WIMP Dark Matter Search Sun Kee Kim Seoul National University For the KIMS collaboration

KIMS Collaboration H.J.Ahn, J.M.Choi, H.Y.Yang, M.S.Yang, S.C.Kim, S.K.Kim, T.Y.Kim, H.S.Lee H.S.Park, I.H.Park, E.I.Won, H.S.Won (Seoul National Univ., Korea) W.K.Kang, Y.D.Kim (Sejong Univ., Korea) M.J.Hwang, H.J.Kim, J.H.Lee, Y.J.Kwon (Yonsei Univ., Korea) I.S.Han, J.H.Keem (Ihwa Womans Univ., Korea ) I.S.Cho, D.H.Choi, S.H.Noh, I.T.Yu (SeongKyunKwan Univ., Korea) S.Y.Choi (Chonbuk National Univ., Korea) P.Ko (KAIST, Korea) M.H.Lee, E.S.Seo (Univ. Maryland, USA) H.B.Li, C.H.Tang, M.Z.Wang (National Taiwan Univ., Taiwan) W.P.Lai, H.T. Wong (Academia Cinica, Taiwan) J.Li, Y.Liu, Q.Yue (Inst. Of High Energy Physics, China) B.Xin, Z.Y.Zhou (Inst. Of Atomic Energy, China) J.J.Zhu(Tsing Hua University, China)

Brief History of KIMS 97 Summer : First discussion on WIMP search(cryogenic detector) 97 Fall : Started R&D on CsI(Tl) for WIMP search 98 Summer : First result at ICHEP98 99 Spring : Started background measurement at Cheongphyung 99 Summer : Started measurement of intrinsic background from crystal, shielding material 99 Fall : Expanded the collaboration 00 Spring : Prototype shielding structure installed 00 Summer : Approval of the proposal for CRI 00 Fall : DMRC established/ KIMS collab. Expanded 01 March : Taiwan, China joined KIMS collab.

The phrase "dark matter" means matter whose existence has been inferred only through its gravitational effects Particle data group

Evidence for the existence of Dark Matter Rotational curves of galaxies Rotation of galaxies in clusters of galaxies … matter 0.1 ~ 0.3 However, lum < 0.01 dark matter = matter - lum NGC6503 K.G.Begeman et. al. Mon. Not. R. Astr. Soc. 249, 523(1991)

Dark Matter Candidates White Dwarfs Brown Dwarfs Neutron Stars Black Holes Baryonic Dark Matter Candidates Hot Dark Matter (HDM) Cold Dark Matter (CDM) Light neutrino ~ few tens of eV Axions ~ 10-5 eV WIMP’s (Weakly Interacting Massive Particles) a) massive neutrino Dirac ~ excluded by Ge Detector Majorana > 20 GeV (LEP) b) SUSY(Super Symmetry) Particles s-neutrino ~ exluded by LEP Neutralino > 30 GeV (LEP) Non-Baryonic

WIMP Certain classes of SUSY model predict Neutralino as LSP : stable, weak interaction scale annihilation cross section gives proper relic density for dark matter  Excellent CDM candidate In MSSM,

How to detect WIMP ? Elastic Sacttering of WIMP off a nuclues in the crytsal WIMP 10-6 ~ 10-10 pb Cs Expected event rate ~ 1/kg/day or less I Recoiled nucleus Energy loss by ionization and lattice vibration

Difficulties Needs large amount of sensitive detector material Cross section is very small : < 10-6 pb Needs large amount of sensitive detector material Recoil energy of nucleus is also very small : 10 ~ 100 keV Qunching effect reduce visible energy further by ~1/10 Detector technique to measure ~ few keV Background is very large : neutrons, -rays, cosmic rays Underground experiment Careful shielding/detector material selection Additional techniques to reject gamma background

WIMP detectors Scintillators : NaI, CsI(Tl), Xe,… Ionization loss → visible light →PMT Relatively cheap to acquire large mass Difficult to reject background - pulse shape discrimination can reject gamma background DAMA, UKDMC, … Low temperature detectors : Si, Ge,… Phonon excitation → temperature change → SC transition Good gamma rejection when combined with ionization loss measurement Expensive to acquire and operate large mass CDMS, CRESST,…

On going experiments DAMA NaI(Tl) crystal 58 kg Gran sasso underground lab. Low threshold ~ 2keV Low background ~ 1 cpd Poor gamma background rejection CDMS Low temperature detector (20mK) Si 100 g, Ge 495g Stanford, 16 m w.e. Threshold ~ 10 keV background ~ 60 cpd (2cpd with veto) Excellent gamma background rejection

CDMS and DAMA results are Status of WIMP Search CDMS limit 1999 CDMS limit 2000 DAMA limit 1996 DAMA 1st positive result based on annual modulation CDMS and DAMA results are not consistent Upgrade DAMA is upgrading to 200 kg CDMS is moving into Soudan mine

WIMP hitting rate depends on season Annual modulation WIMP hitting rate depends on season 30km/s Earth Sun 232km/s

Recoil Energy R : event rate R0: total event rate E0: most probable incident kinematic energy r : kinematic factor, 4MDMT/(MD+MT)2

What determines the sensitivity of WIMP search ? When no signal is observed and background exists, variance of signal is given by B : background rate in cpd(counts/keV/kg/day) M : Mass of the detector in kg, T : Days of data accumulation in days, Q : Quality factor Theory Experiment Eth :Threshold energy

To improve the sensitivity Lager detector mass, longer data taking  easy, but expensive Lower threshold  hard Lower background  very hard Smaller Quality factor  better separation of gamma background  needs new ideas

Sensitivity Mdet=100,300,600 kg Eth=1,2,3 keV Q dependence B = 1,5,10 cpd

CsI(Tl) Crystal Advantage Disadvantages High light yield ~50,000 photons/MeV Pulse shape discrimination Easy fabrication and handling High mass number CsI(Tl) NaI(Tl) Density(g/cm3) 4.53 3.67 Decay Time(ns) ~1050 ~230 Peak emmison(nm) 550 415 Hygroscopicity slight strong Disadvantages Emission spectra does not match with normal bialkali PMT => effectively reduce light yield 137Cs(t1/2 ~30y) ,134Cs(t1/2 ~2y) may be problematic

Amplification & self coincidence Oscilloscope or digitizer CsI(Tl) Detector Unit CsI(Tl) crystal PMT PMT Amplification & self coincidence Oscilloscope or digitizer coincidence Trigger Signal

Typical signals from CsI(Tl) 660 keV g Typical signals from CsI(Tl) 10 keV g 660 keV a

Photoelectron Yield # P.E./keV determines Eth, also with more P.E., better PSD => improve Q

PMT selection RbCs PMT on 3cmx3cmx3cm CsI(Tl) crystal 5.9 keV x-ray No. P.E./keV= nc/5.9 keV 6 p.e./keV

Full size crystal (7x7x30) : Photoelectron yield Full size crystal (7x7x30) : ~ 4 p.e./keV Small crystal(3x3x3) : 5~10 p.e./keV

Pulse Shape Discrimination

Quality factor  : fraction of signal events passing the cut  : fraction of  background passing the cut cut S B Ideal detector  ~ 1 ~ 0 Q << 1 S B Separation variable quality of separation of gamma background from signal

Quality factor NaI(Tl) Ideal detector  ~ 1 ~ 0 Q << 1 CsI(Tl) Measured Energy(keV)

Quenching factor Light yield differs by different incident particles an emprical fomular by Birks Recoiled nucleus yields smaller amount of light than recoiled electron by gammas Quenching factor = Egamma_equivalent/Erecoil Needs to know Quenching factor to extract Erecoil

Neutron Beam Test KIGAM(지질자원연구원) 3.2 MeV p => 2.4 MeV n

QF = Emeasured / Erecoil Quenching factor n QF = Emeasured / Erecoil n

Background Reduction External Background Cosmic ray muons – produce n,   Underground laboratory Environmental radio-isotopes – U, Th, K, …  Heavy shielding (Cu, Pb …) Internal background Within crystals – Rb, 137Cs, …  Material study – chemistry In other components – Shielding, PMT, cables, …  Careful selection of materials

Underground Lab. at Cheongpyung Homyung Mt.(虎鳴山) Reservoir Access tunnel(1.4km) 350m Pukhan River(北漢江) Laboratory Power plant

April 2000

April, 2001 April, 2001

Prototype Shielding 15cm Pb + 10 cm Cu

Background in underground Lab. Cosmic rays : ~ 10-4 relative to the sea level Gamma background measured with HPGe detector ~ 10-4 reduction with 15cm Pb + 10cm Cu * Significant portion of residual background is suspected to be from HPGe itself

Neutron background Measured with 0.5 liter BC501A liquid scintillator ~ 1.7x10-5 /cm2/sec After correcting efficiency and detector volume ~ 5 cpd in CsI detector expected w/o neutron shielding with ~ 30 cm liquid scintillator active shieling < 0.05 cpd can be achieved

Intrinsic background Radioisotopes in the crystal - most important background after the proper shielding 137Cs : half life = 30.07 year (produced by atomic bomb and reactor accidents) Beta decay to 137Ba meta stable state (Q = 1175.6 keV) 2min life time, emitting 661.6 keV gamma Hard to reject => serious background at low energy 134Cs : half life = 2.065 year( produced by cosmic ray) Beta deacy to 134Ba (Q=2058.7 keV) immediate gamma emission Can be rejected easily => not a serious problem 87Rb : half life = 4.75 x 1010 year (27.8% natural abundance) Beta deacy to 87Sr (Q=282.3 keV) no gamma emission Hard to reject => potentially a serious problem => reduction technique in material is known

Background in CsI(Tl) crystals : Single Crystal Institute (Ukraine) : Shanghai Institute of Ceramics (China) : Crismatec (France)

Sources of intrinsic background In the crystals currently available g-rays and b-rays of 87Rb, 134Cs, and 137Cs + GEANT 87Rb 134Cs 137Cs Total <Measured spectrum> 137Cs : ~0.03 Bq/kg, 134Cs : ~0.03 Bq/kg, 87Rb : < 20 ppb

Requirement for 1 cpd 87Rb < 0.2 ppb 137Cs < 0.001 Bq/kg

When do they enter ? 137Cs in powder 134Cs : 605 keV 137Cs : 662 keV CsI powder 134Cs : 3~6x10-2 Bq/kg 137Cs : 3~9x10-2 Bq/kg CsOH/CsNO3 134Cs : 3~4x10-2 Bq/kg 137Cs : 2~3x10-2 Bq/kg 134Cs : 605 keV 137Cs : 662 keV

When do they enter ? Pollucite : Upper bound only <6x10-2 Bq/kg New measurement < 1x10-2 Bq/kg ? Water : 3~6x10-2 Bq/liter

In Pollucite : < 0.009 Bq/kg 137Cs in Pollucite ? In CsI : 0.09 Bq/kg In Pollucite : < 0.009 Bq/kg

Summary of Intrinsic background 134Cs : not a problem - already below the requirement 87Rb : can be reduced 137Cs : does not exist in the pollucite suspected to be inserted during extraction of Cs powder measurement of 137Cs contamination in water - contains 137Cs ~ 0.01 Bq/liter * a lot of water is used in the process of extracting Cs powder ~ 145 liter/ 1kg Cs (from a company) R&D with CsI powder companies is ungoing

CsI(Na) crystal ? More light yield than CsI(Tl) due to better matching with PMT Lower threshold Maybe better PSD(?) Hygroscopic behavior may be a problem ~70% more light yield CsI(Tl) CsI(Na) PSD under investigateion

Summary of detector R&D CsI(Tl) detector Low energy threshold ~ few keV ( photoelectron yield ~ 4 p.e./keV ) g/a separation with pulse shape analysis Neutron beam test : quenching factor measured Background Cosmic ray reduction at 400 m underground : ~1/10000 Gamma-ray background reduction with shielding : ~1/10000 Intrinsic background : 137Cs exists in CsI powder => not in the pollucite (?) => need to understand Cs extraction procedure

Pilot Experiment Purpose : To test the concept of the experiment and to find possible problems for the main experiment One 7cm x 7 cm x 30 cm crystal + 12 surrounding veto crystals + extended prototype shielding will start this month !

Experiment Setup (base line design) CsI(Tl) 9cm x 9cm x 30cm ~ 11 kg PMT (3 inch RbCs) 5x5 = 25 crystals ~ 275 kg

Experimental Setup (base line design) 220 cm 140 cm 140 cm Total weight ~ 35 t

WIMP search Prospect After 1 year data taking with 100 kg CsI(Tl) * 3x3 crystals ~ 100 kg assume 2 keV threshold

Summary & Prospect Underground Lab. at CheongPhyung is being established Environmental background : small enough Comfortable place for long term experiment CsI(Tl) crystal R&D has been carried out Low energy can be measured with good resolution Pulse shape discrimination of -rays : promising Intrinsic background : alsmost understood Prospect Pilot run with ~ 7kg CsI(Tl) will start soon Started engineering design of the detector setup ~100 kg CsI(Tl) crystal → plan to start to data taking this year 1 year data taking would confirm or reject DAMA result

Astroparticle physics session Electroweak session Neutrino session Physics In Collision 2001  June 28-June 30, 2001  http://neutrino.snu.ac.kr/pic2001 The conference will review and update key topics in elementary particle physics, with the aim of encouraging informal discussions experimental results and their implications. New Phenomena session QCD session Heavy flavor session Astroparticle physics session Electroweak session Neutrino session