1. Status of E391a G.Y.Lim IPNS, KEK

2. K L    decay Experimental difficulties  Tiny branching fraction Great number of K L decays  Weak Kinematical constaints Background suppression  From K L decays  K L       Other modes   Beam related events    production  Accidentals  What would be a main obstacle ?  How to estimate backgroud level? KTeV, Phys. Lett. B447(1999) Hyperon decays – lower momentum ? Multi-   events – tighter vetoing ? Neutron events – removing sources ?

3. KEK-PS E391a The first dedicate experiment  To confirm the methodology Large acceptance  Single   (     ) detection Background rejection  Tight vetoing  High P T selection  E391a concepts Pencil beam Hermetic veto system Double decay chamber Highly evacuated decay region Step-by-step approach J-PARC

4. E391a collaboration Joint Institute for Nuclear Research (Dubna), Russia High Energy Accelerator Research Organization, KEK, Japan Department of Physics, Kyoto University, Japan National Defense Academy of Japan, Japan Department of Physics, National Taiwan University, Taiwan Department of Physics, Osaka University, Japan Department of Physics, Pusan National University, Korea Research Center for Nuclear Physics, Osaka University, Japan Faculty of Science and Engineering, Saga University, Japan Department of Physics, University of Chicago, USA Department of Physics, Yamagata University, Japan More than 50 collaborators from 11 institutes from 5 countries

5. Milestones of the E391a Dec.1996: conditionally approved Mar.1999: constructed the beam line July 2001: approved Oct. 2002: engineering run Jan. 2004: finish detector assembling Feb. –June 2004: Data taking Feb. –Mar. 2005 : Run-II Fall 2005 : Run-III (conditionally approved)

6. Detection Principle K L     Nothing  pure CsI calorimeter 4  veto system Clear single  0 with high P T

7. Related to the K L decays K L       Main possible background source  Br(K L   0 )/Br(K L   0  0 )~10 -8  High detection efficiency Hermetic veto system Double decay chamber Low detection threshold High P T selection with pencil beam  Reject dominant multi-  events High energy  -missing Rejection of odd pairing Making a correct inefficiency table for  -detection

8. Related to the Beam Hyperon  decays     n    Short lifetime/low momentum  Length of beam line  o production  n + A   o + n + A  With detector components Clean beam  With Residual gas Evacuated decay region 0.1 S.M. event @ 10 -5 pa SourceEvent#/E391a-Sensitivity Λ at target0.0044 Λ at C6< 0.11 Λ by n with detector<0.002 π by n with residual gas 0.0005 ～ 0.0062 π by n with detector0.014 ～ 0.114 at CC-04 total<0.3

9. Pencil Beam 5 stages of collimators made of heavy metal (tungsten) 2 stages of sweeping magnets Thermal neutron absorber Lead/Be plug for controls gamma/neutron flux Fine alignment using telescope GEANT M.C. agree well to the measurements

10. E391a detector setup K L beam Recycled 576 Un-doped CsI (70X70X300 mm 3 (from E162) 50X50X500 mm 3 (from KTeV)) CC03 (Tungsten + Scin.) Covered by plastic scintillators (Charged Veto (CV))

11. Assembled three parts Detector Integration was finished on Jan 22, 2004

12. Detector inside vacuum To reject  o production by neutron interaction with residual gas Differential pumping technique 80 μm LDPE 15 μm EVAL (aluminized) 15 μm nylon 80 μm LDPE

13. Data taking Run time (physics run)  Run-I : Feb. 16 th – Jul. 1 st 2004: (~60days) KEK 12 GeV PS : Incident protons  2.2 X 10 12 /spill at target  2-sec spill length & 4-sec repetition K L flux in front of detector  5x10 5 /spill  Peak momentum : ~2 GeV/c Trigger : No. of cluster >=2 Energy threshold for a cluster : 60 MeV. DAQ live-time ratio : 75 % Vacuum : ~10 -5 Pa Online event display

14. Calibration Energy & timing  Cosmic-ray muon.  CsI, barrels  Punch-through muon.  Collar counters   0 production at Al target.  Precise calibration of CsI Muon Data (KEK-preprint-2004-85, accepted for publication in NIM.)  0 production at target

15. Normalization channels (without tight vetoing) Invariant Mass of 4  (GeV/c 2 ) Reconstructed vertex of K L (cm) Invariant Mass of 6  (GeV/c 2 ) KL  KL   KL KL  K L   6  events 4  events2  events Vertex-finding cut Vertex-finding cut + fusion-cut P T +decay-position cut Monte-Carlo simulation well reproduces data. Pure kaon sample.  Veto counter study

16. 2  analysis Data without tight veto Reconstructed vertex (cm) P T (GeV/c) M.C. for K L decays ( Without Normalization) K L   K L        K L       K L       o produced at CC02  o produced at CV (?)

17. MC ~material in front of CV?~ CH 2, ~1g/cm 3  10 g/cm 3 & 2 mm-thick P T (GeV/c) Reconstructed vertex (cm)  Well reproduces data distributions.

18. Remove neutron events Kinematical constraints for two gammas 1) Distance between two gammas 2) Energy balance of two gammas Unexpected acceptance loss

19. Veto Optimization ~Main-barrel timing (low E sample)~ K L   pure sample  B.G.sample upstream downstream Backsplash  should NOT veto! ① Real photon hit  should veto. ② Backsplash  should NOT veto. ① ② earlylate

20. Results of 1-Day analysis Intensive study using part of data obtained during 1-day (2 %) B.G. events can be controlled Acceptance loss - Neutron related events - Tight photon vetoing More statistics – 1 week analysis  -  o production at the detector  - To study fiducial region event  Clearer beam condition(Run-II) S.E.S ~ @ 1-day statistics

21. 1-Week Analysis Lager size of data sample  Factor of 5 Deeper understanding about the background events  Another sources ?  Access to the K L decays Detailed M.C. study for veto counters  Pure M.C. + accidental overlay  Reproduce the low energy distribution Finer veto counter tuning Preliminary results will be reported at the KAON 2005 Energy distribution of veto counters

22. Run-II To fix the dropped membrane Install additional collimator in front of detector Minor up-dates of detector systems Apply the Be absorber (better K L /n ratio) Finer tuned DAQ / beam condition Data taking during 40 days from Feb. 2005 Beam-line endpoint Detector upstream section CC00 30cm-long x 40φ (20mm-W / 5mm-Scinti.)

23. Membrane Correction

24. Better quality of data (online plots) Reconstructed vertex (cm) P T (GeV/c) Run-IRun-II Run-II analysisRun-III on this fall ( Conditionally approved )

25. Run-I Run-II PTPT Vertex EE EE (GeV/c) (GeV) (cm) Better quality of data

26. Summary KEK-PS E391a – The first dedicated experiment  To get a guideline for the precise measurement  Lots of list to study (hoping to various suggestions) Successful data taking  To realize a trial of one method  We are on right track Too early to declare To find problem  To fix it  1-day analysis Close to the KTeV limit Feed-back to the Run-II  1-Week analysis Preliminary results at the KAON2005 Better data quality at Run-II Run-III in coming fall

27. Acceptance for K L    Acceptance loss due to kinematical  M.C. Acceptance loss due to veto  Pure Kaon samples  Decay probability is included CutsAcceptance Decay + geometry + skimming5.49x10 -3 E  > 200MeV3.55x10 -3 Distance between 2-  ‘s >50 cm 2.77x10 -3 0.92 < E 1 +E 2 +E 3 /Ecluster <0.981.71x10 -3 0.6< Emax/Ecluster < 0.929.46x10 -4 Energy balance betw. 2-  ‘s< 0.5 7.56x10 -4 300 < Z < 500 cm 0.12< P T < 0.25 GeV/c 3.20x10 -4 Vetos1.21x10 -4

28. Prospect ~Run-II~ Membrane was fixed. Additional collar counter was installed.  to reduce halo-neutron effects. Be absorber was installed in the beam line.  better n/  ratio, less halo-neutron. CC00 Membrane at Endcap W/Scintillator Sandwich(2.5 I ) in front of detector. We are working hard on (and enjoying) this challenging experiment.

29. DAQ and electronics ~1000 channels in total  ADC: 0.05 pC/ch(low gain) 0.4pC/ch(high gain)  TDC: 0.05 ns/ch  MTDC: 0.5 ns/ch up to 16 hit/event Environmental monitors  Temperature  Vacuum  Beam conditions Amp/Disc module  Analog signal -> ADC  Logical signal -> TDC stop  Sum 8 channels -> trigger

30. Kaon Reconstruction (K L  3  0,2  0,  ) No. of reconstructed  -cluster : equal to 2,4,6 (E >= 50MeV) No other cluster ( E < 20 MeV) in CsI  0 reconstruction (assuming  0 -mass  vertex Z) Best chi2 Kinematical cuts - Chi2 < 2 - 2nd Chi2 >20 - Pt < 10(MeV/c) - Beam size< 4cm 11 22 33 44 z1z1 z2z2 beam direction Pt Example : K L  2  0