1. June 2000K_L Reconstruction1 B  J/  K L Reconstruction Beyond 10 34 e + e - Workshop The Homestead Glen Arbor, MI Daniel Marlow BELLE Princeton University June 17, 2000

2. June 2000K_L Reconstruction2 Physics Motivation The CP asymmetry changes sign when one goes from to.Depending on one’s level of faith in the theory, this provides a check, or enhances statistics over the gold-plated mode.

3. June 2000K_L Reconstruction3... Physics Motivation Like is theoretically very clean. Since the K L detection efficiency ( ) is about the same as one can expect about the same detection efficiency for the two modes. The main problem is background (see below). Thus is a “silver-plated” mode.

4. June 2000K_L Reconstruction4 Detection Strategy: Two-Body Kinematics B rest frame  CM Frame Lab Frame Boost The reaction is uniquely identified through measurement of the J/  momentum and the K L direction.

5. June 2000K_L Reconstruction5... Detection Strategy The BELLE flux return incorporates 15 layers of resistive plate chambers (RPCs) that are used to detect K L ’s interacting in the iron.

6. June 2000K_L Reconstruction6... Detection Strategy Require: #RPC layers > 1.OR. #RPC layers > 0.AND. ECsI >0.16 GeV K L hits KLKL

7. June 2000K_L Reconstruction7 Charged Track Veto The reactions and are prominent sources of background. We veto all events with

8. June 2000K_L Reconstruction8 Charged Track Veto As the veto cone widens, the efficiency goes down, but the signal to noise improves.

9. June 2000K_L Reconstruction9 Charged Veto Optimize the figure of merit

10. June 2000K_L Reconstruction10 Kinematic “Fit” Of course this is not really a fit since the system is exactly constrained. We cam, however, determine the B momentum in the CM (Y(4S) frame).

11. June 2000K_L Reconstruction11 Spectrum We fit by allowing the signal and background yields to vary independently. The yield is about ~30% lower than what we expect, a topic under study.

12. June 2000K_L Reconstruction12 Spectra We estimate the non-J/  background by rotating all K L clusters by 180 o in azimuth. These backgrounds are small.

13. June 2000K_L Reconstruction13 Beam Constrained Mass

14. June 2000K_L Reconstruction14 Background Sources Much of the background comes from decays, but other channels also contribute. As noted, non- charmonium backgrounds look to be small.

15. June 2000K_L Reconstruction15 Summary and Conclusions We have observed Most of the background involves real K L ’s produced in inclusive charmonium B decays. The non-trivial background levels will make for an interesting challenge when it comes to extracting CP asymmetries... Stay tuned.