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