Pair Backgrounds and Electron ID for Two-photon Veto

1. Workshop on Forward Region Instrumentation August 26-27, 2004 Pair Backgrounds and Electron ID for Two-photon Veto Takashi Maruyama SLAC

2. Motivation • Beam-beam Pairs in Very Forward Region Measure beam parameters – Pair monitor Measure luminosity – LUMON Detect high energy electrons – BeamCAL •  Veto Capability in BeamCAL.

3. 0.5 0.4 113mrad 0.3 Inst. Mask W 0.2 W 0.1 Pair-LuMon 46mrad QD0 0 LowZ Mask BeamPipe Exit radius 2cm @ 3.5m -0.1 W W -0.2 Support Tube -0.3 ECAL -0.4 HCAL YOKE -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 SiD Forward Masking, Calorimetry & Tracking 2003-06-01 5 Tesla

4. e+ e- Pairs from e+ e- Collisions Pair Energy - NLC <E> = 4.1 GeV 0.01 0.1 1 10 100 Energy (GeV) <L>Tesla = 1.6  <L>NLC

5. Pair Distribution Z = -315 cm Z = +315 cm 5 Tesla e+ 8 cm ≈ 25 mrad 20 mr crossing angle e- Y (cm) Head-on X (cm)

6. Crossing Angle and Serpentine field GLC 7 mrad NLC 20 mrad NLC 20 mrad + Serpentine Y (cm) x (cm)  Pairs spread more in NLC than in GLC. But the smaller crossing angle has other problems: QD design, parasitic beam-beam interference.  Pairs spread more with Serpentine field.

7. Total Pair Energy in BeamCAL vs. Beampipe Radius Tesla energy is scaled down by 1.6 (<L> ratio). NLC R=2.0 cm ≈ Tesla R=1.5 cm NLC Serpentine field adds 50% more energy. GLC energy ≈ ½ NLC energy

8. Pair Energy and RMS R and  dependent Energy / 5 mm x 5 mm RMS Energy / 5 mm x 5 mm Pair energy is much higher than 250 GeV. → 250 GeV electron detection requires pair background subtraction.

9. High Energy Electron Detection in BeamCAL • Beampipe radius: IN 1 cm, OUT 2 cm • Detector: 50 layers of 0.2 cm W + 0.03 cm Si Zeuthen R- segmentation LUMON BeamCAL •Generate 330 bunches of pair backgrounds. •Pick 10 BX randomly and calculate average BG in each cell, <E>background •Pick one BX background and generate one high energy electron. •EBG + Eelectron - <E>background, in each cell •Apply electron finder. OUT IN 11 cm

10. High Energy Electron Detection 250 GeV Electron Pair Background BG 250 GeV e- Deposited Energy (arb. Units) Ebg + Eelectron - <Ebg> Si Layers

11. Electron finder 2 distribution N. Graf •  Use first several layers as shield. • Use towers past layer 10 as seeds for a fixed-cone algorithm to cluster cells. • - physical size of shower • doesn’t change • - simplifies geometry handling • - single pass through the data • Cuts on cluster width and longitudinal shower 2. 250 GeV e- Background Cut at 450.

12. Electron detection effciency N. Graf The detection efficiency remains high with increasing numbers of bunches. Fake rate (all cluster energies): ~5% increses to ~20% at 2 crossings. Fakes are concentrated in hot spots, not uniform in phi. Reduce fake rate with cuts on fiducial region or energy, depending on analysis. Expect rejection to improve with further study.

13. Two Photon Veto in smuon Search For M > 8 GeV, Pt > 6 GeV is >90% efficient. No effect from pair background, crossing angle. For 2 < M < 8 GeV, Pair background, incoming beam hole affect veto efficiency. Veto efficiency in head-on collision is higher. g*g*→m+m- Before veto After veto DM = 5 GeV DM = 8 GeV Smuon signal (arb. scaled) Incoming beam hole Pt (di-muon) (GeV)

14. Pairs Timing Pairs arrival time Energy deposition time in Si layers E > 100 MeV Time (ns) E < 100 MeV Si Layer Time (ns) Time (ns) • 90% of E >100 MeV pairs arrive within 10 ps. • Low energy pairs are slower, but their contribution to BeamCAL signal is negligible.. • Shower development in BeamCAL is < 100 ps. Pair Energy (GeV)

15. Summary •  veto capability is important for SUSY dark matter scenarios. • High energy electrons can be detected in the pair background. • For 2 < M < 8 GeV, the veto efficiency is affected by the crossing angle. • Shower development in BeamCAL is fast (< 100 ps).