1. Current estimation of the background level Ken Sakashita ( Osaka University ) for the E391a collaboration 1.Overview/Motivation 2.4  data analysis 3.2  data analysis 4.Conclusion

2. Overview K L   0 decay (BR SM ~ 3x10 -11 ) –2  + nothing –Require high Pt of  0 Expected Background –K L  2  0 decay ( BR ~ 10 -3 ), 2  missing –Neutron interaction ( n+(material)   0 + …) Key points –How to detect extra photon with high effeciency Hermetic photon veto –How to reduce neutron interaction High vacuum system (10 -5 Pa) Clean pencil beam

3. Motivation Study of the performance of the  vetoes –Use 4  data –Background(K L  3  0, 2  missing) + Signal(K L  2  0 ) –photon rejection by each vetoes Study of 2  data –Study of the background by using 1-day data sample –Understand events lies in the outside region

4. Motivation   KLKL MB photon veto   KLKL     KLKL     KLKL     A)K L   0 B)K L  2  0, 2  missing C)K L  2  0 D)K L  3  0, 2  missing CsI calorimeter

5. 4  analysis Reconstruction of 4  data –# of reconstructed  -cluster : exactly equal to 4 (E >= 13MeV) –No other cluster ( thr. = 5 MeV) –  0 reconstruction (assume M  0  vertex Z) –Best chi2 –Kinematics cut Chi2 < 2 Pt 2 < 1x10 -3 (GeV/c) 2 Beam size < 5cm 11 22 33 44 z1z1 z2z2 2  0 momentum beam direction Pt

6. 4  analysis M 4  plot after kinematics cut(w/o photon veto) M 4  ~ M KL Signal(K L  2  0 ) M 4  < M KL Bkg(K L  3  0 ) Estimation S/N - fitting w/ G+p1 Data

7. Study of  vetoes Performance of MainBarrel photon veto –Simple cut (E dep < Threshold), no timing information R(N sig ) S/N Acceptance loss (no timing info.) Factor ~3 improvement

8. 4  analysis Apply MB veto + other photon vetoes Final M 4  plot –K L  3  0 Contamination ~ 4.7% Data

9. Consideration on 4  data Remaining Background –K L  3  0 + Fusion –Result of the Monte Carlo simulation of K L  3  0 Performance of the photon vetoes –M 4  plot shows current veto performance –However, we need more detail study to estimate the background level –Study of fusion also necessary Data K L  3  0 MC before vetoes after vetoes  

10. 2  analysis Apply the photon vetoes E  > 200MeV Zvtx distribution, Acoplanarity angle distribution Beam direction 11 22 

11. Neutron Monte Carlo Assume a core neutron hits a material at the Zvtx = 550 cm –Membrane in front of the Charged veto counter Halo neutron –Single neutron hits CsI and make 2 associated cluster 1g/cm3 0.2mm-thick n n CsI Hadronic shower  2 clusters CsI CV

12. Neutron MC w/ lose vetoes –Halo n MC well agree with data –Assume the factor 0.22 (material), core n MC also agree with data Halo n MC Core n MC (in right plot : w/ factor 0.22) data Data/MC Neutron Monte Carlo

13. 2  analysis  selection –A kind of cluster shape cut ( ) –Distance between 2  -clusters Signal MC Halo n MC Signal MC R  > 0.9 D > 40 cm

14. 2  analysis applying photon vetoes  selection w/o acoplanarity angle cut

15. 2  analysis Background study (MonteCarlo simulation) High statistics for K L  2  0 and K L   decays Small statistics for other decays and neutron MC Core neutron’s events located in the z=550cm

16. Summary Study of 4  data –Good performance of the photon vetoes –Need more detail study of the photon vetoes Use timing information / Estimate accidental loss –K L  3  0 contamination ~ 4.7% –In order to estimate the background level, we need more detail study of 4  data Current status of 2  analysis –Events in the side band region Single halo neutron hits in the CsI  2 associated cluster Core neutron makes events at the Zvtx=550 cm –We need shape study in order to separate n/  –We need more statistics of Monte Carlo simulation

17. Backup slides

18. Vacuum system Differential pumping system Region-1 : < 0.1Pa Region-2 : ~ 10 -5 Pa Separated by thin membrane

19. 2  analysis  selection (Data) –Distance between 2 clusters –R  Data R  > 0.9 D > 40 cm Data

20. Neutron Monte Carlo Momentum distribution from GEANT simulation Core neutronHalo neutron Pb abs. C1 C2C3 C4 C5 C6

21. 4  analysis Best chi2 Kinematics –Pt 2, Beam size chi2 < 2 R beam < 5 cm Pt 2 <1 x 10 -3 4  data

22. Study  vetoes(4  data) Other vetoes –Typically 1 MeV E dep cut (w/o timing information) R(N sig )S/N