1. XMASS 2012 Shanghai Particle Physics and Cosmology Symposium Hiroshi Ogawa (ICRR, Univ. of Tokyo) Sep. 16 th, 2012

2. XMASS experiment Dark matter Double beta Solar neutrino 2 pp/7Be chain: n+e  n+e 136Xe  136Ba + 2e- χ+Xe  χ+Xe Multi purpose low-background and low-energy threshold experiment with liquid Xenon Large photon yield Low threshold Scintillation wavelength (175 nm, detected directly by PMT) High density (~3 g/cm 3 ) Compact detector Large Z (=54) Self-Shielding ~-100 ℃ liquid : easy to use. Purification (distillation) No long life radioactive isotope  Liquid Xenon :

3. Direct Detection Principle From the density of dark matter in the galaxy: Every liter of space: 10-100 WIMPs, moving at 1/1000 the speed of light Less than 1 WIMP/week will collide with an atom in 1kg material WIMPs elastically scatter off nuclei in targets, producing nuclear recoils. Xe (A=131) is one of the best target. R 0 : Event rate F: Form Factor should be calculated in each nuclei Maxwellian distribution for DM velocity is assumed. v 0 :dispersion V :velocity onto target, V E : Earth’s motion around the Sun Spin independent case: Larger A is higher event rate Dark Matter （ WIMP ） Deposit Energy

4. XMASS : single phase detector –Large volume and simple structure, operation. 1 ton scale xenon detector, 100kg for fiducial volume. –Background reduction technique : Self shielding Reconstruction by hit pattern of PMTs –High light yields & Large photon coverage (15 pe/keV) Low energy threshold (< 5 keVee ~ 25 keVNR ) for fiducial volume Lower energy threshold: 0.3 keV for whole volume –Large Scalability, simple to construct. Characteristics of XMASS Self shielding 1ton 10ton

5. XMASS detector 5

6. XMASS detector : site KamLAND Super-K XMASS (Lab-C) CANDLES IPMU Lab1 CLIO NEWAGE Lab2/EGad Kamioka Mine Tokyo Kamioka Mine 10m 11m Water tank Refrigerator 72 20inch PMTs (veto) Elec. hut 72 20-inch PMTs are installed to veto cosmic-ray muon (<10 -6 for thr-mu, 10 -4 for stop-mu). Water is active shield for muon induced neutron and also passive shield for gamma-ray and neutron from rock/wall.

7. Structure of XMASS detector Detector –Single phase (scintillation only) liquid Xenon detector –Operated at -100 o C and  0.065MPa –100 kg fid. mass, [835 kg inner mass (0.8 m  )] –Pentakis-dodecahedron  12 pentagonal with 5 triangle –630 hexagonal & 12 round PMTs with 28-39% Q.E. –photocathode coverage: > 62% inner surface 1.2m diameter

8. Detector construction 2010 1 st application of WC tank for WIMP search By fall, 2010 8

9. Detector performance 9

10. Calibration –Source Rod (57Co, 241Am, 137Cs, 109Cd, 55Fe) –External sources: 60Co, 137Cs, 232Th, Neutron Normal Data taking (physics runs) Change of the physical condition of Xenon. –High/Low pressure run Change of the refractive index of Xe –O2 runs: change of the absorption length –Boiling runs: create convection inside of the detector PMT gain reduced run Evaluate the signal from radon daughter in xenon. Gas run -Important to identify the surface BG BG measurement of the detector parts （ attach the material at the end of the calibration source rod ） Al, GORE-TEX, Cu, Ni plate Data taking : commissioning run Nov.2010-June.2012

11. Detector response for a point-like source (~WIMPs) 57 Co source @ center gives a typical response of the detector. 14.7p.e./keV ee (  2.2 for S1 in XENON100) The pe dist. well as vertex dist. were reproduced by a simulation well. Signals would be <150p.e. exp shape. total photo electron data MC 122keV 136keV 59.3keV of W ~4% rms data MC reconstructed vertex +15V RI source with rod

12. RI in PMT Activity per 1PMT(mBq/PMT) 238U-chain0.70+/-0.28 232Th-chain1.51+/-0.31 40K-chain9.10+/-2.15 60Co-chain4.09+/-0.22 ~200keV Energy spectrum data and PMT MC ~1MeV ~50keV MC: PMT  rays Observed data ~x100 Measured Spectrum (Whole Volume) and background evaluation Gamma from PMTs  RI of PMT are measured by HPGe detector per PMT parts. (+40% for 60Co)  Data has large excess in <200keV compared with data.  ~x100 difference in ~5keV Gamma from PMTs had been assumed as mainly background sources

13. ~1MeV ~50keV MC: PMT  rays MC: PMT Al 238U & Pb210 Surface 210Pb Observed data ~1MeV Measured Spectrum (Whole Volume) and background evaluation Aluminum Seal RI from PMT aluminum seal –Aluminum sealing used for the PMT between quarts window and metal body. It contains 238U-230Th and 210Pb-206Pb. 210Pb on detector surface 222Rn daughter put btw. construction work These background are faced to liquid xenon. X-ray and beta from RI emit to Liquid xenon directly.

14. 14 GORE-TEX: between PMT and holder used for a light seal. It contains 0  6±3% of modern carbon. –GORE-TEX might explain the background below 5keV But parameters ( ex. transparency of light inside of GORE-TEX) are not well known We will remove GORE-TEX in future detector refurbishment Measured Spectrum (Whole Volume) and background evaluation Below 5keV 14C in GORE-TEX MC: GORE-TEX Modern C: 7.5% LXe inside scintillate 0.3mm photon att. ~5keV MC: GORE-TEX Modern C: 7.5% LXe inside scintillate 0.1mm photon att. ~5keV m DM = 30GeV  SI = 1.4x10 -41 cm 2 m DM = 18 GeV  SI = 1.6x10 -41 cm 2

15. Background estimates MaterialMeasured RI and activityMethods of the measurements PMTs (per PMT) 238U: 0.704 ± 0.282 mBq 232Th: 1.51 ± 0.31 mBq 60Co: 2.92 ± 0.16 mBq 40K: 9.10 ± 2.15 mBq HPGe detector measurement for each parts and whole PMT PMT aluminum (210g) 238U-230Th: 1.5 ± 0.4 Bq 210Pb: 5.6 ± 2.3 Bq 232Th: 96 ± 18 mBq 235U: ~67 mBq HPGe detector measurement.  By calculation Detector surface 210Pb: ~40 mBq Alpha candidates using FADC data Surface: PMT window 59%, PMT Al 7.0% PMT rim 7.0%, GORETEX 3.7%, Cu 23.3% (surface 7.8%, wall 14.2%, bottom 1.3%) GORE-TEX for PMTs (120g) 14C: 0.4 ± 0.2 Bq (6±3% of modern carbon) 210Pb: 26.5 ± 11.9 mBq 14C: modern carbon measurement. 210Pb: Ge measurement. Internal RI in xenon 85Kr: <2.7 ppt 214Pb: 8.2 mBq 85Kr : API-MS measurement 214Pb : ~222Rn concentration in detector

16. Low background even with the surface BG Our BG is still quite low, even with the extra surface BG! In principle, the surface BG can be eliminated by vertex reconstruction. Optimization of the reconstruction program is on going to minimize a possible leakage to the inner volume. E. Aprile, 2010 Princeton XMASS full volume Our sensitivity for the low mass WIMP signals at low energy without reconstruction will be shown. Evens/kg/day/keV 16

17. Physics analysis 17

18. Physics analysis (sensitivity study) Whole Volume Analysis (Large BG, Large target mass of 835 kg, low energy threshold, no reconstruction) 1) 4 hit threshold analysis Low mass region search 2) 1keVee threshold analysis Check DAMA modulation 3) Axion DM (super-WIMPs) 4) Solar Axions Fiducial Volume analysis 1) Standard WIMPs search (> 5 keV) Event reconstruction/reduction Standard WIMPs: Prefer heavy mass Low BG Light WIMPs & ALP: Low threshold BG less important

19. Physics analysis: Low energy, full volume analysis for low mass WIMPs The dark matter signal rapidly increase toward low energy end. The large p.e. yield enables us to see light WIMPs. Try to set absolute maxima of the cross section (predicted spectrum must not exceed the observed spectrum). The largest BG at the low energy end is the Cherekov emission from 40 K in the PMT photo cathodes. Light WIMPs & ALP: Low threshold BG less important Selection criteria for data:  Triggered by the inner detector only (no water tank trigger)  RMS of hit timing <100ns (rejection of after pulses of PMTs)  Cherenkov rejection  Time difference to the previous/next event >10ms

20. Detail of the Cherenkov rejection Basically, separation between scintillation lights and Cherenkov lights can done using timing profile. (# of hits in 20ns window) / (total # of hits) = “head total ratio” is a good parameter for the separation. Total # of hits 20ns Cherenkov likeScintillation like 20 Cherenkov events peaks around 1  scintillation ~ 0.5 Low energy events observed in Fe55 calibration source as well as DM simulation (t=25ns) show similar distributions. Efficiency ranges from 40% to 70% depending on the p.e. range. 0 40 80 120 160 200 (PE) Head total ratio Black: real data Blue: DM MC

21. p.e. distribution after each cut 6.8 days x 835kg data The Cherenkov events are efficiently reduced by the cut. We analyze the events above > 0.3 keVee for entire volume 21 ID only event (trigger ID == 1) dT > 10ms RMS of hit timing < 100ns Cherenkov cut

22. Uncertainties Major uncertainty is the scintillation efficiency of nuclear recoil in liquid xenon. Uncertainties of the trigger thre. (hard trig. 4hits), cut eff., and energy scale are also properly taken into account. Scintillation efficiency as a function of energy E. Aprile et al., PRL 105, 131302 (2010) 22

23. Spectrum and Sensitivity Spectrum shows that observed data and MC WIMPs signal with best fit per WIMPs mass. Sensitive to the allowed region of DAMA/CoGeNT. Some part of the allowed regions can be excluded. DAMA CoGeNT XMASS 23 WIMP cross section on nucleon (cm 2 ) XMASS observed energy [keVee] Count/day/kg/keVee GeV Preliminary

24. Progress of annual modulation analysis 1-4 keVee(Xe)  2-6 keVee (Na) Check the DAMA modulation QF(Na)  0.25, Leff(Xe)  0.15 2  6keVee(Na)  8  24 keV NR  1  4keVee(Xe) –but 1/30 sensitivity  recoil shape, A 2  2 : 10.8 for flat, 23.8 for a modulation Preliminary 2013 data taking (after refurbishment) will cover the maximum and minimum flux region.

25. Dark matter axion search in XMASS The DAMA signal may be due to electromagnetic interaction of WIMPs to the NaI detectors by such as a non-relativistic axion dark matter. See J. Collar, arXiv: 0903.5068 XMASS can search for dark matter axion by detection through axio-electric effect. We show a preliminary result based on 6.8 days x 835kg data. g aee

26. Limit on the axio-electric coupling DAMA allowed CoGeNT CDMS XMASS g aee Sensitivity with spectrum fitting with background MC Preliminary (keVee) Observed spectrum Expectation for m a =3keV Events/keV/kg/day We obtained a limit on the axio-electric coupling (g aee ) by comparing the observed spectrum and the expectation. The result can be further improved above 5keV by fitting spectrum.

27. Solar axion search in XMASS Observed spectrum MC axion signals Abs upper limit g aee =4.5e-11 XMASS can also search for solar axion emitted owing to Bremsstrahlung and Compton processes in the Sun, by detection through axio-electric effect. We obtained a limit on axio-electric coupling (g aee ) Solar axion flux ( Derbin et al., arXiv:1206.4142 ) g aee =10 -10

28. Allowed mass < 200eV for KSVZ <2eV for DFSZ Original figure from Derbin et al., 1206.4142 XMASS Limit by DM XMASS Limit by solar axion 4.5e-11 Solar limit 2.8e-11 Limits on g aee from XMASS (DM+solar) Preliminary

29. Future plan 29

30. Plan: Refurbishment work Tuning of reconstruction/reduction is on going but for better sensitivity, removing the origins of BG must be done. To reduce the BG caused by Aluminum, we are planning to cover the part and surfaces by copper rings and plates: BG > 5keV must be reduced significantly (1/100 of current BG). Schedule: start physics run from 2013

31. Expected sensitivity with fiducialization XENON100 CDMSII XMASS 2keVee thre. 100d Black:signal+BG Red:BG Expected energy spec. 1 year exposure   p =10 -44 cm 2 50GeV WIMP Spin Independent XMASS 5keVee thre. 100d Initial target of the energy threshold was ~5keVee. Because we have factor ~3 better photoelectron yield, lower threshold = smaller mass dark matter may be looked for. 31

32. XMASS1.5 (2015-) Total mass: 5 tons Fiducial mass: 1 ton  100kg Backgrounds –No dirty aluminum –No GORETEX –Less surface 210 Pb New PMTs. Expect 10 -5 dru Sensitivity s SI 5 keV) Low threshold analysis could reach a few x 10 -42 cm 2 around a few GeV region. XENON100 CDMSII EDELWEISSII DAMA CoGeNT

33. Summary The XMASS 800kg detector was constructed and started commissioning late 2010. We completed commissioning data-taking and physics analyses are on-going. BG level is not as low as originally expected, but now the composition is well understood above 5keV. Some preliminary results on dark matter and axion searches are shown. More results will come later. By improving software (reconstruction/BG reduction) and hardware refurbishment, we aim at the dark matter search with the original sensitivity. And XMASS1.5 will progress. Physics run is planed from 2015.

34. XMASS collaboration ICRR, University of TokyoK. Abe, K. Hieda, K. Hiraide, Y. Kishimoto, K. Kobayashi, Y. Koshio, S. Moriyama, M. Nakahata, H. Ogawa, H. Sekiya, A. Shinozaki, Y. Suzuki, O. Takachio, A. Takeda, D. Umemoto, M. Yamashita, B. Yang IPMU, University of TokyoJ. Liu, K. Martens Kobe UniversityK. Hosokawa, K. Miuchi, A. Murata, Y. Ohnishi, Y. Takeuchi Tokai UniversityF. Kusaba, K. Nishijima Gifu UniversityS. Tasaka Yokohama National University K. Fujii, I. Murayama, S. Nakamura Miyagi University of Education Y. Fukuda STEL, Nagoya UniversityY. Itow, K. Masuda, H. Takiya, H. Uchida Kobe UniversityK. Ohtsuka, Y. Takeuchi Seoul National UniversityS. B. Kim Sejong UniversityN.Y. Kim, Y. D. Kim KRISSY. H. Kim, M. K. Lee, K. B. Lee, J. S. Lee

36. Cryogenic 700L Liq. Storage liquid pump Condenser 360W gas pump filters Calibration line Cable line filters Outer vacuum 857kg 10 m 3 x 2 gas storage emergency gas pump 100L/min evaporator Water tank Liquid circulation ~5L/min Gas circulation <30L/min XMASS Xenon operating system  Xenon keeping : Pulse tube refrigerator.  Xenon circulation : Gas phase: < 30 L/min Liquid phase: ~ 5 L/min  Xenon collection : 700L liquid storage tank. 10m3 x 2 gas storage tank.  We successed the xenon filling/collection for 5 times with keeping the amount and quality of xenon.

37. Internal RI in xenon 100 5001000 Time difference (  s) Fitting with an expected decay curve 1 st event ( 214 Bi  ) 2 nd event ( 214 Po  ) 222Rn 220Rn Radon concentration: 222Rn : 8.2+/-0.5mBq 220Rn : <280microBq (90%C.L.) 220Rn candidate 216Po candidate ~accidental 164us 145ms ※ 214Pb, 212Pb(daugther of radon) is source of background in low energy. These background is remained even after fiducial volume cut. 8.2mBq ～ 2x10-4dru contribution.