(direct) Dark Matter Searches and XMASS experiment  DM search: status, prospects and difficulties for large scale experiments  XMASS experiment S. Moriyama Kamioka Observatory Institute for Cosmic Ray Research March 3 rd 2007, KEKTC7

 A lot of evidences on the existence of DM 1. Orbital velocities of galaxies in clusters 2. Rotational speed of galaxies 3. Gravitational lensing of background objects by galaxy clusters 4. Temperature distributions of hot gas in galaxies and clusters of galaxies 5. CMB power spectrum analysis in WMAP 6. The Bullet cluster 7. Large scale map for dark matter 8. … Dark Matter in the Universe  A lot of evidences on the existence of DM WMAP Cosmic Microwave Background power spectrum decomposed matter, cold dark matter, and dark energy NASA The Bullet cluster: NASA Red: ordinary matter from X-ray Blue: mass from gravitational lensing All of them are astrophysical observations. “Direct detection” is an urgent issue now.

WIMP Dark Matter  Particles thermally generated at high temperature decouple when the expansion rate ~ the interaction rate.  Weakly interacting, heavy, neutral, and stable particles can be CDM.  (WIMPs != SUSY here) E.W. Kolb and M.S. Turner, The Early Universe X=m/Temperature (time  ) Comoving number density N equillibrium Increasing

Dark Matter in the Milky Way  Also, the Milky Way (our galaxy) has a large amount of Dark Matter. R.P.Olling and M.R.Merrifield MNRAS 311, 369- (2000) Buldge Steller disk Dark Halo

Motion of WIMPs around us Earth Sun Annual, sidereal modulation DM density ~0.3GeV/cc 100GeV WIMPs  1 WIMP / 7cm cubic,  =10 5 /cm 2 /sec The solar system is moving at ~230km/sec. The earth is moving at ~30km/sec. The earth is rotating. WIMPs are randomly moving with Maxwell distribution ~270km/sec “Wind” of Dark Matter

(direct) Detection method  Since they are neutral and stable, what we can expect is only a collision with ordinary matter.  Electron recoil does not give enough energy but nuclear recoil gives ~100keV if m DM ~O(100GeV). Dark Matter particles Energy deposit

What is the best target? Recoil spectrum (spin independent)  Ge and Xe, and Ar give similar rates.  Annual modulation should be seen. Si Ge Xe Si Xe Ge Red: differential, Blue: integrated Target Xe Assuming quenching factor of 0.2 June Dec. R.J.Gaitskell, Ann. Rev. Part. Sci., 54 (2004) 315. WIMP mass 100GeV Diff. ~6%

Candidate: Neutrarino in the Supersymmetric Model  SUSY solves the hierarchy problem, stability of boson mass, and it can predict the gauge coupling unification. The prime paradigm beyond the standard model.  Even if SUSY particles are discovered in LHC, direct detection of DM should be done INDEPENDENTLY. SM particlesSUSY particles Quark Lepton Higgs Gauge particles udcstb e e  g W  Zudcstb ~ ~ ~ ~ ~ ~ e e   ~ ~ ~ ~ ~ ~ H+H+H+H+ H-H-H-H- H10H10H10H10 H20H20H20H20 ~ ~ ~ ~ gW Z0Z0Z0Z0 ~ ~ ~ ~ A0A0A0A0 H+H+H+H+ H-H-H-H- H10H10H10H10 H20H20H20H20 Neutralino

(1pb = cm 2 ) Expected cross section to nucleon  Large space of  SI for proton.  SNOWMASS benchmark, many of them: pb General MSSM  SI for proton (pb) M  (GeV) Y.G.Kim et al

Mission in the real world  Once we make detector, we have 10~100Hz background by ambient gamma rays and internal radioactive contaminations, and neutrons.  What we need to realize is “1count/100day detector”.  reduction needed

DM Searches: past NaI(Tl) at Gran Sasso Exposure 107 ton day Annual modulation over 7 years 6.3sigma C.L. annual modulation 2-6keV data: / cpd/kg/keV T0=140+/-22 days (exp th day) T=1.00+/-0.01 year No modulation: 7x10 -4 (  2 /d.o.f.=71/37) DAMA: NaI(Tl) scintillator LIBRA experiment will reject/confirm the result. astro-ph/

DM searches: recent, CDMS (Ge)  Blind analysis done.  World best limit obtained: ~O(kg) 4x250g Ge 2x100g Si BG from , e: E Q ~E P Signal : E Q ~0.3E P Ionaization E Q Bolometric E P Rise time of Ep, timing diff. btw E P & E Q 1 st run Oct.03-Jan.042 nd run Feb.04-Aug.04 Recoil energy (keV) BG band Signal band ~E Q /E P

CDMS to SuperCDMS: 25kg seems to be straight forward

DM searches: XENON (2 phase LXe) XENON10: 10kg target first results soon! XENON100: 100kg target construction M. Yamashita IDM2006 Liquid xenon scintillator with charge coll. Rare gas liquid: scale up, purification easy

DM searches: WARP/ArDM (Liquid Ar) P.Benetti astro-ph/ S2/S1 + difference of scintillation waveform for e and nuclear recoil e n Integrated waveforms 1s1s Log(S2/S1)  n-like e-like  Pulse shepe parameter  e-like n-like  CDMSII WARP Need to remove 39Ar (0.8Bq/kg) 100 liter detector soon?

Future: larger scale Exp. General MSSM  SI for proton (pb) M  (GeV) Y.G.Kim et al B. Sadoulet KEKTC6 ~4events/100ton/year  pb is really challenging!! ~4events/100kg/year Xe recoil E >25keV  +5-10year ~ -10 pb CDMSII (current) Sorry for approximate line  2007 CDMS pb DAMA  2003 DAMA pb

pb detection requires keV Super-K KamLAND (>0.8MeV) Heidelberg Moscow Kamioka Ge ZEPLIN before PSD cut DM signal for LXe 100GeV 10-6pb QF=0.2 DAMA NaI Events/kg/keV/day BG required For pb CDMSII Atm, solar neutrino NC nuclear recoil appear

Difficulties appear ~ pb  Scale-up is always accompanied with new background sources which are caused by: 1. Low E threshold 2. Surface effect 3. Impurities in the targets 4. Insufficient rej. eff. 5. Neutron background  Small signals  Surface  &  rays  39 Ar, 85 Kr  Large # of BG >10 8  muon, parts inside neutron ? Target (Ge, LAr, LXe) Neutron BG will be dominated <10 -9 pb ~Full target scheme i.e. Ge, XENON, WARP single neutron events cannot be discriminated Neutron background

neutron e,  Strategy for larger target, high sensitivity  XMASS!  Neutron shielding by large amount of target itself.  Currently, “gamma rejection” is extensively studied. CDMS, XENON, and WARP have good discriminations with multiple information.  However, <10 -9 pb, neutron will be most serious BG. It cannot be discriminated. N. Spooner, DM2004

MC:  outside Xenon Blue : γ tracking Pink : whole liquid xenon Deep pink : fiducial volume U-chain gamma rays XMASS (liquid Xe sci.) at Kamioka Self shielding with large target for gamma rays 80cm dia. 800 kg  All volume 20cm wall cut 30cm wall cut (10ton FV) Large self-shield effect External  ray from U/Th-chain  BG can be efficiently reduced in < 500keV low energy region 1MeV02MeV3MeV 250cm dia. 23 ton BG normalized by mass

Self shield for fast neutrons (5MeV)  Assumption: irreducible BG is only single neutron scattering  1/100 reduction is possible in 30cm thickness. Red: single recoil 250cm dia. 23 ton Fiducial volume Single recoil Vertex distribution Evis>5keV Distance from the center (cm) FV # of events (cm -3 ) n

100kg Prototype FV 3kg 800kg detector FV 100kg 23 ton detector, FV 10t ～ 30cm ～ 80cm ～ 2.5m R&D Dark matter search Multipurpose detector (solar neutrino,  …) “Full” photosensitive, large volume detector Realize ~FULL COVERAGE Neutron BG not serious Neutron shield 30cm outer layer >1/100

Vertex reconstruction Large photoelectron Yield can be obtained ~5 p.e./keV Accurate vertex Reconstruction based on light pattern possible Liq. Xe (31cm) 3 MgF 2 window 54 2-inch low BG PMTs 16% photo- coverage Hamamatsu R8778 Confirmation with a prototype detector done.

Demonstration with a prototype detector I DATA MC A B C +++ Reconstruction works well Data well reproduced by MC 30cm cube Gamma ray injection from collimators

Confirmed Self shielding >2 orders of magnitude agreements Gamma rays Z= +15Z= Cs: 662keV DATA MC PMT saturation cm-15+15cm ~1/200 ~1/10 Reconstructed Z Arbitrary Unit Co: 1.17&1.33MeV DATA MC Reconstructed Z Demonstration with a prototype detector II

Detail design of the 800kg detector GEANT4 base simulation done 6cm RMS for boundary of FV  Good performance

Background of the 800kg detector  Background origins Neutrons: enough by water shield PMT  rays: low BG PMT Radioactive impurities: distillation PMT  n RI in LXe

Background from PMTs ( 238 U  ) 1.8 mBq- 238 U/PMT, most clean PMT ever made (achieved) c.f. usual “low” activity PMT ~O(1Bq/PMT) BG <100keV 5cm self shield: ~10 -3 /day/kg/keV 10cm self shield: ~10 -4 /day/kg/keV 20cm self shield: ~10 -5 /day/kg/keV (2events/100kg/5keV/year) Fiducial cut Red: 100kg Normalized by volume (day -1 kg -1 keV -1 )

Radioactive contamination Measured with the prototype detector U, Th, Kr near to the goal. Within reach. Internal origin of background Target values to achieve our goal 238 U: < 1x g/g 232 Th: < 2x g/g 85 Kr: < 1ppt from reactors (9±6) x g/g Further reduction by filter < 23 x g/g Upper limit, use filter 3.3±1.1 ppt Distillation system (original)

Sensitivity of the 800kg detector x 100 sensitive than CDMSII XMASS FV 0.5ton year (100kg, 5yr) 3  discovery W/O any pulse shape info. Plots exept for XMASS Gaitskell & Mandic

Near future ~5years  Ge, Ar and Xe go down to pb  Aim to detect DM ~ Cross section to nucleon (pb)

+10yr, sensitivity of DM exp. < pb  ~10ton volume  Thick outer shield (~30cm) will reduce neutron BG for further >100! Cross section to nucleon (pb)

Summary  Direct detection of DM, WIMPs, is an urgent issue.  Solid state & Noble gas liquid are the world trend for large scale experiments.  Search with <10 -9 pb, neutron will be most serious background, and it should be shielded by the target volume itself.  XMASS  R&D efforts for XMASS done.  XMASS at Kamioka is going to improve factor 100 in the sensitivity (~10 -9 pb).  Serious competition in the world. Need to start now!