1. FCAL Workshop Munich -17 October 2006FCAL Workshop Munchen -17 October 2006 Four-fermion processes as a background in the luminosity calorimeter M.Pandurović I. Božović-Jelisavčić M.Pandurović I. Božović-Jelisavčić “Vinča“ Institute of Nuclear Sciences “Vinča“ Institute of Nuclear Sciences Belgrade, Serbia Belgrade, Serbia

2. FCAL Workshop Munich -17 October 2006 Reminder about physics background properties -Four-fermion processes, e + e -  e + e - ff bar via NC are dominated by two-photon exchange, and they are the main source of physics background for luminosity measurement -Outgoing e + e - pairs are emitted along the beam pipe carrying the most of energy, while low energetic ff bar pairs are distributed over a wider polar angle range -Total cross section of e + e -  e + e - ff bar is decreasing with sqrt(s) E [GeV] ——— electrons ——— muons Ecm [GeV] cross setion [nb] leptons hadrons e+e-  e+e - μ + μ -

3. FCAL Workshop Munich -17 October 2006 Simulation tools Signal consists of 10 5 Bhabha events generated with the cross-section of (4.58±0.02) nb Background “Leptonic” background are 10 6 e + e -  e + e - l + l - (l=e,μ) four-lepton events, with the cross- section of (1.54±0.03) nb “Hadronic” background are 10 5 e + e -  e + e - qq bar (q=u,d,s,c,b) events, with the cross-section of (1.14  0.02) nb Simulation is done for 500 GeV centre-of-mass energy Bhabha generator: BHLUMI integrated into Detector simulation: BARBIE V4.1 Background generator: WHIZARD V1.40

4. FCAL Workshop Munich -17 October 2006 - Background track rate ~ 10 -3 is in agreement with the independent study of two-photon production of four-electrons using Vermasseren event generator (L.Suszycki, FCAL Workshop, Tel Aviv, September 2005) BhabhaBackground Track rates Bhabha ~ 8. x 10 -3 tracks/BX “Leptonic” background ~ 1. x 10 -3 tracks/BX “Hadronic” background ~ 2. x 10 -3 tracks/BX in luminosity calorimeter } 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7  Hadrons —— Leptons 10 -3

5. FCAL Workshop Munich -17 October 2006 Assumptions - Ideal reconstruction: We are taking generated information on tracks that are, according to Geant based tracking routine, associated with direct or indirect hits in the Lumical. Effect of magnetic field is included. - Background generation: - Invariant mass of outgoing e + e -, ffbar pair is larger than 1 GeV/c 2 - All outgoing particles are generated in the polar angle range (0.05 deg. - 179.95) deg. - Minimal Q transferred by photon exchange is no less than 1 GeV/c - Only tree level diagrams - Signal |θ|<5.27 - All tracks are produced within (1.43<|θ|<5.27)deg. - Both s and t channels are included as well as vacuum polarization option and initial state radiation

6. FCAL Workshop Munich -17 October 2006 Isolation cuts Isolation cuts are applied on any pair of tracks that entered opposite sides of the Lumical. The cuts are based on well known topology of Bhabha events: - Acolinearity cut |Δθ|< θ cut (θ cut = 0.06, 0.1,0.2, 0.5) deg -Acomplanarity cut |Δφ|<5 deg -Relative energy cut (E F +E B )/2E beam >0.75 -Energy balance cut |E F(R) -E B(L)  |<0.1 E min (relative cut) E min =min(E F(R) -E B(L) )

7. FCAL Workshop Munich -17 October 2006 Bhabha Background 10 -3 10 -4 10 -5 10 -6 — Hadrons — Leptons 10 -5 — Hadrons — Leptons 10 -5 10 -3 10 -4 10 -5 10 -6 10 -7

8. FCAL Workshop Munich -17 October 2006 Bhabha Background 10 -3 10 -4 10 -5 10 -6  10 -3 10 -4 10 -5 10 -6 10 -7 — Hadrons — Leptons 10 -5  — Hadrons — Leptons 10 -5

9. FCAL Workshop Munich -17 October 2006 Bhabha Leptonic background Hadronic bacground Signal efficiency Rejection rate |Δ θ |<0.06 deg 81.87 %95.20 %95.27 % |Δφ|<5 deg 97.96 %89.53 %90.42 % Ebal<0.1 E min 90.61 %94.58 %95.45 % total 80.6 % 99.3899.78 ‘Detector performance’ cuts

10. FCAL Workshop Munich -17 October 2006 |Δ θ |< Signal efficiency B/S 0.594.67 %8.6 x 10 -4 0.2 89.71 % 6.2 x 10 -4 0.1 84.81 % 4.0 x 10 -4 0.06 80.78 % 2.6 x 10 -4 0.06 * 80.57 %1.3 x 10 -4 Efficiency vs. B/S ratio * Here we applied Ebalance cut instead of Erelative cut A combination of |Δφ|<5 deg, |Δθ| (varied), Erelative /Ebalance cut

11. FCAL Workshop Munich -17 October 2006 Illustration of the effect of cuts Event has been selected if there is at least a pair of tracks at the opposite side of LumiCal. Projection of hits at the front plane of the Lumical is illustrated –before (left) and after different set of cuts (right): Hadronic background Leptonic background 10 -4 10 -5 10 -4 10 -5 10 -6 |Δφ|<5 deg, |Δθ|<0.5, Erelative cut |Δφ|<5 deg, |Δθ|<0.06, Ebalance before

12. FCAL Workshop Munich -17 October 2006 Conclusion - Due to the characteristic topology of Bhabha events, it is possible to keep the background from e + e -  e + e - ffbar processes within the required 10 -4 of background to signal ratio, with the loss of efficiency of app. 15 % - Though we initially have app. two times more “hadronic” background than one from purely leptonic processes, it is possible to reject it efficiently with the suggested set of cuts. - The most important outcome of this study of physics background is that, in principal, it is possible to separate signal from background to the required level of 10 -4. The exact set of cuts is to be finally defined once more realistic simulation - full track reconstruction is available. Then it would make sense to eventually look into other finer effects like overlapping diagrams in event generators, etc. - It could be worthwhile, to prepare an ILC note as a ‘state of art’ document that would, apart from this study, also include part on detector simulation (B. Pavlik) and other similar studies (L. Suszycki, S.Kananov).