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Current estimation of the background level

This report provides an overview of the current estimation of the background level for the E391a collaboration, including the motivation, data analysis, and conclusions. The study focuses on the detection of extra photons with high efficiency and reducing neutron interaction.

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Current estimation of the background level

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  1. Current estimation of the background level Ken Sakashita ( Osaka University ) for the E391a collaboration • Overview/Motivation • 4 g data analysis • 2 g data analysis • Conclusion

  2. Overview • KL p0nn decay (BRSM ~ 3x10-11) • 2g + nothing • Require high Pt of p0 • Expected Background • KL 2p0 decay (BR ~ 10-3) , 2g missing • Neutron interaction ( n+(material)  p0 + …) • 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 g vetoes • Use 4 g data • Background(KL3p0, 2g missing) + Signal(KL2p0) • photon rejection by each vetoes • Study of 2 g data • Study of the background by using 1-day data sample • Understand events lies in the outside region

  4. Motivation • KL2p0 • KLp0nn g CsI calorimeter g g g KL KL g g MB photon veto • KL3p0 , 2g missing • KL2p0 , 2g missing g g g g g g KL KL g g g g

  5. 4 g analysis • Reconstruction of 4 g data • # of reconstructed g-cluster : exactly equal to 4 (E >= 13MeV) • No other cluster ( thr. = 5 MeV) • p0 reconstruction (assume Mp0 vertex Z) • Best chi2 • Kinematics cut • Chi2 < 2 • Pt2 < 1x10-3 (GeV/c)2 • Beam size < 5cm g1 g2 z1 g3 z2 2p0 momentum Pt g4 beam direction

  6. 4 g analysis • M4g plot after kinematics cut(w/o photon veto) Data M4g < MKL Bkg(KL3p0) M4g ~ MKL Signal(KL2p0) Estimation S/N - fitting w/ G+p1 Data

  7. Study of g vetoes • Performance of MainBarrel photon veto • Simple cut (Edep < Threshold), no timing information S/N R(Nsig) Factor ~3 improvement Acceptance loss (no timing info.)

  8. 4 g analysis • Apply MB veto + other photon vetoes • Final M4g plot Data Data • KL3p0 Contamination ~ 4.7%

  9. g g Consideration on 4 g data • Remaining Background • KL 3p0 + Fusion • Result of the Monte Carlo simulation of KL  3p0 • Performance of the photon vetoes • M4g plot shows current veto performance • However, we need more detail study to estimate the background level • Study of fusion also necessary Data KL3p0 MC before vetoes after vetoes

  10. 2 g analysis • Apply the photon vetoes • Eg > 200MeV • Zvtx distribution, Acoplanarity angle distribution Q g1 Beam direction g2

  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 CsI CV n CsI Hadronic shower  2 clusters n 1g/cm3 0.2mm-thick

  12. data data Neutron Monte Carlo • 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 Core n MC (in right plot : w/ factor 0.22) Data/MC Halo n MC

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

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

  15. 2 g analysis • Background study (MonteCarlo simulation) • High statistics for KL2p0 and KLgg decays • Small statistics for other decays and neutron MC • Core neutron’s events located in the z=550cm

  16. Summary • Study of 4 g data • Good performance of the photon vetoes • Need more detail study of the photon vetoes • Use timing information / Estimate accidental loss • KL3p0 contamination ~ 4.7% • In order to estimate the background level, we need more detail study of 4g data • Current status of 2 g 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/g • We need more statistics of Monte Carlo simulation

  17. Backup slides

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

  19. 2 g analysis • g selection (Data) • Distance between 2 clusters • Rg Data D > 40 cm Data Rg > 0.9

  20. Neutron Monte Carlo • Momentum distribution from GEANT simulation C4 C6 C5 C2 C3 C1 Pb abs. Core neutron Halo neutron

  21. 4 g data 4 g analysis • Best chi2 • Kinematics • Pt2, Beam size chi2 < 2 Rbeam < 5 cm Pt2<1 x 10-3

  22. Study g vetoes(4g data) • Other vetoes • Typically 1 MeV Edep cut (w/o timing information) R(Nsig) S/N

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