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Status of KIMS

Status of KIMS. S.S. Myung (SNU) For the KIMS Collaboration. Results from 4 crystal array Status of 12 crystal array. KIMS Collaboration. H.C.Bhang, J.H.Choi, D.W.Kim, S.C.Kim, S.K.Kim, J.H.Lee,H .S.Lee, S.E.Lee, J. Lee, S.S.Myung Seoul National University U.G.Kang, Y.D.Kim, J.I. Lee

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Status of KIMS

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  1. Status of KIMS S.S. Myung (SNU) For the KIMS Collaboration • Results from 4 crystal array • Status of 12 crystal array

  2. KIMS Collaboration H.C.Bhang, J.H.Choi, D.W.Kim, S.C.Kim, S.K.Kim, J.H.Lee,H .S.Lee, S.E.Lee, J. Lee, S.S.Myung Seoul National University U.G.Kang, Y.D.Kim, J.I. Lee Sejong University H.J.Kim, J.H.So, S.C.Yang Kyungpook National University M.J.Hwang, Y.J.Kwon Yonsei University I.S.Hahn Ewha Womans University Y.H.Kim, K.B.Lee, M. Lee KRISS J.Li Institute of High Energy Physics D. He, X.Li, Q.Yue Tsinghua University

  3. Composition of Universe Mostly Dark matter, Dark energy Known ~ 4% property of neutrinos not known very well

  4. Galactic Dark Matter WIMP Cs I Recoiled nucleus rotation velocity curve Direct detection of galactic dark matter Recoil energy ~ 10’s keV Event rate < 1/kg/day Neutralinos the strongest WIMP candidate because they hardly interact and are stable Density of dark matter around the sun ~ 0.3 GeV /cm3 ( ~ 5 x 10-28 kg/cm3)

  5. KIMS WIMP detection techniques Energy lost by recoiled nucleus converts to phonons, photons, ion pairs Photon(light) Phonon(heat) NaI(Tl), CsI(Tl), Xe W W Cryogenic detector Ge, Si _ + + _ _ 2 phase Xe + N Ionization HPGe detector

  6. Yangyang Underground Laboratory(Y2L) 3.5 hours by car

  7. KIMS Yangyang Underground Laboratory Korea Middleland Power Co. Yangyang Pumped Storage Power Plant Vertical Depth ~700m ~2 km

  8. OFHC Cu 10cm : 3t Detector and Shielding

  9. Why CsI(Tl)? • Sensitive to both SD and SI WIMP interactions • Ge, Xe not sensitive to SD proton coupling •  complimentary to CDMS, XENON-10 •  direct check of DAMA signal by I-127 recoil • Easy to get large mass with an affordable cost •  annual modulation study • High light yield ~60,000/MeV • Pulse shape discrimination •  better gamma rejection than NaI(Tl) • Easy fabrication and handling

  10. Detector • CsI(Tl) Crystal 8x8x30 cm3 (8.7 kg) • 3” PMT (9269QA) : Quartz window, RbCs photo cathode (Green enhanced) • ~5 Photo-electron/keV Total charge spectrum for 59.5 keV Am241 Resolution @ 59.5 keV : 7.3%

  11. Internal background Radioisotopes in the crystal • 137Cs: 10 mBq/kg, • 0.35 cpd/mBq/kg @ 10 keV • 134Cs: 20 mBq/kg • 0.07 cpd/mBq • 0.005 cpd/mBq • 87Rb : 10 ppb • 1.07 cpd/ppb Geant Simulation Cs-137 reduction – use ultra pure water in powder prodcution; ~1.7 mBq/kg Rb reduction - recrystalization method ; < 1ppb Latest crystals are from ~2 cpd level powder

  12. Other Detector Components • Muon Detector • 4 coverage muon detector : 28 channels • Liquid Scintillator(5%) + Mineral Oil (95%) = 7 ton • Measured Muon flux =2.7 x 10 –7 /cm2/s • Position resolution : σx, ~ 8 cm • Reconstructed muon tracks with hit information • Muon veto efficiency ~99.9% • Neutron detector • 1 ~ 1.2 liter BC501A liquid scintillator x 3 • n/gamma separation using PSD • E_vis > 300 keV • Measured neutron flux (outside shield) •  8 x 10 –7 /cm2/s ( 1.5 < E neutron < 6 MeV )

  13. Study of Muon induced neutrons • Coincidence between Muon and Neutron detectors • n/gamma separation using PSD • Neutron energy: 0.4MeV<E<2.75MeV • Measured : (3.8±0.7)10-2 counts/day/liter • GEANT4 : (2.0±0.2) 10-2 counts/day/liter

  14. Neutron calibration 300 mCi Am/Be source  neutron rate 7 x 105 neutrons /sec  a few 100 neutrons/sec hit 3cm  3cm  3cm crystal ( sample )  Quenching factor of Recoil Energy NR mean time distribution Tag γ(4.4MeV) to measure TOF and energy of neutrons Neutron sample Gamma Full size Gamma sample <t>=(S Ai ti)/S Ai Note: their shapes are Well matching

  15. Data analysis for Wimp Search Data used for this analysis

  16. Data analysis for Wimp Search Calibration and control data samples Neutron ~ 500 kg days (at 4~6 keV) Gamma (using 137Cs) ~ 1100 kg days (0501A), 1650 kg days(0501B) 910 kg days (0510A), 840 kg days(0510B) PMT only ~350 kg days for each crystal with the PMTS used for each crystal clear box PMT PMT

  17. PMT noise Reduction of PMT noise log(τf) Decay time fit result for one event LL2-LL1 Gamma Neutron PMT only(dot-dashed) WIMP search DATA

  18. remove multi-crystal events 0.7 cpd bkg reduction Analysis Method: Event selection 3-5 keV Asymmetry Cut Gamma PMT background WIMP search DATA 5-7 keV

  19. Efficiency Compton events Neutron

  20. Event rates after cuts WIMP search data with cut S0501A S0501B B0510A B0510B Efficiency Corrected

  21. NR event rate estimation n g Electron recoil Nuclear recoil Best fit • Fit the WIMP search DATA with PDF function from gamma and neutron calibration data •  extract NR events rate • Modeling of Calibration data with asymmetric gaussian function

  22. NR Event rates Extracted NR S0501A S0501B B0510A B0510B

  23. Geant4 expectation 90% CL limit Total Rate: Extracted NR is fitted with simulated WIMP signal for each WIMP mass

  24. Cross-section upper limits Spin-Independent SD, Wimp-Proton PRL 99, 091301 (2007) ρD=0.3 GeV/c2/cm3 v0=220km/s, vesc=650km/s Systematic uncertainty ~15% Nuclear recoil of 127I of DAMA signal region is ruled out unambiguiously

  25. 12 crystal array published PLB(2006) Pilot run published PRL(2007) Engineering run Total crystals 8.7 kg x 12 = 104.4 kg started data taking background level 2~4 cpd/keV/kg (preliminary)

  26. 5us Study of Muon Coincidence signal Muon coincidence event rate: ~6evt/hr for 12 detectors Muon coincidence event is very high energy event and triggers multi-crystals. It requires a few tens of ms for scintillation to disappear. High Energy muon event in CsI detector (sec)

  27. Muon Coincidence signal Event rate of muon tail event -> muon tail event is defined as the event which shows up within 30ms from the start of Muon coincidence event. Event rate of muon tail event for det0 20days data Multi hit event is rejected. 8ms dead time after high energy event is applied by hardware setting. cpd keV(electron recoil equivalent E)

  28. Short remarks on Annual Modulation Study in KIMS Have been taking data with 100 kg array for ~7 months AM study without applying PSD can be done With 100kg CsI(Tl), ~3 cpd background level if no AM: one-year-run -> upper limit on AM amplitude < 0.01 cpd/keV/kg level with 90% CL if AM amplitude ~ 0.02 cpd/keV/kg two years data is required to confirm with more than 3σ significance

  29. Summary and Prospects • Result using 3409 kg days data • PRL 99, 091301 (2007) • DAMA signal region is ruled out for both SD and SI interactions • at WIMP mass > 20 GeV • Most stringent limit on SD interactions for pure proton case • Successfully reduced internal backgrounds of CsI(Tl) crystals (latest powder ~ 2cpd) • 12 full size crystals(8x8x30cm3) ~ 100 kg • Current shielding can accomodate 250 kg • 100 kg crystals installed in the shield and data taking was started •  Annual modulation search

  30. Very small WIMP signal Elastic Sacttering of WIMP off a nuclues in the detector Very small 10-6 ~ 10-10 pb Expected event rate ~ 1/kg/day or less Very small Recoil energy < 100 keV Measured energy < 10 keV due to quenching R : event rate R0: total event rate E0: most probable incident kinematic energy r : kinematic factor, 4MwMN/(MW+MN)2

  31. Analysis Method: Event selection • Multi hit event rejection • Fit quality cut • fitted τf • log likelihood value for two exponential fit and one exponential fit • ratio of fitted mean time to the calculated mean time • Short component rejection • ratio of tail (t> 10 μs) to the whole amplitude • Asymmetry cut

  32. Effect of cuts on MT distribution Data Gamma Base cut Fit quality Asym tail PMT Neutron

  33. Background level Previous run cpd preliminary PMT noise cut applied Muon tail events rejected Cut efficiency not corrected keV

  34. WIMP Nucleus Scattering Cross Section ρχ=galatic halo density vE=earth velocity in galatic frame v0=sun velocity in galatic frame • Spin Independent • Spin Dependent

  35. Latest WIMP search analysis results WIMP Nucleus Cross section Scalar: Spin independent interaction -> coherent amplification via constituent nucleon Axial: Spin dependent interaction -> coupling depends on proton & neutron spin expectation value

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