A WIMP dark matter search with CsI ( Tl ) crystal

1. A WIMP dark matter search with CsI(Tl) crystal SeungCheon Kim Department of Physics & Astronomy Seoul National University Seminar@Fermilab (2012-07-31)

2. Outline A brief introduction about dark matter A Description about KIMS experiment A study about the background The recent results The prospect

3. Dark matter ? 1936 Zwicky first proposed the existence of dark matter, inferred from the motion of galaxies in coma cluster. => The total mass of the luminous matter is too small to explain the velocity of the galaxy in the cluster! 107-108K What is holding such hot gases ? The recent x-ray measurement of the cluster also requires the invisible matter! Hubble image of coma cluster

4. A pile of evidences for dark matter !!! Gravitational Lensing Rotation curve for Galaxy 99 % of people believe the dark matter… cosmic microwave background Bullet cluster Big Bang Nucleosynthesis

5. What we know about darkmatter Dark (invisible) • no EM interaction rarely interacting very stable Massive • gravitationally evident non-relativistic (cold– most, not necessarily all of them) Influencing the structure formation of the Universe

6. What we know about dark matter Overwhelmingly abundant • the majority of the matter constituents in the universe Exotic • Not made of the known matter Need of the new theory other than SM (multifunctioning particle – not invented for DM – favored)

7. WIMP (Weakly Interacting Massive Particle)- a good dark matter candidate Ω: energy density of the universe Ω= ΩDM+ ΩΛ + Ωbaryon = 1 for the flat universe ΩDM = 0.222 ± 0.026 from WMAP The weakly interacting relic particles can survive the annihilation and so exist in this level of the density. • Supersymmetry and Univ-ersal extra dimension • theories provide good • candidates of this kind. • lightest neutralino • lightest kluza-klein • particle

8. WIMP (Weakly Interacting Massive Particle)- a good dark matter candidate And, it is testable with the experiment, even though very challenging. It has been expected WIMP will deposit a few tens keV in the detector absorber. The event rate will be very small. Background reduction & discrimination is essential. σχ-nucleon =10-4pb

9. KIMS (Korea Invisible Mass Search) A WIMP search experiment using CsI(Tl) scintillators It has been carried out at Yangyang underground lab(Y2L), South Korea. Seoul National University: H.C.Bhang, J.H.Choi, K.W.Kim, S.C.Kim, S.K.Kim, J.H.Lee, J.I.Lee, J.K.Lee, M.J.Lee, S.J.Lee, J.Li, X.Li, S.S.Myung, S.L.Olsen, I.S.Seong Sejong University: U.G.Kang, Y.D.Kim Kyungpook National University: H.J.Kim, J.H.So, S.C.Yang Yonsei University: M.J.Hwang, Y.J.Kwon EwhaWomans University: I.S.Hahn Korea Research Institute of Standard Sciences: Y.H.Kim, K.B.Lee, M. Lee Tsinghua University : Y.Li, Q.Yue, J. Li

10. CsI (Tl) scintillator ? Widely used, well-known detector Easy handling Large scintillation yield : 60,000/MeV technique electron recoil • Low hygroscopicity • Pulse Shape Discrimination • Distinguishing • nuclear recoil event • at the statistical basis nuclear recoil

11. CsI (Tl) scintillator ? Large atomic number, Cs (133), I (127) Good for A2 scaling High spin expectation value for proton Sensitive to SD interaction

12. CsI (Tl) scintillator ? However, it has some inherent background. 137Cs artificial radioisotope , half-life of 30 years beta decay & delayed (2.5min) gamma of 662 keV 134Cs Produced by the capture of neutron from the cosmic ray muon, half-life of 2 years, beta decay & several prompt gamma rays 87Rb It exists in the pollucite (ore for Cs). Natural abundance of 27.84 %, beta decay

13. CsI (Tl) scintillator ? It contains iodine (I) like DAMA/LIBRA experiment. • Cross-checking the WIMP-I interaction scenario for DAMA positive signal. So, it’s very good idea to search for WIMP using CsI (Tl) scintillators if one can manage its internal background .

14. CsI (Tl) detector for KIMS One detector module : one CsI Crystal + 2 PMTs Crystal size: 8x8x30 cm3 (8.7 kg) (Beijing Hamamatsu Photon Techniques Inc.) Background level: 2-3 counts/keV/kg/day (cpd) PMT : 3” PMT (9269QA), RbCsphoto cathode (green extended) Event window is 40µs. Digitized with 400MHz FADC Am241 calibration • data • simulation (GEANT4.9.5) Light yield:5-6 p.e/keV Eth = 3 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 ...

15. Detector array • 3 x 4 detector array • The total mass : 103.4 kg • Multiple hit events • Vetoed for DM search • But, references for calibration • & event selection E of det10 134Cs E [keV] β- 4+ 1969.87 27.3% 3+ 1643.28 2.50% 4+ 1400.55 70.1% 1167.93 2+ 0.033% 2+ 604.70 0.10% 0+ 0.0 134Ba Decay for 134Cs(T1/2=2.06yr) Measurement: 2.053±0.145yr Total E except det10

16. KIMS Detector system 12 x CsI (Tl) crystal Copper shield (10 cm) Polyethylene (5 cm) Lead shield (15 cm) Neutron shield & MuonVeto (30 cm mineral oil) N2 gas flow inside the Cu shield

17. Experimental site: Yangyang Underground Laboratory (Y2L) (Upper Dam) YangyangPumped Storage Power Plant Y2L Seoul (Power Plant) (Lower Dam) KIMS(DM Search) AMoRE(Double Beta Decay Experiment) Minimum depth : 700 m / Access to the lab by car (~2km)

18. The Experimental hall Muon flux : 2.7 x 10-7/cm2/s (2000 m.w.e) Neutron flux : 8 ×10−7/cm2/s for 1.5-6 MeV neutrons Rn background in the air : 1-2 pCi/l Clean room system Online-Monitoring of temperature, humidity, power stability, Rn background level

19. Background study Multiple hit only Crystal : 134,137Cs, 87Rb PMT : 238U, 232Th, 40K Surface : progenies of 222Rn α spectrum

20. PMT PMT Clean acryl box Background study Surface alpha (SA) background • These events makes trouble • in pulse shape discrimination. • SA => Characterize its PDF • PMT events • Find cuts to minimize its • contamination. Scintillation at PMT body pmt0 pmt1

21. Background at the low energy A few tens of recoil energy will be measured as a few keV because quenching. The current background level below 10 keV : 2.8 counts/keV/kg/day (cpd) + PMT dark current

22. Rejection of events produced from PMT itself=> PMT body scintillation & dark current (PMT events) PMT generates events by itself. • PMT body scintillation + After pulse + random coincidence + etc. => It can be problematic for the low energy, rare phenomena search. After pulse • A delayed (~1 μs) pulse following the primary pulse • The atom in the PMT vacuum ionized by the accelerating photo-electron drifts to the photo- cathode producing a big signal (spike-like).

23. PMT PMT Clean acryl box Afterpulses in PMT Dummy Detector Signal size => # of SPE pmt0 pmt1 Afterpulse by postive ion(H,He,N2 ...) in PMT Time span btw neighboring SPEs(us)

24. PMT PMT Clean acryl box PMT events rejection From the PMT dummy detector data, We have developed the event selection condition to minimize the PMT events contamination. The Efficiency for nuclear recoil (NR) events (WIMP candidate) : ~30% tail events Compton events

25. Pulse Shape Discrimination (PSD) Discriminating the event type by the pulse shape Enabling the estimation of NR event rates statistically PSD parameter at 3 keV Reference data for PSD Surface alpha (SA) : from the Radon-contaminated crystal Nuclear Recoil (NR) : by irradiating the test crystal with Am-Be neutron source Electron Recoil (ER) : compton scattering NR ER SA

26. Opening the data… Period: September 2009 – August 2010 => 317.15 live days Exposure: 24524.3 kg•days After applying the PMT noise selection cut, Assuming the data are composed of SA, ER and NR events Estimating the fraction NR event rates with PSD. Candidate events for WIMP : NR occurring in one detector

27. The estimation of the NR events rate Pdf = f0 x FNR + f1 x FSA + (1-f0-f1) x Fgamma The posterior pdffor f0 & f1 is obtained from Bayesian analysis method.

28. The estimated NR event rates for 12 detectors SA The positive signals at higher energy bins seem correlated with SA contamination. The PSD PDF of NR at higher energy bins might be contaminated with gamma ray caused by the inelastic scattering. => need of further study For the conservative claim, in the combining all the detectors, the detectors with high SA contamination are excluded. 90 % limit 68 % interval

29. The overall NR events rate det0, 8, 11 excluded Total exposure: 24524.3 kg days No meaningful excess of NR events is observed.

30. KIMS NR event rates & DAMA/LIBRA annual modulation amplitude • The annual modulation amplitude from DAMA • At 2-4keV, 0.0183±0.0022 counts/day/kg/keV • At the corresponding energy range (3.6-5.8 keV) • in KIMS, (QF for DAMA =0.08 adopted) • The 90 % C.L limit is 0.0098 counts/day/kg/keV • This is well below the DAMA modulation amplitude. • Any scenario with Iodine for DAMA is not consistent • with KIMS results !!! DAMA/LIBRA The spectrum of the amplitude of the annual modulation

31. KIMS & DAMA in iDM model Cs I Expected spectrum for iDMfor I & Cs target => M=70 GeV, δ=116keV, vesc=500 km/s, ve=235km/s, σ=0.93e-3 pb For 70 GeV mass WIMP

32. The new exclusion limits from KIMS Spin-dependent proton interaction Spin-Independent interaction Published in PRL 108, 181301 (2012)

33. Annual modulation study in KIMS The analysis of 2.5 years data is almost completed. PSD was not taken into account. There’s time-varying background from 134Cs (2year half-life). The results will be reported very soon. 3-6 keV Sep. 2009 ~ Feb. 2012

34. Prospect of KIMS experiments New PMTs are under test. • ~10 p.e/keV ? Decreasing the energy threshold • Reduced 238U, 232Th, 40K Decreasing current background level by ~ 1 cpd The treatment of the surface background • Polishing or surrounding with active veto layer ? Adding low background NaI (Tl) scintillators is being planed.

35. Summary 12 CsI (Tl) scintillators (103.4kg) have been running at Yangyang underground lab. Most of backgrounds of the detector has been understood. The improved limit of WIMP-nucleon interaction has been derived based on 1 year data with PSD. Annual modulation study based on 2.5 years was almost done and will be reported very soon. New upgrade is being planed.

36. Back up

37. Event rate of WIMP in the detector WIMP nucleon cross section Coherent enhancement along nucleus number density of WIMP nuclear form factor number of target nuclei The effect by the velocity Distribution of WIMP If σn = 10-5pb, for 100 GeV WIMP and Iodine target, The expected event rate is 1-2 counts/keV/kg/day ,considering quenching.

38. CsI (Tl) detector for KIMS One detector module : one CsI Crystal + 2 PMTs Crystal size: 8x8x30 cm3 (8.7 kg) (Beijing Hamamatsu Photon Techniques Inc.) Background level: 2-3 counts/keV/kg/day (cpd) PMT : 3” PMT (9269QA), Electron tube RbCs photo cathode (green extended) Am241 calibration 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 5-6 p.e /keV

39. KIMS Detector system Neutron shield(30cm mineral oil) Lead shield (15cm) Polyethylene(5cm) Copper (10cm) CsI(Tl) Scintillator Neutron detector Muon detector (Neutron sheild) N2 gas flow inside the Cu shield

41. Annual Modulation Studies (w/o PSD) • Total DAQ rate is under 6Hz . • 2.5 year data to see annual modulation ; 75.53 ton∙days • The temperature of detector array is 20 - 21.6 OC depending on the position, and it is maintained stably with a maximum fluctuation of around 0.2 OC. Sep. 2009 ~ Feb. 2012

42. 3-6 keV • Annual modulation amplitude is obtained including the exponential decay of 134Cs. • Annual modulation amplitude is consistent with null. • The 90% upper limit of the amplitude is comparable to DAMA’s annual modulation signal (0.0189 cpd/kg/keV) • We are trying to give final numbers shortlyin this summer.