The XMASS 800kg Experiment Jing LIU IPMU, Univ. of Tokyo TAUP2011, Munich XMASS collaboration: Kamioka Observatory, ICRR, Univ. of Tokyo ： Y. Suzuki, M. Nakahata, S. Moriyama, M. Yamashita, Y. Kishimoto, Y. Koshio, A. Takeda, K. Abe, H. Sekiya, H. Ogawa, K. Kobayashi, K. Hiraide, A. Shinozaki, S. Hirano, D. Umemoto, O. Takachio, K. Hieda IPMU, University of Tokyo ： K. Martens, J.Liu Kobe University: Y. Takeuchi, K. Otsuka, K. Hosokawa, A. Murata Tokai University: K. Nishijima, D. Motoki, F. Kusaba Gifu University ： S. Tasaka Yokohama National University ： S. Nakamura, I. Murayama, K. Fujii Miyagi University of Education ： Y. Fukuda STEL, Nagoya University ： Y. Itow, K. Masuda, H. Uchida, Y. Nishitani, H. Takiya Sejong University ： Y.D. Kim KRISS: Y.H. Kim, M.K. Lee, K. B. Lee, J.S. Lee

The XMASS Experiment XMASS 800kgJing TAUP20112 Xenon MASSive detector for Solar neutrino (pp/ 7 Be) Xenon neutrino MASS detector (double beta decay) Xenon detector for weakly interacting MASSive Particles ~100 kg LXe prototype ~800 kg LXe direct dark matter search ~26 ton LXe Multi-purpose LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe PMT LXe inside

The XMASS Experiment XMASS 800kgJing TAUP20113 Xenon MASSive detector for Solar neutrino (pp/ 7 Be) Xenon neutrino MASS detector (double beta decay) Xenon detector for weakly interacting MASSive Particles ~100 kg LXe prototype ~800 kg LXe direct dark matter search ~26 ton LXe Multi-purpose LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe PMT LXe inside , n, ,… or  ? PMT LXe scintillation

The XMASS Experiment XMASS 800kgJing TAUP20114 Xenon MASSive detector for Solar neutrino (pp/ 7 Be) Xenon neutrino MASS detector (double beta decay) Xenon detector for weakly interacting MASSive Particles ~100 kg LXe prototype ~800 kg LXe direct dark matter search ~26 ton LXe Multi-purpose LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe PMT LXe inside PMT LXe

The XMASS Experiment XMASS 800kgJing TAUP20115 Xenon MASSive detector for Solar neutrino (pp/ 7 Be) Xenon neutrino MASS detector (double beta decay) Xenon detector for weakly interacting MASSive Particles ~100 kg LXe prototype ~800 kg LXe direct dark matter search ~26 ton LXe Multi-purpose LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe PMT LXe inside

The XMASS Experiment XMASS 800kgJing TAUP20116 Xenon MASSive detector for Solar neutrino (pp/ 7 Be) Xenon neutrino MASS detector (double beta decay) Xenon detector for weakly interacting MASSive Particles ~100 kg LXe prototype ~800 kg LXe direct dark matter search ~26 ton LXe Multi-purpose LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe PMT LXe inside

The XMASS Experiment XMASS 800kgJing TAUP20117 Xenon MASSive detector for Solar neutrino (pp/ 7 Be) Xenon neutrino MASS detector (double beta decay) Xenon detector for weakly interacting MASSive Particles ~100 kg LXe prototype ~800 kg LXe direct dark matter search ~26 ton LXe Multi-purpose LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe PMT LXe inside

Structure of XMASS 800kg detector XMASS 800kgJing TAUP20118 PMT

Structure of XMASS 800kg detector XMASS 800kgJing TAUP20119 PMT Pentakis dodecahedron ~10 PMTs in one triangle 642 PMTs in total  ~0.8 meter PMT photo-cathodes cover ~62% inner surface

Reconstruct interaction point from PMT hit pattern XMASS 800kgJing TAUP Colored photo cathodes indicating number of p.e. (photoelectrons) recorded by PMTs Interaction point (vertex) can be reconstructed from the PMT hit pattern.

LXe self-shielding XMASS 800kgJing TAUP Simulation:  into LXe water LXe E  [keV] Attenuation length of  [cm]

Where is it? XMASS 800kgJing TAUP m rock overburden (2700 m water equiv.): Muon: 6.0x10 -8 /cm -2 /s/sr Neutron: 1.2x10 -6 /cm -2 /s 360m above the sea Horizontal access: 15 minutes drive from office, too easy to get in! No excuse to avoid 24-hour shift  Kamioka underground observatory

XMASS 800kgJing TAUP New experimental hall, Aug meter 20 meter

XMASS 800kgJing TAUP water tank, Nov m x 10 m

XMASS 800kgJing TAUP Frame of clean room in water tank, Mar. 2008

XMASS 800kgJing TAUP PMT mounting in clean room, Dec. 2009

XMASS 800kgJing TAUP PMT mounting finished, Feb. 2010

XMASS 800kgJing TAUP Water filling, Sep. 2010

XMASS 800kgJing TAUP Commissioning started, Nov. 2010

Scintillation light yield :: calibration system XMASS 800kgJing TAUP Top PMT can be pulled out Source changed here Z position of source is controlled by a motor on top at <1 mm accuracy x y z 57 Co, 241 Am, 109 Cd, 55 Fe, 137 Cs  4mm  0.15mm  for 57 Co

Scintillation light yield: 15.9  1.2 p.e./keV ( 57 Co at center) XMASS 800kgJing TAUP Data MC 122 keV 136 keV 59.3 keV (W) Number of photoelectrons Arbitrary unit

Position & energy resolution (122keV  from 57 Co) Data Reconstructed vertices for various source positions Position resolution (RMS): 1.4 z = 0 cm 1.0 z =  20 cm Reconstructed energy [keV] Arbitrary unit Data MC 122 keV 136 keV 59.3 keV (W) RMS ~4% XMASS 800kgJing TAUP y [cm] z [cm] Data MC Arbitrary unit

Background under control? External – Comic ray: underground, muon veto – Ambient gamma & neutron: water shielding – PMT radiation: LXe self-shielding Internal – Rn: material screening, clean room filled with Rn free air – Kr: distillation XMASS 800kgJing TAUP201123

Xe water X [cm] y [cm] Ambient  and n: pure water tank,  ~10 meter XMASS 800kgJing TAUP Pure water tank (large enough for 26 ton LXe) equipped with 20 inch PMTs on the wall as active muon veto and passive ambient  and n shielding 20 inch PMTs Water tanks LXe sphere 10 7 neutrons, simulation  <<  from PMT, n<<10 -4 /d/kg

LXe copper cryostat Calibration pipe   n n PMT radiation: Ultra low background PMTs XMASS 800kgJing TAUP Neutron: <1.2x10 -5 keV

PMT & PMT holder radiation: LXe self-shielding XMASS 800kgJing TAUP Energy [keV] Counts [dru ] Simulation:  into LXe fiducial volume: r<20cm, 100 kg LXe BG/PMT [mBq] U chain 0.70  0.28 Th chain 1.51  K< Co 2.92  0.16

85 Kr (Q  =687keV) : distillation XMASS 800kgJing TAUP Kr LXe intake LXe outlet Gas Kr outlet K. Abe et al. for XMASS collab., Astropart. Phys. 31 (2009) 290 Kr can be boiled out from LXe 0.1 ppm  ~1ppt (~1 ton in 10 days)

Kr concentration XMASS 800kgJing TAUP Xenon sample bg sample 1 bg sample 2 Kr concentration: < 2.7 ppt (90% C.L.) Measured by gas chromatography + API mass spectrometer

222 Rn XMASS 800kgJing TAUP p0 * exp(-t/  ) + p1,  : decay constant Time difference [  s] 1 st event ( 214 Bi  ) 2 nd event ( 214 Po  ) Tail due to saturation 214 Po decays with 164  s half life. It can be identified by time coincidence between two consecutive events: Bi  decays into 214 Po Po  decays into 210 Pb x10 3 Number of photoelectrons Events 8.2  0.5 mBq Events

220 Rn XMASS 800kgJing TAUP p0 * exp(-t/  ) + p1,  : decay constant 216 Po decays with 140 ms half life Time difference [ms] Events x10 3 Number of photoelectrons 1 st event 2 nd event <0.28 mBq (90%C.L.)

Target sensitivity of WIMP-nucleon XS (spin independent) XMASS 800kgJing TAUP XENON100 CDMSII XMASS 1yr Expected energy spectrum assuming 1 year exposure flat background (10 -4 dru)   = cm 2 M WIMP = 50 GeV L eff = 0.2 Black: signal + background Red: background (10 -4 dru)

Conclusion The XMASS 800kg detector is a single phase LXe scintillation detector Construction of the 800kg detector finished last winter Commissioning runs are on going to confirm the detector performance and low background properties – Energy resolution and vertex resolution were as expected. ~1cm position resolution and ~4% energy resolution for 122 keV . – Radon and Kr background are close to the target values. The physics results are on the way XMASS 800kgJing TAUP201132

XMASS 800kgJing TAUP201133

PMT holer made of OFHC copper

3 Steps in reconstruction Step1 – Search the map grid where likelihood becomes smallest. Step2 – By linear interpolation, calculate likelihood at finer “interpolated grid” in Cartesian coordinate. – 1.5cm interval, 5x5x5 finer grid points are evaluated. Result grid of step1 1.5cm    PMT pe  L) (pe+1) )exp( Log()  Ln likelihood is calculated from the expected pe and observed pe. Using gamma distribution. Treatment of saturated PMT, cumulative probability of gamma distribution. z x y

Integral spectrum Assumption 1x10^-44 cm^2 100 kg X 1 year exposure WIMP Mass [GeV] No. of Events keVr(5keVee) energy threshold 100kg x 1 year

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