1. The analysis status of WIMP search at KIMS Seung Cheon Kim (DMRC,SNU) yongpyung workshop 2010

2. WIMP? Galaxy rotation curve Bullet clusterX-ray from Hot cluster Non-ordinary, Weakly-interacting, Stable, Massive (non-relativistic) Particle required.->WIMP Dark matter exists, and it’s very likely to be a WIMP. 2

3. WIMP candidate? LSP( lightest supersymmetric particle ) Introduced from SUSY naturally. By R parity conservation, lightest supersymmetric particle(LSP) can be the stable, WIMP. Relic dark matter density for large scale structure can be explained well. LKP( lightest Kaluza-Klein particle) Theory with Universal Extra Dimensions introduces lightest KK( Kaluza-Klein) particle. It becomes stable thanks to Kaluza-Klein parity. Massive sterile neutrino, axino... We have plenty of candidates for WIMP from various motivations. 3

4. How to sense WIMP? WIMP recoils nucleus. It is expected to deposit around a few tens keV. Thermal signal Scintillation, e-h pair Since it interacts rarely, Background event from radioisotope impurity or cosmic shower must be reduced seriously.  Selection of Radioisotope free material  Location at Underground laboratory WIMP seems testible with the state-of-the-art experimental technique. 4

5. WIMP search of KIMS WIMP search using CsI(Tl) scintillator =>12 detectors (104.4kg) YangYang underground Lab(Y2L) =>Located in Yangyang pumped storage power plant =>700m minimum depth, 2000m water-equivalent =>accessible by car (tunnel~2km) 5

6. KIMS’ main detector: CsI(Tl) scintillator CsI crystal? Easy to get large mass with an affordable cost High light yield ~60,000/MeV Easy fabrication and handling Enabling pulse shape discrimination -> Statistically, nuclear recoil event rates can be estimated Sensitive to both SD and SI WIMP interactions A(Cs) =133, A(I) =127 IsotopeJAbun 133 Cs7/2100%-0.3700.003 127 I5/2100%0.3090.075 73 Ge9/27.8%0.030.38 129 Xe1/226%0.0280.359 131 Xe3/221%-0.009-0.227 19 F1/2100%0.441-0.109 6

7. KIMS’ main detector: CsI(Tl) scintillator CsI crystal? Radioisotope in CsI crystal 137 Cs :from processing water in manufacturing factory Using “ultra clean water” reduced to ~1.7mBq/kg 134 Cs :produced by neutron capture of 133 Cs induced by cosmic ray 87 Rb : most dominant background at the low energy can be reduced by recrystalization technique =>potential for further background level reduction Geant Simulation Latest crystals are from ~2 cpd level powder 7

8. Full range background spectrum ▬ Total events ▬ Multiple hit events => These can be used for various calibrations. E of det10 Total E except det10 Energy spectrum of one detector Spectrum of total energy sum of whole detectors 605 KeV 134 Cs, 660 KeV 134 Cs 796 KeV 134 Cs 1401 KeV 134 Cs 1970 KeV 134 Cs 1168+802 = 1970 KeV 1365+605 = 1970 KeV β decay 8

9. CsI(Tl) crystal detector One detector module : one CsI Crystal + 2 PMTs PMT : 3” PMT (9269QA), Quartz window, RbCs photo cathode (green extended) Crystal size: 8x8x30 cm 3 (8.7 kg) (Beijing Hamamatsu Photon Techniques Inc.) Trigger condition: In 2us, 2 more photons in each PMT + high energy event 8ms dead time after high energy event 30ms dead time after muon coincidence Event window is 40µs. Digitized with 400MHz FADC Muon event sec 9 Event window is 40µs. Digitized with 400MHz FADC

10. 10 Relative intensity..... Am calibration -> ~5 p.e /keV Am241 calibration E(keV) 13.9keV Np L  X-ray 17.8keV Np L  X-ray 20.8keV Np L  X-ray 26.35keV gamma Cs, I X –ray escape 59.54 gamma

11. Neutron shield(30cm mineral oil) Lead shield (15cm) Polyethylene(5cm) Copper (10cm) CsI(Tl) Scintillator Neutron detector Muon detector (Neutron sheild) KIMS Detector system N 2 gas flow inside the Cu shield 11

12. PMT noise limits the sensitivity at the low energy level experiment seriously. Source of PMT noise =>Thermionic Emission =>Afterpulse: the drift of ionized residual gas in the PMT toward photocathode excitation of metastable state of photocathode or glass => Cerenkov radiation from Cosmic ray or background radio-isotope => Fluorescence of glass PMT noise event is asymmetric along 2PMTs in size and time. PMT noise event rejection 12

13. Characters of PMT noise event Clean acryl box PMT  Data from PMTs + acryl box system We have 1-2 month data for this. Event rate: a few tens to a few hundreds events per day 13 pmt noise eventtypical scintillation event called as clusters Usually one cluster equals one s.p.e. But, pmt noise event includes abnormly big cluster larger than typical cluster. pmt0 pmt1

14. Signal size => # of SPE Time span btw neighboring SPEs(us) pmt0pmt1 Afterpulses in PMT-only -detector Afterpulse by postive ion(H,He,N2...) in PMT Time btw neighboring SPEs Afterpulses by positive ions Backscattering by electron 14

15. Pmt0/biggest cluster Biggest cluster Pmt noise rejection cut 1 => pmt/biggest cluster >3 for both pmts Crystal installed pmt only 15 Pmt1/biggest cluster det0det1 det2 det4 det3 det5

16. Pmt noise rejection cut 2 => abs(pmt0.t0-pmt1.t0)<0.3 Pmt noise rejection cut 3  Reject large asymmetric event in signal size btw 2 PMTs High energy event tail Total events Multiple hit events Total events Events passing all the cuts 16

17. PMT noise event rejection cut And with other cuts ( multiple hit veto …) For 49days pmt-only-detector data, After applying the cut developed above, Less than 30 events survived for almost detectors  < 0.025 count/kg/day at 3-4keV Total Cut efficiency at 3-4keV : 50-60% 17

18. Mean time calibration for Nuclear recoil and electron recoil for pulse shape discrimination Fitted by asymmetric gaussian Neutron with test crystal Gamma with test crystal Compton evts(gamma) with Y2L detector (det0) 18

19. Fit results of mt distribution by asymmetric gaussian Compton scattering event Neutron with test crystal Gamma with test crystal 19

20. Current Nuclear recoil(NR) rate limit assuming null observation of WIMP For SET18(57days), NR evt number by simulation at 3-4keV det0: 2.704  54.27  18.76  3.76 det1: -1.02  64.56  24.49  5.35 det3: -39.03  76  43.38  8.5 det5: -5.1  60.02  17.04  4.2 det6: 4.47  50.15  10.97  3.57 det7: -1.47  59.83  17.27  4.44 det8: -4.38  56.38  16.8  4.34 det9: -3.2  64.7  21.06  5 det10: -7.5  66.79  19.4  6.0 det11: -14.1  55.32  16.75  3.82 statistical error systematic error from mt of electron recoil systematic error from mt of nuclear recoil Electron recoil systematic error, ~ 0.05cpd at 3-4keV. Statistical error, ~0.05cpd NR recoil systematic error, ~0.01cpd 20

21. Data summary Now, 460 days data collected for analysis => Statistical limit ~ 0.018cpd/kev/kg background level ~ 3 count/kg/keV/day. =>background level becomes lower than before by more than 1cpd level. Better understanding about PMT noise background => considerable amounts of data in PMT noise are afterpulses. Effective PMT noise cut developed. => better efficiency With above improvements, =>By factor of 5-6, Improvement of sensitivity expected. 21

22. Inelastic dark matter(iDM) analysis with KIMS data iDM is proposed to reconcile the DAMA observation and null results from other experiments. (Tucker-Smith, Weiner 2001) There exists an excited state  * of the dark matter particle with a mass m  * - m  =  ~ 100keV. Elastic scatterings off of the nucleus,  N ->  N are suppressed, compared with the inelastic scatterings  N ->  * N.

23. Minimum velocity for recoil E, E R m N : mass of target Nucleus µ:reduced mass of WIMP/target Nucleus  :mass splitting between  and  * ERER  min

24. Three key features *Spectrum of events is dramatically changed, suppressing or eliminating low energy events. ->Spectrum of Conventional WIMP rises exponentially at low energy. But iDM peaks at E R ~20keV or above. ERER rate Conventional WIMP spectrum iDM spectrum 20keV

25. Three key features *Heavier targets favored over lighter targets. For a given E R and , Heavier target mass -> lower  min -> greater available range of velocity I(DAMA) is better than Ge(CDMS).

26. Three key features *modulation of the signal enhanced significantly. Mainly high velocity WIMPs are detectible. In these high velocity region, WIMP numbers is changing rapidly. visible to DAMA visible to DAMA and CDMS f(v) Neal Weiner, NYU; IDM 2008 Modulation ratio of DAMA/LIBRA according to 

27. Benchmark Points (S.Chang et al 2008) Vesc =500km/s, V0=220km/s statistical limit of our current data collection ~0.018 dru

28. Benchmark Points (S.Chang et al 2008) Expected energy spectrum simulation for KIMS. The peak is above our threshold energy, 3keV! E measured Eee(keV)

29. 29 σ = 4.47 ±0.035 E = 59.55 ± 0.04 Resolution; 7.51% P.E yield : 6.4 per keV Electron Tubes – 9269QA =>pmt used currently HAMAMATSU – R6233 =>new PMT known as high QE PMT test for next upgrade σ = 3.86 ±0.048 E = 59.56 ±0.06 Resolution; 6.48% p.E yield :9.8 per keV More P.E yield, Lower Energy threshold! Dark current rate will be tested. Am241 calibration

30. Current status and plan Our latest limit is incompatible with spin independent interaction of Iodine of DAMA/LIBRA. (PRL 99, 091301 (2007) ) 30 Still, most stringent in spin dependent proton interaction around 100GeV of dark matter mass KIMS PICASSO(2009) COUP(2008) DAMA/LIBRA(2008)

31. Current status and plan ~ 460days data collected for analysis. => Statistical limit ~ 0.018cpd/keV/kg expected => Factor of 5-6 improvement of limit expected Various background events under study. iDM scenario is studied with data. High QE PMT test is going on. 31