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Annual Modulation Study of Dark Matter Using CsI(T l ) Crystals In KIMS Experiment

Annual Modulation Study of Dark Matter Using CsI(T l ) Crystals In KIMS Experiment. J.H. Choi (Seoul National University) 2012. 8. 13. SUSY2012, Beijing. Dark Matter. Today’s Concordance Model of Universe -  CDM  K,0 = 1 – (  m,0 +  r,0 +  ,0 )

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Annual Modulation Study of Dark Matter Using CsI(T l ) Crystals In KIMS Experiment

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  1. Annual Modulation Study of Dark Matter Using CsI(Tl) Crystals In KIMS Experiment J.H. Choi (Seoul National University) 2012. 8. 13. SUSY2012, Beijing

  2. Dark Matter • Today’s Concordance Model of Universe - CDM • K,0 = 1 – (m,0+ r,0+ ,0 ) • = 1 – (b,0 + cdm,0+ ,0 + ,0 + ,0 ) •  : Dark Energy : 73% • CDM (Cold Dark Matter) : 23% • Baryon : 4% • Super Symmetry Theory • WIMP(Weakly Interacting Massive Particle)

  3. Milky Way Galaxy ~ 230km/s <2MASS> • The Milky Way has a large amount of dark matter halo • The mass density of dark matter is about 0.3 GeV/cm3near the Sun • The Milky Way rotates with the period of about 220 million years • The sun revolves at the speed of about 230 km/s 2MASS

  4. Annual Modulation 30km/s arXiv:1101.5205v1[astro-ph.CO]17Jan2011 Annual Modulation of Velocity : ~ 6% 30km/s < Recoil Energy > < Differential Event Rate > < A.M . Rate> < 3% < 7%

  5. Annual Modulation - KIMS DAMA KIMS Spin Independent(WIMP-nucleon) Spin dependent (WIMP-proton) • DAMA reported the annual modulation amplitude of 0.0183 cpd/kg/keV(2~4keV) by the modulation fitting method • KIMS reported the cross-section limit from the PSD(Pulse Shape Discrimination) method • KIMS should see the same modulation if iodine is scattered by WIMP Eur. Phys. J. C 67 (2010) 39

  6. Y2L Underground Laboratory Yangyang Underground Laboratory Depth : 700m (2000 mwe)

  7. Detector Structure Detector Shielding • CsI Crystal Detectors • - The 4x3 array of 12 CsI(Tl) crystals is installed in the innermost • - Size of 8x8x30 cm, Mass of 8~9 kg(Total 103.4 kg) • - Each crystal is mounted with 2 PMTs on the both side • Energy Resolution • - 16~23%(FWHM) for 59.54 keV • Quenching Factor • - CsI • - Na : 30%, I : 9%, Ca : 8%, F : 12% 12 x CsI(Tl) crystal Copper shield Polyethylene Lead shield Moderator(Muon Det.) CsI(Tl) crystal mounted PMTs

  8. Electronics • Electronics system digitize scintillation signals with 400MHz FADCs, which have a 10bit dynamic range and 40.96μs window • Digitized signals are acquired by DAQ(Data AcQuisition) Program, which is working on ROOT based linux system • DAQ rate is under 6Hz(0.5 million events/day) • We are monitoring and managing the stability in real time with network and alarm systems (electric power, room temperature, crystal’s surface temperature, etc.) 400MHz FADC DAQ Program Stabilities Signal

  9. Data Taking • Analysis is conducted with 30 month data from Sep. 2009 to Feb. 2012 • DAQ time does not correspond to a full of month for each month because of a unavoidable problems, such as an outage of electricity or an abnormal termination of computers • Total exposure is 75.53 ton∙dayswith 103.4 kg of CsI(Tl) crystals Sep. 2009 ~ Feb. 2012 Sep. 2009 ~ Feb. 2012

  10. Noise & Background • We are receiving 0.5 million events everyday • The expected rate of WIMP signal is under 1 counts/kg/day considering DAMA • Most of signals come from noise or background • Noise : electronics cables or circuits • electronics noise • PMT’s thermionic electrons from photocathode • PMT’s afterpulses • Background : scintillation signals from particles • Internal background (137Cs, 134Cs, 87Rb, ……) • External background (238U, 232Th, 40K, ……) • Cosmic background ( muon, ……) MC Simulation of Background

  11. Event Selection Efficiency • Multiple events survival probability after all the cuts are used to estimate the efficiency

  12. Energy Spectrum (Single) X • Analysis was done by 11 detector except DET10 because of higher background rate

  13. Data Fitting < 3-6 keV > • Fitting Function Decay Constant Modulation Adecay : Initial level (134Cs) t0 : Initial time – fixed to 1 Sep. 2009 τ : Decay constant (134Cs) – fixed to 2.065 y Bkg : Constant background level A : Annual Modulation Amplitude ω : Period – fixed to 1 year t1 : Annual Modulation Peak – fix to 2 Jun. X 2 Jun. • Parameter Correlation - χ2 fitting Bkg [cpd/kg/keV] Amplitude [cpd/kg/keV] Amplitude [cpd/kg/keV] Adecay [cpd/kg/keV] Bkg [cpd/kg/keV] Adecay [cpd/kg/keV]

  14. Averaged Result of 11 CsI(Tl) crystals Adecay Bkg • Adecay is typical beta spectrum of 134Cs • The background rate of 2 keV bin is relatively higher than other energy bin • The amplitude of annual modulation corresponds to zerowith 90% C.L. • The averaged amplitude of annual modulation from 3 keV to 6 keV is 0.0008±0.0068 cpd/kg/keV Amplitude

  15. KIMS & DAMA KIMS • DAMA reported the annual modulation amplitude of 0.0183±0.0022 cpd/kg/keV(8.3; 2~4 keV) • 2~4 keVof DAMA corresponds to 3.24~5.41 keV of KIMSby 9% quenching factor of I • The amplitude of annual modulation is 0.0008±0.0068 cpd/kg/keV (1; 3~6 keV) in KIMS • The 90% C.L. upper limit of amplitude of annual modulation is 0.0119 cpd/kg/keV (3~6 keV) in KIMS 3.24 keV 5.41 keV DAMA

  16. Conclusion • The analysis was done with 2.5 years data of 75.53 tondaysusing CsI(Tl) cryatals • The background level of single hit events is under 3 cpd/kg/keVpassing 7 event selection conditions • The amplitude of annual modulation is consistent with zero by 90% C.L. • The amplitude of annual modulation is 0.0008±0.0068 cpd/kg/keV (1) for 3-6 keV in KIMS experiment • The 90%CL upper limit is 0.0119cpd/kg/keV(3-6 keV) in KIMS experiment, which is contradictory to the interpretation of WIMP-iodine scattering for DAMA modulation data

  17. Thank You

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