEA-PIG

1. Beam-Beam interaction SIMulation: GUINEA-PIG CARE 05, Geneva, November 2005 C. Rimbault, LAL Orsay

2. Beam-Beam interaction SIMulation :GUINEA-PIG • 1. Introduction: BBSIM tasks overview • 2. Incoherent e+e- pair background study • BBSIMs predictions: GUINEA-PIG Vs CAIN • Impact of beam parameter sets on microVertex Detector background • 3. In progress: Bhabhas in GUINEA-PIG

3. Beam-Beam SIMulation • When beams collide: • Energy loss in the form of synchrotron radiation: beamstrahlung • Secondary backgrounds • Electromagnetic : e+ + e-→ gg → e+e- … • Hadronic : e+ + e-→ gg → hadrons • Electromagnetic deflections • effect on backgrounds (pairs ...) • effect on luminosity measurements ? (Bhabha scattering) • e+ e- spin depolarisation effects • GUINEA-PIG check-up & benchmarking • Comparison with simulation codes • General Beam-Beam interactions: CAIN • Dedicated codes: BDK, BHLUMI ... • Improvement, adding new options/ phenomena • Management (Web doc, version updating...)

4. Incoherent Pair Creation Processes x-section exact calculation Weizäcker-Williams approximation 1. Breit-Wheeler :gg-> e+e- 2. Bethe-Heitler : ge±-> e± e+e- 3. Landau-Lifshitz : e+e-->e+e- e+e- Equivalent photon spectrum, associated to a virtuality parameter, Q Simulation inputs GuineaPig&Cain: Tracking1, Beam Size Effect2 , Q2max=s/4,me2 BDK: e+e-  e+e-e+e-, smin= 4me2 used as a reference for the LL process Emin= 5 MeV ; Beam parameter set: USSC 500 GeV ; VD: r1=15mm, B=4T (LDC) 1Tracking : Deflection of low energy pairs due to the field of the opposite beam. 2Beam Size Effect: Reduction of cross section due to the position uncertainty for the virtual photons with low Pt.

5. Deflection of the pairs Pt vs q GuineaPig After Deflection Before Deflection Pt > 5 MeV/c mostly due to electromagnetic deflections

6. GuineaPig /CAIN Energy spectra BW BHLL Qualitative agreement between Guinea-Pig and CAIN Phase space Pt vs q

7. Landau-Lifshitz : Comparison with BDK GuineaPig BDK Pt Cain Energy theta No Beam Size Effect, No Deflection Very good agreement forGuinea-PigandBDK

8. e+ + e- production (effective) cross sections E > 5 MeV without & with Beam Size Effect

9. e+ + e- production (effective) cross sections E > 5 MeV without & with Beam Size Effect

10. IPC particles reaching the VD (LDC) Pt vs q Comparison Deflection / No Deflection Pt > 5 MeV & q > 20 mrad VD bkg does not come from magnetic deflection

11. Events reaching the VD effective s(mb) without & with Beam Size Effect

12. Events reaching the VD effective s(mb) Where does the difference between GUINEA-PIG and CAIN comes from ? without & with Beam Size Effect

13. Origin of the difference GuineaPig / CAIN : Q2max Same virtuality limit, same results : agreement between GP & CAIN at low virtuality Is it correct to choose s/4 as the virtuality upper limit ?

14. The photon virtuality spectrum in BDK Qg1 Vs Qg2 - BDK Virtuality spectrum 24% GuineaPig BDK 67% me Nice agreement between GuineaPig & BDK both at low and large virtuality BKD prediction at low virtuality: sprod= 24 mb ; sVD = 12 mb ~CAIN results

15. Impact of beam parameter sets on VD backgroundfor r1 = 15 mm Nominal Low Power Pairs reaching the VD for an inner layer radius of 15 mm and different magnetic fields: 216 mb88 mb 59 mb 39 mb 145 mb 77 mb 50 mb Pt (GeV/c) Pt (GeV/c) for B=2T + + + for B=3T + + for B=4T + for B=5T q (rad) q (rad) Pairs deflection limit for Nominal option: for the Low Power option this limit is higher !  too close to the background inflation region • A careful choice is required for Low Power option: • Incompatibility with Vertex Detector designs at low B and small radius • 2T: not safe with ri = 15 mm

16. Bhabha scattering & electromagnetic deflections e+ e+ e+ e+ + + ... g g e- e- e- e- • Bhabhas are used to measure the luminosity: Lint= NBhabha/sBhabha • Bhabha cross section : • Beam-Beam effectEM deflections •  Modification of the angular distribution ? •  Modification of the theoretical cross section ? •  Would it be possible to estimate Lwith DL/L < 10-4 ?

17. Bhabha scattering & electromagnetic deflections: VeryPrelimary results • 106 Bhabhas produced with BHLUMI, √s=500 GeV, 25≤q ≤90 mrad: • GUINEA-PIG EM deflection treatment (same as for the e+e- pairs) • for the analysis : 30≤q ≤75 mrad, no cut on energy Effect of the deflection on angles (qx1-qx0) (mrad) (qy1-qy0) (mrad) (q1-q0) (mrad)

18. Bhabha scattering & electromagnetic deflections: Verypreliminary results • 106 Bhabhas produced with BHLUMI, √s=500 GeV, 25≤q ≤90 mrad: • GUINEA-PIG EM deflection treatment (same as for the e+e- pairs) • for the analysis : 30≤q ≤75 mrad, no cut on energy Effect of the deflection on angles 0.21e-04 0.14e-04 -0.10e-02 (qy1-qy0)/qy0 (10-2) (qx1-qx0)/qx0 (10-2) (q1-q0)/q0 (10-2)

19. Bhabha scattering & electromagnetic deflections: Verypreliminary results NBh bef. def. = 41815 NBh added by def. = 14 NBh suppressed by def. = 206 q1(mrad) q0before deflection q1after deflection q0(mrad) Difference between theoretical and “real” nb of Bhabhas : -0.46%

20. Summary • Incoherent e+e- pairs • GUINEA-PIG modelisation is more convenient than CAIN: • me is a too small limit for the photon virtuality. • Be careful with pair accumulation region after deflection : • LowPower  constraints on VD design (B, radius, readout) • 2T detector concept needs a VD inner radius >20 mm • reference: EUROTeV-Report-2005-016-1 • First study of EM Deflection effect on Bhabha scattering: • Effect > 10 × 10-4 •  Necessity to know well the impact of EM deflection •  Tool will be provided (BHLUMI + GUINEA-PIG)