WIMP search Result from KIMS experiments

Kim Seung Cheon (DMRC,SNU)

WIMP

( Weakly Interacting Massive Particle)

search

One of the strong candidates of dark matter ,introduced from the supersymmetry theory Interacting rarely -> Below 1 cpd (count/kg/keV) Expected recoil energy to the target nucleus -> a few keV - a few tens keV By measuring nuclear recoil energy, We can observe WIMP scattering.

KIMS’ WIMP search tool -> CsI(Tl) crystal scintillator High light yield : ~60000 photons / MeV Easy fabrication & Handling Easy to have the large mass with an affordable cost ,enabling study of seasonal variation of WIMP interaction Better Pulse Shape Discrimination than NaI(Tl) Sensitive to spin dependent and independent interaction of WIMP both -> Complementary to other pioneering Spin independent WIMP Search (CDMS, Xenon-10)

CsI crystal detectors & data collected CsI(Tl) Crystal 8x8x30 cm 3 (8.7 kg) x 4 3” PMT (9269QA) : Quartz window, RbCs photo cathode ~5 Photo-electron/keV Crystal S0501A S0501B B0510A B0510B Total p.e./keV 4.6

4.5

5.9

5.9

Mass (kg) 8.7

8.7

8.7

8.7

34.8

Data (kg·days) 1147 1030 616 616 3409

Neutron calibration 300 mCi Am/Be source  a few 100 neutrons/sec hit 3cm  3cm  3cm crystal (sample)  Quenching factor of Recoil Energy Nuclear Recoil mean time distribution

Neutron sample Gamma Full size Gamma sample

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

PMT data without crystal PMT noise significantly limits the sensitivity at the low energy range.

PMT only data taken ~350 kg days for each crystal with the PMTs used for each crystal to understand the PMT noise event PMT Clean acryl box PMT

PMT data without crystal WIMP search data PMT only data

Event selection Likelihood fit with F(t) = 1/τ f exp(-(t-t 0 )/τ f )+r/ τ s exp(-(t-t 0 )/τ s ) Gamma Neutron PMT only (dot-dashed) WIMP search DATA

Event selection Gamma PMT only (dot-dashed) WIMP search DATA 3-5keV Asymmetry = (PMT1 –PMT2)/total PMT 5-7keV

Event selection coincidence cut spectrum of the sum of energy of all the crystals

Effect of cuts on MT distribution

Data Gamma Neutron PMT

Log(MT) distribution at various cut stage

Selected Events WIMP search Data after cut WIMP search Data after efficiency correction

Extraction of Nuclear Recoil event Neutron calibration gamma calibration Fit the WIMP search data with PDF function from gamma and neutron calibration data  extract NR events rate

Extraction of Nuclear Recoil event Estimated nuclear recoil rates

WIMP Nucleus Cross section Assuming WIMP forms a spherical halo around our galaxy.

has a Maxwellian velocity distribution.

-> Interaction rate of WIMP with the target nucleus (according to WIMP mass & Cross-section) WIMP density ρ D =0.3 GeV/c 2 /cm 3 Earth’s velocity v 0 =220km/s Local galactic escape velocity v esc =650km/s I Cs σ WIMP-p = 7.2x10

-6 pb WIMP Mass = 50 GeV

Simulation of expected energy spectrum Fix σ WIMP-p = 7.2x10

-6 pb, WIMP Mass = 50 GeV I Cs GEANT4 Simulation considering Quenching factor, Energy resolution, form factor, detector character

Limit on WIMP nucleus cross section 90% CL Central value  

W A

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 

W A

Spin independent limits WIMP density ρ D =0.3 GeV/c 2 /cm 3 Earth’s velocity v 0 =220km/s Local galactic escape velocity v esc =650km/s Nuclear recoil of 127 I of DAMA signal region is ruled out.

PRL 99, 091301 (2007)

Spin dependent limits Pure proton case Pure neutron case PRL 99, 091301 (2007)

Conclusion of this data analysis New results using 3409 kg days data Published in PRL 99, 091301 (2007) DAMA signal region is ruled out for both SD and SI Interactions at WIMP mass > 20 GeV World best limit on SD interactions for pure proton case

Current status 12 crystals(104.4kg) installed. 1 st Calibration run was over.

Started data taking.

Current background level cpd (count/kg/day) 2-3 cpd keV

prospect 12 full size crystals(8x8x30cm 3 ) ~ 100 kg installed ->Current shielding can accommodate 250 kg.

Data taking was started.

Internal backgrounds reduced to 2cpd level.

After 1-2 year operation, Annual modulation of WIMP wind can be studied.