Annual Modulation Study of Dark Matter Using CsI(Tl) Crystals In KIMS Experiment J.H. Choi (Seoul National University) 2012. 8. 13. SUSY2012, Beijing
Jan 05, 2016
Annual Modulation Study of Dark Matter Using CsI(Tl) Crystals In KIMS
Experiment
J.H. Choi (Seoul National University)2012. 8. 13.
SUSY2012, Beijing
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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)
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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/cm3 near 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>
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Annual Modulation
Annual Modulation of Velocity : ~ 6%
arXiv:1101.5205v1[astro-ph.CO]17Jan2011
< 3%
< Recoil Energy > < Differential Event Rate > < A.M . Rate>
30km/s
30km/s
< 7%
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Annual Modulation - KIMS
Eur. Phys. J. C 67 (2010) 39
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
DAMA KIMS
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Y2L Underground Laboratory
Yangyang Underground LaboratoryDepth : 700m (2000 mwe)
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Detector Structure• 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%
CsI(Tl) crystal mounted PMTs
Detector Shielding
Moderator(Muon Det.)
Lead shield
Polyethylene
Copper shield
12 x CsI(Tl) crystal
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Electronics
400MHz FADC
• 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.)
DAQ Program Stabilities
Signal
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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∙days with 103.4 kg of CsI(Tl) crystals
Sep. 2009 ~ Feb. 2012
Sep. 2009 ~ Feb. 2012
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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
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Event Selection Efficiency
• Multiple events survival probability after all the cuts are used to estimate the efficiency
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Energy Spectrum (Single)
• Analysis was done by 11 detector except DET10 because of higher background rate
X
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Data Fitting
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.
< 3-6 keV > • Fitting
Function
Decay
Constant
Modulation
2 Jun.
X• Parameter Correlation - χ2 fitting
Adecay [cpd/kg/keV]
Bkg
[cp
d/k
g/k
eV
]
Adecay [cpd/kg/keV] Bkg [cpd/kg/keV]
Am
plit
ud
e [
cpd
/kg
/keV
]
Am
plit
ud
e [
cpd
/kg
/keV
]
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Averaged Result of 11 CsI(Tl) crystals
• 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 zero with 90% C.L.
• The averaged amplitude of annual modulation from 3 keV to 6 keV is 0.0008±0.0068 cpd/kg/keV
Adeca
y
Bkg
Amplitude
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KIMS & DAMA
• DAMA reported the annual modulation amplitude of 0.0183±0.0022 cpd/kg/keV(8.3; 2~4 keV)
• 2~4 keV of DAMA corresponds to 3.24~5.41 keV of KIMS by 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
KIMS
DAMA
3.24 keV
5.41 keV
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Conclusion
The analysis was done with 2.5 years data of 75.53 tondays using CsI(Tl) cryatals
The background level of single hit events is under 3 cpd/kg/keV passing 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
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Thank You