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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(Tl) 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 ) • = 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)
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
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%
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
Y2L Underground Laboratory Yangyang Underground Laboratory Depth : 700m (2000 mwe)
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
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
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
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
Event Selection Efficiency • Multiple events survival probability after all the cuts are used to estimate the efficiency
Energy Spectrum (Single) X • Analysis was done by 11 detector except DET10 because of higher background rate
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]
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
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
Conclusion • The analysis was done with 2.5 years data of 75.53 tondaysusing 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