Status of KIMS

S.S. Myung (SNU)

For the KIMS Collaboration

• Results from 4 crystal array• Status of 12 crystal array

KIMS Collaboration

H.C.Bhang, J.H.Choi, D.W.Kim, S.C.Kim, S.K.Kim, J.H.Lee,H .S.Lee, S.E.Lee, J. Lee, S.S.MyungSeoul National University

U.G.Kang, Y.D.Kim, J.I. LeeSejong University

H.J.Kim, J.H.So, S.C.Yang Kyungpook National University M.J.Hwang, Y.J.KwonYonsei University

I.S.Hahn Ewha Womans University

Y.H.Kim, K.B.Lee, M. Lee KRISS

J.LiInstitute of High Energy Physics D. He, X.Li, Q.YueTsinghua University

H.C.Bhang, J.H.Choi, D.W.Kim, S.C.Kim, S.K.Kim, J.H.Lee,H .S.Lee, S.E.Lee, J. Lee, S.S.MyungSeoul National University

U.G.Kang, Y.D.Kim, J.I. LeeSejong University

H.J.Kim, J.H.So, S.C.Yang Kyungpook National University M.J.Hwang, Y.J.KwonYonsei University

I.S.Hahn Ewha Womans University

Y.H.Kim, K.B.Lee, M. Lee KRISS

J.LiInstitute of High Energy Physics D. He, X.Li, Q.YueTsinghua University

Mostly Dark matter, Dark energy

Composition of Universe

02.002.1 matter

Known ~ 4%

property of neutrinosnot known very well

Galactic Dark Matter

Density of dark matter around the sun ~ 0.3 GeV /cm3 ( ~ 5 x 10-28 kg/cm3)

rotation velocity curve

WIMP

Cs

IRecoiled nucleus

Direct detection of galactic dark matter

Recoil energy ~ 10’s keVEvent rate < 1/kg/day

Neutralinos the strongest WIMP candidate because they hardly interact and are stable

WIMP detection techniques

WW

N

Phonon(heat) Photon(light)

Ionization

+ _+ __+

Energy lost by recoiled nucleus converts to phonons, photons, ion pairs

HPGe detector

NaI(Tl), CsI(Tl), Xe

2 phase XeCryogenic detector

Ge, Si

KIMS

Yangyang Underground Laboratory(Y2L)Yangyang Underground Laboratory(Y2L)

3.5 hours by car

Yangyang Underground LaboratoryKorea Middleland Power Co.

Yangyang Pumped Storage Power Plant

~2 km

Vertical Depth ~700m

KIMS

OFHC Cu 10cm : 3t

Detector and Shielding

Why CsI(Tl)?

• Sensitive to both SD and SI WIMP interactions Ge, Xe not sensitive to SD proton coupling complimentary to CDMS, XENON-10 direct check of DAMA signal by I-127 recoil

• Easy to get large mass with an affordable cost annual modulation study

• High light yield ~60,000/MeV

• Pulse shape discrimination better gamma rejection than NaI(Tl)

• Easy fabrication and handling

• Sensitive to both SD and SI WIMP interactions Ge, Xe not sensitive to SD proton coupling complimentary to CDMS, XENON-10 direct check of DAMA signal by I-127 recoil

• Easy to get large mass with an affordable cost annual modulation study

• High light yield ~60,000/MeV

• Pulse shape discrimination better gamma rejection than NaI(Tl)

• Easy fabrication and handling

Detector

•CsI(Tl) Crystal 8x8x30 cm3 (8.7 kg)

• 3” PMT (9269QA) : Quartz window, RbCs photo cathode (Green enhanced)

• ~5 Photo-electron/keV

Total charge spectrum for 59.5 keV Am241 Resolution @ 59.5 keV : 7.3%

Internal background

Radioisotopes in the crystal

137Cs : 10 mBq/kg,

0.35 cpd/mBq/kg @ 10 keV

134Cs : 20 mBq/kg

0.07 cpd/mBq

0.005 cpd/mBq87Rb : 10 ppb

1.07 cpd/ppb

Geant Geant SimulationSimulation

Cs-137 reduction – use ultra pure water in powder prodcution; ~1.7 mBq/kg

Rb reduction - recrystalization method ; < 1ppb

Latest crystals are from ~2 cpd level powder

Muon Detector •4 coverage muon detector : 28 channels • Liquid Scintillator(5%) + Mineral Oil (95%) = 7 ton • Measured Muon flux = 2.7 x 10 –7 /cm2/s • Position resolution : σx, ~ 8 cm • Reconstructed muon tracks with hit information• Muon veto efficiency ~99.9%

Neutron detector•1 ~ 1.2 liter BC501A liquid scintillator x 3 •n/gamma separation using PSD•E_vis > 300 keV•Measured neutron flux (outside shield) 8 x 10 –7 /cm2/s ( 1.5 < E neutron < 6 MeV )

Other Detector Components

Study of Muon induced neutrons

•Coincidence between Muon and Neutron detectors • n/gamma separation using PSD•Neutron energy: 0.4MeV<E<2.75MeV

• Measured : (3.8±0.7)10-2 counts/day/liter • GEANT4 : (2.0±0.2) 10-2 counts/day/liter

Neutron calibration

300 mCi Am/Be source neutron rate 7 x 105 neutrons /sec a few 100 neutrons/sec hit 3cm 3cm 3cm crystal

( sample ) Quenching factor of Recoil Energy NR mean time distribution

NeutroNeutronnsamplesample

GammaGammaFull Full sizesize

GammaGammasamplesample

Tag γ(4.4MeV)to measure TOF and energy of neutrons

Note: their shapes areWell matching

<t>=( Ai ti)/ Ai

Data analysis for Wimp Search

Data used for this analysis

Crystal p.e./keV Mass(kg) Data(kg·days)

S0501A 4.6 8.7 1147

S0501B 4.5 8.7 1030

B0510A 5.9 8.7 616

B0510B 5.9 8.7 616

Total 34.8 3409

Calibration and control data samples

Neutron ~ 500 kg days (at 4~6 keV)Gamma (using 137Cs) ~ 1100 kg days (0501A), 1650 kg days(0501B)

910 kg days (0510A), 840 kg days(0510B)PMT only ~350 kg days for each crystal with the PMTS used for each crystal

Data analysis for Wimp Search

clear boxPMT PMT

Reduction of PMT noise

GammaNeutronPMT only(dot-dashed)WIMP search DATA

PMT noise

Decay timelog(τf)

LL2-LL1

fit result for one event

Analysis Method: Event selection

remove multi-crystal events

0.7 cpd bkg reduction

Gamma PMT background WIMP search DATA

Asymmetry Cut3-5 keV

5-7 keV

Efficiency

NeutronCompton events

S0501AS0501BB0510AB0510B

Efficiency CorrectedEfficiency Corrected

WIMP search data with cutWIMP search data with cut

Event rates after cuts

Modeling of Calibration data with asymmetric gaussian function

NR event rate estimation

• Fit the WIMP search DATA with PDF function from gamma and neutron calibration data

extract NR events rate

n

Electron recoilNuclear recoilBest fit

Extracted NR

S0501AS0501BB0510AB0510B

NR Event rates

90% CL limit

Geant4 expectation

Total Rate:

Extracted NR is fitted with simulated WIMP signal for each WIMP mass

Nuclear recoil of 127Iof DAMA signal region

is ruled out unambiguiously

Nuclear recoil of 127Iof DAMA signal region

is ruled out unambiguiously

ρD=0.3 GeV/c2/cm3

v0=220km/s, vesc=650km/s

Systematic uncertainty ~15%

Cross-section upper limits

PRL 99, 091301 (2007)

Spin-Independent SD, Wimp-Proton

Name Weight (kg)

Data(kg

days)

S0406 6.7 237

S0501A 8.7 1147

S0501B 8.7 1030

B0510A 8.7 616

B0510B 8.7 616

B0511 8.7

B0601 8.7

B0605A 8.7

B0605B 8.7

B0606A 8.7

B0606B 8.7

B0609 8.7

B0609 8.7

published PLB(2006)

background level2~4 cpd/keV/kg (preliminary)

Total crystals 8.7 kg x 12 = 104.4 kg

started data taking

Pilot run

Engineering run

published PRL(2007)

12 crystal array

Study of Muon Coincidence signal

Muon coincidence event rate: ~6evt/hr for 12 detectors

Muon coincidence event is very high energy event and

triggers multi-crystals.

It requires a few tens of ms for scintillation to disappear.

(sec)

High Energy muon event in CsI detector

5us

Event rate of muon tail event

-> muon tail event is defined as the event which shows up within

30ms from the start of Muon coincidence event.

keV(electron recoil equivalent E)

cpd

20days data

Multi hit event is rejected.

8ms dead time after high energy event is applied by hardware setting.

Event rate of muon tail event for det0

Muon Coincidence signal

Short remarks on Annual Modulation Study in KIMS

• Have been taking data with 100 kg array for ~7 months

• AM study without applying PSD can be done

• With 100kg CsI(Tl), ~3 cpd background level

if no AM:

one-year-run -> upper limit on AM amplitude

< 0.01 cpd/keV/kg level with 90% CL

if AM amplitude ~ 0.02 cpd/keV/kg

two years data is required to confirm

with more than 3σ significance

• Result using 3409 kg days data PRL 99, 091301 (2007) DAMA signal region is ruled out for both SD and SI interactions at WIMP mass > 20 GeV Most stringent limit on SD interactions for pure proton case • Successfully reduced internal backgrounds of CsI(Tl) crystals

(latest powder ~ 2cpd)– 12 full size crystals(8x8x30cm3) ~ 100 kg– Current shielding can accomodate 250 kg

• 100 kg crystals installed in the shield and data taking was started Annual modulation search

Summary and Prospects

rEE

R

RerE

R

dE

dR0/

0

0

)χ~

χ~

( 01

01 NN

Elastic Sacttering of WIMP off a nuclues in the detector

10-6 ~ 10-10 pb

Expected event rate ~ 1/kg/day or less

WIMP signal

R : event rateR0: total event rateE0: most probable incident kinematic energyr : kinematic factor, 4MwMN/(MW+MN)2

Recoil energy < 100 keVMeasured energy < 10 keV due to quenching Very

small

Very small

Very small

Analysis Method: Event selection

• Multi hit event rejection• Fit quality cut

• fitted τf

• log likelihood value for two exponential fit and one exponential fit

• ratio of fitted mean time to the calculated mean time

• Short component rejection• ratio of tail (t> 10 μs) to the whole amplitude

• Asymmetry cut

Effect of cuts on MT distribution

Data Gamma

Neutron PMT

Base cutFit qualityAsymtail

Background level

PMT noise cut applied

Muon tail events rejected

Cut efficiency not corrected

cpd

keV

preliminary

Previous run

• WIMP Nucleus Scattering

• Spin Independent

• Spin Dependent

ρρχχ=galatic halo density =galatic halo density vvEE=earth velocity in galatic =earth velocity in galatic frameframevv00=sun velocity in galatic frame=sun velocity in galatic frame

Cross Section

WIMP Nucleus Cross section

Scalar: Spin independent interaction

-> coherent amplification via constituent nucleon Axial: Spin dependent interaction

-> coupling depends on proton & neutron spin expectation value

WA

Latest WIMP search analysis results

Isotope J Abun <Sp> <Sn>133Cs 7/2 100% -0.370 0.003127I 5/2 100% 0.309 0.07573Ge 9/2 7.8% 0.03 0.38129Xe 1/2 26% 0.028 0.359131Xe 3/2 21% -0.009 -0.22719F 1/2 100% 0.441 -0.109