1. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Recent Results from Super-Kamiokande Hiroyuki Sekiya ICRR, University of Tokyo on behalf of the Super-Kamiokande Collaboration ICHEP 2008 Philadelphia, USA

2. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 2 Super-Kamiokande Collaboration 130 authors 36 institutions 5 countries

3. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 3 Super-Kamiokande 50kton water Cerenkov detector 1km (2.7km w.e) underground in Kamioka zinc mine. 11129 50cm PMTs in Inner Detector 1885 20cm PMTs in Outer Detector ~5-20~20-50~1 Solar ν MeV Relic SN ν GeVTeV Atmospheric ν ~100 Physics targets of Super-Kamiokande Proton decay

4. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 4 History1996199719981999200020022003200420052006200720080120062009 SK-I (1996-2001) 11,146 ID PMTs (40% coverage) 1,885 OD PMTs SK-II (2003-2005) 5182 ID PMTs (19% coverage) Acrylic shields added SK-III (2006-2008) 11,129 ID PMTs (40% cov.) OD segmentation (top/barrel/bottom) Coming soon: SK-IV (2008-... ) Replace DAQ electronics

5. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 5 Solar neutrino Total flux Day/night Seasonal flux variation Spectrum distortion 8 B neutrino measurement by elastic scattering: (sensitive to all ν flavors) PhaseEnergy response Energy threshold SK-I6p.e./MeV5.0MeV SK-II3p.e./MeV7.0MeV SK-III6p.e./MeV5.0MeV→4.5MeV SK-IV6p.e./MeV4.0MeV Measure/Observe Reconstruct Recoil electron energy Recoil angle relative to the Sun

6. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Solar neutrino flux 6 Live time (days) Energy Range (MeV) Number of signal eventsFlux x 10 6 cm -2 sec -1 SK-III 289 6.5-20.0 3378.9 (stat only)In preparation SK-II 791 7.0-20.0 7212.8 (stat) (sys)2.38± 0.05 (stat) (sys) SK-I 1496 5.0-20.0 2240± 226 (stat) (sys)2.35 ± 0.02 (stat) ± 0.08 (sys) +82.7 -81.1 + 152.9 -150.9 +483.3 -461.6 +784 -717 +0.16 -0.15 SK-III 289 days Full Final sample 6.5 - 20 MeV, 22.5 kton Signal: Preliminary Extract number of signal events by fit signal + background shapes to cosθ sun distribution SK-III flux seems consistent with SK-I & SK-II flux Derive neutrino flux cosθ sun distribution

7. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Energy Spectrum arXive:0804.4312 7 Consistent with flat SK-II is consistent with SK-I SK-II spectrum: SK-II 791 days Energy-correlated errors SK-I average

8. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Oscillation Analyses (SK only) 8 SK Exclusion RegionsSK Allowed Regions SK-I only SK-II only SK-I + SK-II SK-I only SK-II only SK-I + SK-II 8 B flux constrained to SNO Salt Phase NC flux Based on SK energy spectrum shape SK-II contributes to widen the region. S.N. Ahmed et al., PRL92 (2004) 181301

9. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Solar global analysis+KamLAND 9 9 SK-I + SK-II + SNO + radiochemical KamLAND (arXiv:hep-ex/0801.4589v2) SNO data: 371-day salt phase (CC & NC fluxes) 306-day pure D 2 O phase (A D-N ) Radiochemical data: Homestake SAGE GALLEX KamLAND is consistent with solar global Combined experimental data allow us to measure the oscillation parameters in this framework......but we would like to observe predicted upturn at low energy if the parameters are in this region… Best fit

10. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Prospects for SK-III/SK-IV In order to see it, we must: reduce statistical errors reduce energy-correlated sys errors (0.5 x SK-I) → lower backgrounds &energy threshold Low energy upturn ~10% effect in Super-K Work in progress... Ratio (data/ssm) SK-III, 5years (sin 2 ,  m 2 ) = (0.30,7.9x10 -5 ) E (MeV) 10

11. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 11 SK-I SK-III SK-III background rate lower than SK-I in central region Z R2R2 0 SK-III Backgrounds cosθ sun Radon in the water from the material (Tank/PMT/FRP) SK-I SK-III …promise for future lowering of analysis threshold

12. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Solar 95% 99.73% KamLAND Solar+KamLAND SK-III/IV up-turn sensitivity x10 -4 Year 2121 significance σ Target: 2 sigma level upturn discovery (or exclusion) for 3 years observation Assuming: -achieved SK-III Background level -0.5x energy correlated systematic error of SK-I

13. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 13 Atmospheric neutrino interactions in SK 13,000 k m 15 km Atmospheric Neutrinos Fully-ContainedPartially-Contained Upward Stopping Muon Upward Through- going Muon Event categories in Super-kamiokande

14. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 14 zenith angle analysis – Use many subsamples of data – Look for zenith angle distortion L/E analysis – Use much more selective subsample of data – Require good L/E resolution – Look for first oscillation dip Oscillation analyses downwards （ L=10~100 km ） upwards （ L=up to 13000 km ） Θ cosΘ

15. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 15 Updates in Analyses Re-analysis of SK-I and SK-II data due to many changes/improvements: Changed to agree with K2K measurement. Effect: Increase number of events Effect: Small change in lepton momentum distributions Effect: Suppression in forward direction of lepton scattering angle Effect: Reduction in number of multiple- π events Effect: Better data/MC agreement for various quantities Simulation atmospheric neutrino flux model: Honda06 neutrino interaction model (neut) QE: M A = 1.2 GeV 1 π (resonant): M A = 1.2 GeV Add Δ → N γ Add lepton mass effects in CC1 π 1 π (coherent): Rein & Sehgal with lepton mass correction DIS: GRV98 PDF with Bodek-Yang correction detector simulation more detailed model of light reflections and scattering better OD tuning Reconstruction improved ring counting Other higher MC statistics re-evaluate and add systematic uncertainties Effect: Reduced systematic errors Increase from 100 yrs to 500 yrs

16. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Updated Zenith Angle distribution 16 Monte Carlo (no oscillations) Monte Carlo (best fit oscillations) cos θ zenith Datasets SK-I FC/PC: 1489 days SK-I Upmu: 1646 days SK-II FC/PC: 799 days SK-II Upmu: 828 days

17. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 17 Zenith Angle Analysis: SK-I + SK-II Best fit: Δ m 2 = 2.1 x 10 -3 eV 2 sin 2 2 θ = 1.02 χ 2 = 830.1 / 745 d.o.f. χ 2 fit in bins of zenith angle with 90systematic error pull terms to account for uncertainties in: Neutrino flux Cross sections Event reconstruction Data reduction

18. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Updated L/E distribution 18 Datasets SK-I FC/PC μ -like: 1489 days SK-II FC/PC μ -like: 799 days Use only event categories with good L/E resolution: Partially-contained muons Fully-contained muons Compare against: Neutrino decoherence (5.0 σ ) Neutrino decay (4.1 σ ) Grossman and Worah: hep-ph/9807511 Lisi et al.: PRL85 (2000) 1166 Barger et al.: PRD54 (1996) 1, PLB462 (1999) 462

19. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 19 L/E Analysis: SK-I + SK-II Best fit: Δ m 2 = 2.2 x 10 -3 eV 2 sin 2 2 θ = 1.04 χ 2 = 78.9 / 83 d.o.f. 90% C.L. allowed region sin 2 2 θ > 0.94 1.85x10 -3 < Δ m 2 < 2.65x10 -3 eV 2 χ 2 fit to 43 bins of log 10 (L/E) with 29 systematic error terms

20. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 20 SK-IV: DAQ Upgrade SK-IV Installation begins August 2008 to be completed by mid-September ~6-month commissioning period before T2K beam

21. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 21 New DAQ readout scheme Current readout module New readout module Trigger logic 12 PMT signals per module 24 PMT signals per module Hitsum Trigger (1.3 μsec x 3kHz) Readout (backplane) Hardware trigger by hit information (HITSUM) 1.3 μsec event window clock Periodic trigger (17 μsec x 60 kHz) Readout (Ethernet) Record every hit by 60kHz periodic timing signal x 17 μs TDC window Variable event window by software trigger SK-I,II,III DAQ scheme: No hardware trigger. Instead record all hits and apply software triggers. SK-IV DAQ scheme:

22. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Summary Super-Kamiokande (I+II+III) have been operated successfully. More than 10 years dataset for atmospheric & solar neutrino data are accumulated. Study “Standard Model” oscillation physics - help constrain solar parameters - precisely measure atmospheric parameters Will continue to observe every predicted effect and measure mixing angles. – SK-IV will start from this September with upgraded electronics 22

23. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Extra Slides 23

24. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 24 ๏ Simplified detector operations unified readout scheme for ID and OD ๏ Increased reliability/performance - fewer discrete components - improve energy resolution wider dynamic range - improve multiple-hit capability efficient ID of μ -decay electrons - reduce SPE hit threshold low E solar ν’ s γ -tagging for proton decay - improve supernova burst capability ๏ Ethernet-based readout increased bandwidth and reduced dead time build DAQ system from commodity network devices! SK-IV: DAQ Upgrade

25. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 25 SK-IV Installation begins August 2008 to be completed by mid-September ~6-month commissioning period before T2K beam

26. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Low energy events in SK 26 SK-I 10 MeV electron SK-II 10 MeV electron Simulated event Using SK-II improved algorithm Energy response Vertex resolution for 10 MeV electron SK-I~6 p.e./MeV~70 cm → 60 cm SK-II~3 p.e./MeV~100 cm SK-III~6 p.e./MeVin preparation

27. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 BG reduction for SK-III solar ν analysis 27 100% trigger efficiency at 5 MeV Preliminary SK-III reduction tools Datasets: ✤ Full Final (FF) sample Livetime: 288.9 days Energy > 6.5 MeV ✤ Radon Reduced (RR) sample (shown) → periods of high radon activity removed Livetime: 191.7 days Energy > 5 MeV Run period shown: Jan. 24, 2007 - Mar. 2, 2008 Good agreement of SK-III with SK-I final data sample

28. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Energy Spectra 28 Consistent with flat SK-II is consistent with SK-I SK-I spectrum: SK-II spectrum: SK-I 1496 days Energy-correlated errors SK-II 791 days Energy-correlated errors

29. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Time Variation of Flux 29 SK-I SK-II Seasonal Variation Consistent with expected variations due to eccentricity of Earth’s orbit SK-IISK-I SK-I (binned) Consistent with zero SK-I asymmetry: SK-II asymmetry: Day/Night Asymmetry

30. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 SK-I 1496 days Energy-correlated errors Prospects for SK-III+SK-IV 30 In order to see it, we must: reduce statistical errors reduce energy-correlated sys errors (0.5 x SK-I) → lower backgrounds &energy threshold arXiv:hep-ph/0405172v6 Low energy upturn ~10% effect in Super-K

31. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Event Categories 31 Event rates consistent across all phases of SK Event Category Event Rate (events/day) SK-ISK-II SK-III (Preliminary) Fully Contained (FC)8.18 ± 0.078.22 ± 0.108.31 ± 0.22 Partially Contained (PC)0.61 ± 0.020.54 ± 0.030.57 ± 0.06 Upward-stopping μ (Upstop)0.25 ± 0.010.28 ± 0.020.24 ± 0.03 Upward-thrugoing μ (Upthru)1.12 ± 0.031.07 ± 0.041.11 ± 0.06 SK-III run period: July 29, 2006 - present Fully-ContainedPartially-Contained Upward Stopping Muon Upward Through- going Muon

32. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 SK-I 1 GeV electron SK-I 1 GeV muon SK-II 1 GeV electron SK-II 1 GeV muon 32 Atmospheric ν in SK

33. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 33 SK-III Atmospheric ν Zenith Distributions No oscillation analysis yet, but zenith angle distortion clearly visible SK-III data Monte Carlo (no oscillations) \ >25,000 atmospheric ν events in SK-I + II + III

34. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 34 Atmospheric ν Analyses Oscillation: Zenith angle (2-flavor) L/E Non-standard interactions Poster by G. Mitsuka: “Limit on Non-Standard Interactions from the Atmospheric Neutrino Data in Super-Kamiokande” Zenith angle (3-flavor) (Phys. Rev. D 74, 032002 (2006)) ν τ appearance (Phys. Rev. Lett. 97, 171801 (2006)) MaVaNs (Phys. Rev. D 77, 052001 (2008)) Exotic scenarios: LIV, CPT, Sterile 3-flavor with solar term Non-oscillation: Nucleon decay searches Poster by H. Nishino: “Search for proton decays via p → e + π 0 and p → μ + π 0 in Super-Kamiokande” WIMP search Poster by T. Tanaka “Search for Indirect Signal of WIMPs in Super-Kamiokande”

35. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 35 Atmospheric ν Analyses Exotic Scenarios Neutrinos frequently set stringent limits, although not usually testing exactly the same parameters. e.g., cosmic ray spectrum LIV < 10 -15, NMR LIV < 10 -22 K 0 K 0 bar CPTV < 10 -18

36. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Zenith Angle Analysis 36 400 bins for SK-I 350 bins for SK-II χ 2 fit in bins of zenith angle with systematic error pull terms: Data binned according to: event type + momentum + zenith angle where 90 systematic error terms to account for uncertainties in: Neutrino flux Cross sections Event reconstruction Data reduction Datasets SK-I FC/PC: 1489 days SK-I Upmu: 1646 days SK-II FC/PC: 799 days SK-II Upmu: 828 days

37. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 37 Zenith Angle Analysis: SK-I + SK-II Best fit: Δ m 2 = 2.1 x 10 -3 eV 2 sin 2 2 θ = 1.02 χ 2 = 830.1 / 745 d.o.f.

38. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 L/E Analysis:SKI+SKII 38 χ 2 fit to 43 bins of log 10 (L/E) with 29 systematic error terms Datasets SK-I FC/PC μ -like: 1489 days SK-II FC/PC μ -like: 799 days Use only event categories with good L/E resolution: Partially-contained muons Fully-contained muons Compare against: Neutrino decoherence (5.0 σ ) Neutrino decay (4.1 σ ) Grossman and Worah: hep-ph/9807511 Lisi et al.: PRL85 (2000) 1166 Barger et al.: PRD54 (1996) 1, PLB462 (1999) 462

39. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 39 L/E Analysis: SK-I + SK-II Best fit: Δ m 2 = 2.2 x 10 -3 eV 2 sin 2 2 θ = 1.04 χ 2 = 78.9 / 83 d.o.f. 90% C.L. allowed region sin 2 2 θ > 0.94 1.85x10 -3 < Δ m 2 < 2.65x10 -3 eV 2

40. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 40 SK-II Low E reconstruction Vertex resolution improves with new algorithms developed for SK-II

41. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 41 Target: 2 sigma level upturn discovery (or exclusion) for 3 years observation SK Sensitivity to Solar Upturn To achieve this, we must: enlarge fiducial volume while maintaining low background reduce energy-correlated systematic errors

42. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 42 Oscillation Analysis Spectrum Energy correlated systematic error Time variation Function for energy correlated systematic errors 8 B spec. shape energy scale energy resolution

43. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 43 Unbinned Time Variation Analysis Solar Signal Shape Solar  Flux Time-Variatio n Background Shape Event Energy Event “Time” # Signal Events # Backgrounds in each energy bins 21 Energy bins Likelihood for solar neutrino extraction

44. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 44 Solar Neutrinos at Super-K SK-III data agree well with SK-I - Analyses are in progress - Low background in central region of detector shows promise for future lowering of analysis threshold SK-I + SK-II results are consistent SK will continue work to hopefully observe low energy upturn

45. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 45 Using SK-II improved algorithm Low energy events in Super-K SK-I 10 MeV electron SK-II 10 MeV electron Simulated event Energy response Vertex resolution for 10 MeV electron SK-I~6 p.e./MeV~70 cm → 60 cm SK-II~3 p.e./MeV~100 cm SK-III~6 p.e./MeVin preparation

46. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 46 Ring likelihood OldNew sub-GeV multi-GeV Phys. Rev. D 71, 112005 (2005) Reduced systematic uncertainty: XX% (old) XX% (new) Better separation of single vs. multi-ring events

47. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 47 Better data/MC agreement for PC and FC datasets Ring Counting New Old

48. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 48 ID/OD Optical Segmentation PCPC Corner Clippers

49. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 49 L/E and Zenith Angle: SK-I + SK-II Stronger constraint on Δ m 2 with high resolution sub-sample of data

50. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 50 L/E Analysis (SK-I re-analysis cf. published result) Best fit oscillation parameters PRL93, 101801 (2004) best fit: (1.00, 2.4 x 10 -3 )

51. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 51 Zenith Analysis Systematic Errors

52. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 52 Zenith Analysis Systematic Errors

53. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 53 L/E Systematic Errors Normalization Multi-mu / PC relative norm Flux: anti numu/numu ratio Flux: up/down Flux: horiz/vertical Neutrino flight length energy spectrum Sample-by-sample multi-GeV QE cross section Single-pi cross section DIS cross section Coherent pi cross section NC/CC FC reduction PC reduction Ring counting PID 1-ring Energy calibration Up-down asym of E calibration Nuclear effects Non-nu BG (cosmic ray mu) PC stop/thru separation QE and Single-pi MA DIS (Bodek-Yang correction) Solar activity Single-meson pi0/pi+pi- PC stop/thru bottom PC stop/thru barrel

54. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Search for proton decays via p  e +  0 and p   +  0 in Super-Kamiokande 54

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61. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 61

62. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 3 flavor oscillation using SK-1 Assume One mass scale dominance Δm 2 12 ＝ 0 Earth matter effect play an important role. Normal mass hierarchy (90%CL) – sin 2 θ 13 < 0.14 – 0.37< sin 2 θ 23 < 0.65 Inverted mass hierarchy(90%CL) – sin 2 θ 13 < 0.27 – 0.37< sin 2 θ 23 < 0.69 62

63. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Normal mass hierarchy 63

64. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Inverted mass hierarchy 64

65. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 SN & GADZOOKS! 65

66. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Super-K: Expected number of events ~7,300 e +p events ~300 +e events ~360 16 O NC  events ~100 16 O CC events (with 5MeV thr.) for 10 kpc supernova Neutrino flux and energy spectrum from Livermore simulation (T.Totani, K.Sato, H.E.Dalhed and J.R.Wilson, ApJ.496,216(1998))

67. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 S.Ando, NNN05 Supernova Relic Neutrinos S.Ando, Astrophys.J.607:20-31,2004.

68. H.Sekiya, Jul 30 th 2008, Philadelphia, ICHEP2008 Super-K results so far Flux limit VS predicted flux T.iida, poster #90.5

69. e can be identified by delayed coincidence. Neutron tagging in water Cherenkov detector e e+e+ p n   Positron and gamma ray vertices are within ~50cm. 2.2MeV  -ray  T = ~ 200  sec Another possibility n+p →d +  Number of hit PMT is about 6 in SK-III p Gd n+Gd →~8MeV   T = ~30  sec GADZOOKS! (J.Beacom and M.Vagins) Phys.Rev.Lett.93:171101,2004 Add 0.2% GdCl 3 in water

70. This apparatus deployed in the SK tank. BGO 13 cm 18 cm 5 cm BGO 0.2 % GdCl 3 Solution Am/Be GdCl 3 test vessel Test neutron tagging at Super-K α + 9 Be → 12 C * + n 12 C* → 12 C +  (4.4 MeV) n + p → …… → n + Gd → Gd +  (totally 8 MeV) BGO signal (prompt signal (large and long time pulse)) Look for Cherenkov signal (delayed signal) H.Watanaba, poster #94

71. Cherenkov signal of Gd gamma rays τ = 23.7±1.7μs [msec] Number of Events 0 Time from prompt 100  s Vertex position 92% within 2m dR [cm] Energy spectrum Measured time, vertex and energy distributions are as expected from the MC simulation. H.Watanaba, poster #94

72. Selection criteria of delayed signal: (1)Vertex position within 2m (2)Energy of delayed signal > 3MeV (3)Time after the prompt within 60  sec. (4)Ring pattern cuts Selection efficiency is ~74%. With 90% capture eff. by 0.2% Gd,  Tagging efficiency is 67% 92 % Tagging efficiency and BG reduction While the chance coincidence prob. is estimated to be ~2×10 -4 Recon. Energy [MeV] Energy spectrum MC simulation It almost satisfy the requirement to remove remaining spallation background at 10 MeV. SRN predictions Current single BG rate (mainly due to spallation and solar 8 B) H.Watanaba poster #94

73. Possibility of SRN detection Relic model: S.Ando, K.Sato, and T.Totani, Astropart.Phys.18, 307(2003) with flux revise in NNN05. If invisible muon background can be reduced by neutron tagging Assuming invisible muon B.G. can be reduced by a factor of 5 by neutron tagging. By 10 yrs SK data, Signal: 33, B.G. 27 (E vis =10-30 MeV) SK10 years (  =67%) Assuming 67% detection efficiency.

74. Items to be studied before introducing gadolinium to SK  Effect to water transparency Water transparency should be long enough to do various physics at SK.  Water purification system Current water purification system remove ions. So, it must be modified to purify water without removing gadolinium.  Material effects Corrosion by gadolinium solution should be checked.  How to introduce/remove How to mix gadolinium uniformly in the tank. How quickly/economically/completely can the Gd be removed?  Ambient neutron level in the tank Does it cause significant increase singles in trigger rate[for solar analysis]? In order to study those things, we will construct a test tank (6~10m size) in the Kamioka mine.