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H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
Recent Results from Super-Kamiokande
Hiroyuki SekiyaICRR, University of Tokyo
on behalf of the Super-Kamiokande Collaboration
ICHEP 2008Philadelphia, USA
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 2
Super-Kamiokande Collaboration
• 130 authors• 36
institutions• 5 countries
H.Sekiya, Jul 30th 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
GeV TeV
Atmospheric ν
~100
Physics targets of Super-Kamiokande
Proton decay
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 4
History
1996 1997 1998 1999 2000 2002 2003 2004 2005 2006 2007 20080120 06 2009
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
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 5
Solar neutrino
• Total flux• Day/night• Seasonal flux variation• Spectrum distortion
8B neutrino measurementby elastic scattering:
(sensitive to all ν flavors)
Phase Energy response
Energy threshold
SK-I 6p.e./MeV 5.0MeV
SK-II 3p.e./MeV 7.0MeV
SK-III 6p.e./MeV 5.0MeV→4.5MeV
SK-IV 6p.e./MeV 4.0MeV
Measure/Observe
Reconstruct• Recoil electron energy• Recoil angle relative to the Sun
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
Solar neutrino flux
6
Live time (days)
Energy Range (MeV)
Number of signal events Flux x 106 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 sample6.5 - 20 MeV, 22.5 ktonSignal:
Preliminary
Extract number of signal events by fit signal + background shapes to cosθsun distribution
SK-III flux seems consistent with SK-I & SK-II flux
Derive neutrino flux
cosθsun distribution
H.Sekiya, Jul 30th 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
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
Oscillation Analyses (SK only)
8
SK Exclusion Regions SK Allowed Regions
SK-I onlySK-II onlySK-I + SK-II
SK-I onlySK-II onlySK-I + SK-II
8B flux constrained to SNO Salt Phase NC flux
Based on SK energy spectrum shapeSK-II contributes to widen the region.
S.N. Ahmed et al., PRL92 (2004) 181301
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
Solar global analysis+KamLAND
99
SK-I + SK-II + SNO + radiochemical
KamLAND (arXiv:hep-ex/0801.4589v2)
SNO data: 371-day salt phase (CC & NC fluxes) 306-day pure D2O phase (AD-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 energyif the parameters are in this region…
Best fit
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
Prospects for SK-III/SK-IV
In order to see it, we must:reduce statistical errorsreduce energy-correlated sys errors (0.5 x SK-I) → lower backgrounds &energy threshold
Low energy upturn ~10% effect in Super-K
Work in progress...
Rat
io (
data
/ssm
) SK-III, 5years(sin2q, Dm2) = (0.30,7.9x10-5)
E (MeV)
10
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 11
SK-I SK-IIISK-III background rate lower than SK-I in central region
Z
R20
SK-III Backgrounds
cosθsun
• Radon in the water from the material (Tank/PMT/FRP)
SK-ISK-III
…promise for future lowering of analysis threshold
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
Solar
95%99.73% KamLAND
Solar+KamLAND
SK-III/IV up-turn sensitivity
x10-4
Year
2
1
sig
nif
ican
ce 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
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 13
Atmospheric neutrino interactions in SK
13,000 km
15 kmAtmospheric Neutrinos
Fully-Contained Partially-Contained Upward Stopping Muon
Upward Through-going Muon
Event categories in Super-kamiokande
H.Sekiya, Jul 30th 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 analysesdownwards
( L=10~100 km)
upwards
( L=up to 13000 km)
Θ
cosΘ
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 15
Updates in AnalysesRe-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
Simulationatmospheric neutrino flux model: Honda06neutrino interaction model (neut)QE: MA = 1.2 GeV1π (resonant): MA = 1.2 GeV Add Δ → Nγ Add lepton mass effects in CC1π1π (coherent): Rein & Sehgal with lepton mass correctionDIS: GRV98 PDF with Bodek-Yang correctiondetector simulationmore detailed model of light reflections and scatteringbetter OD tuning
Reconstructionimproved ring counting
Otherhigher MC statistics re-evaluate and add systematic uncertainties
Effect: Reduced systematic errors
Increase from 100 yrs to 500 yrs
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
Updated Zenith Angle distribution
16
Monte Carlo (no oscillations)Monte Carlo (best fit oscillations)
cos θzenith cos θzenith
cos θzenith
DatasetsSK-I FC/PC: 1489 daysSK-I Upmu: 1646 daysSK-II FC/PC: 799 daysSK-II Upmu: 828 days
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 17
Zenith Angle Analysis: SK-I + SK-II
Best fit:Δm2 = 2.1 x 10-3 eV2
sin2 2θ = 1.02χ2 = 830.1 / 745 d.o.f.
χ2 fit in bins of zenith angle with 90systematic error pull termsto account for uncertainties in:Neutrino flux Cross sectionsEvent reconstruction Data reduction
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
Updated L/E distribution
18
DatasetsSK-I FC/PC μ-like: 1489 daysSK-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 (5.0σ)Neutrino decay (4.1σ)Grossman and Worah: hep-ph/9807511Lisi et al.: PRL85 (2000) 1166Barger et al.: PRD54 (1996) 1, PLB462 (1999) 462
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 19
L/E Analysis: SK-I + SK-II
Best fit:Δm2 = 2.2 x 10-3 eV2
sin2 2θ = 1.04χ2 = 78.9 / 83 d.o.f.
90% C.L. allowed regionsin2 2θ > 0.941.85x10-3 < Δm2 < 2.65x10-3 eV2
χ2 fit to 43 bins of log10(L/E) with 29 systematic error terms
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 20
SK-IV: DAQ UpgradeSK-IV Installation begins August 2008
to be completed by mid-September
~6-month commissioning period before T2K beam
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 21
New DAQ readout scheme
Current
readout
module
New
readout
module
Trigger
logic12 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
clockPeriodic 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:
H.Sekiya, Jul 30th 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
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 24
๏ Simplified detector operationsunified readout scheme for ID and OD
๏ Increased reliability/performance- fewer discrete components
- improve energy resolutionwider dynamic range
- improve multiple-hit capabilityefficient ID of μ-decay electrons
- reduce SPE hit thresholdlow E solar ν’sγ-tagging for proton decay
- improve supernova burst capability
๏ Ethernet-based readoutincreased bandwidth and reduced dead timebuild DAQ system from commodity network devices!
SK-IV: DAQ Upgrade
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 25
SK-IV Installation begins August 2008
to be completed by mid-September
~6-month commissioning period before T2K beam
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
Low energy events in SK
26
SK-I10 MeV electron
SK-II10 MeV electron
Simulated event Simulated event
♱Using SK-II improved algorithm
Energy responseVertex 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./MeV in preparation
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
BG reduction for SK-III solar ν analysis
27
100% trigger efficiency at 5 MeVPreliminary SK-III reduction tools
Datasets:✤ Full Final (FF) sampleLivetime: 288.9 daysEnergy > 6.5 MeV
✤ Radon Reduced (RR) sample (shown)→ periods of high radon activity removedLivetime: 191.7 daysEnergy > 5 MeV
Run period shown: Jan. 24, 2007 - Mar. 2, 2008
Good agreement of SK-III with SK-I final data sample
H.Sekiya, Jul 30th 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
H.Sekiya, Jul 30th 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
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
SK-I 1496 days
Energy-correlated errors
Prospects for SK-III+SK-IV
3030
In order to see it, we must:reduce statistical errorsreduce energy-correlated sys errors (0.5 x SK-I)→ lower backgrounds &energy threshold
arXiv:hep-ph/0405172v6Low energy upturn ~10% effect in Super-K
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
Event Categories
31
Event rates consistent across all phases of SK
Event CategoryEvent Rate (events/day)
SK-I SK-IISK-III(Preliminary)
Fully Contained (FC) 8.18 ± 0.07 8.22 ± 0.10 8.31 ± 0.22
Partially Contained (PC) 0.61 ± 0.02 0.54 ± 0.03 0.57 ± 0.06
Upward-stopping μ (Upstop) 0.25 ± 0.01 0.28 ± 0.02 0.24 ± 0.03
Upward-thrugoing μ (Upthru) 1.12 ± 0.03 1.07 ± 0.04 1.11 ± 0.06
SK-III run period: July 29, 2006 - present
Fully-Contained Partially-Contained Upward Stopping Muon
Upward Through-going Muon
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
SK-I1 GeV electron
SK-I1 GeV muon
SK-II1 GeV electron
SK-II1 GeV muon
32
Atmospheric ν in SK
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 33
SK-III Atmospheric ν Zenith Distributions
No oscillation analysis yet, but zenith angle distortion clearly visible
SK-III dataMonte Carlo (no oscillations)
\
>25,000 atmospheric ν events in SK-I + II + III
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 34
Atmospheric ν Analyses
Oscillation:Zenith angle (2-flavor)L/ENon-standard interactionsPoster 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))
ντ appearance (Phys. Rev. Lett. 97, 171801 (2006))
MaVaNs (Phys. Rev. D 77, 052001 (2008))
Exotic scenarios: LIV, CPT, Sterile3-flavor with solar term
Non-oscillation:Nucleon decay searchesPoster by H. Nishino:“Search for proton decays via p → e+ π0 and p → μ+ π0 in Super-Kamiokande”
WIMP searchPoster by T. Tanaka“Search for Indirect Signal of WIMPs in Super-Kamiokande”
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 35
Atmospheric ν Analyses
Exotic ScenariosModel Exclusion level or limit
νμ → νs oscillation SK-I+II: 7.3σ
Admixture (2+2 hierarchy) SK-I+II: 23% allowed
Decay I (sin4θ + cos4θ e-αL/E) SK-I+II: 17σ
Decay II (sin2θ + cos2θ e-αL/2E)2 SK-I+II: 3.9σ
Decay Limit (GeV2) SK-I+II: 6.5 x 10-23
Decoherence ((1+e-βL/E)/2) SK-I+II: 4.2σ
Decoherence Limit (GeV) SK-I+II: 6.0 x 10-24
LIV Limit SK-I+II: 1.2 x 10-24
CPTV Limit (GeV) SK-I+II: 0.9 x 10-23
MaVaNs (various models) SK-I: 3.5-3.8σ
Non-Standard Interactions See poster by G. Mitsuka
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
K0K0bar CPTV < 10-18
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
Zenith Angle Analysis
36
400 bins for SK-I 350 bins for SK-II
χ2 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 sectionsEvent reconstruction Data reduction
DatasetsSK-I FC/PC: 1489 daysSK-I Upmu: 1646 daysSK-II FC/PC: 799 daysSK-II Upmu: 828 days
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 37
Zenith Angle Analysis: SK-I + SK-II
Best fit:Δm2 = 2.1 x 10-3 eV2
sin2 2θ = 1.02χ2 = 830.1 / 745 d.o.f.
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
L/E Analysis:SKI+SKII
38
χ2 fit to 43 bins of log10(L/E) with 29 systematic error terms
DatasetsSK-I FC/PC μ-like: 1489 daysSK-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 (5.0σ)Neutrino decay (4.1σ)
Grossman and Worah: hep-ph/9807511Lisi et al.: PRL85 (2000) 1166Barger et al.: PRD54 (1996) 1, PLB462 (1999) 462
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 39
L/E Analysis: SK-I + SK-II
Best fit:Δm2 = 2.2 x 10-3 eV2
sin2 2θ = 1.04χ2 = 78.9 / 83 d.o.f.
90% C.L. allowed regionsin2 2θ > 0.941.85x10-3 < Δm2 < 2.65x10-3 eV2
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 40
SK-II Low E reconstruction
Vertex resolution improves with new algorithms developed for SK-II
H.Sekiya, Jul 30th 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 backgroundreduce energy-correlated systematic errors
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 42
Oscillation Analysis
SpectrumEnergy correlated systematic error
Time variation
Function for energy correlated systematic errors
8B spec. shape
energy scale
energy resolution
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 43
Unbinned Time Variation Analysis
Solar Signal Shape
Solar ν Flux
Time-Variation
Background Shape
EventEnergy
Event“Time”
# SignalEvents
# Backgroundsin each energy bins
21 Energy bins
Likelihood for solar neutrino extraction
H.Sekiya, Jul 30th 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
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 45
♱Using SK-II improved algorithm
Low energy events in Super-K
SK-I10 MeV electron
SK-II10 MeV electron
Simulated event Simulated event
Energy responseVertex 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./MeV in preparation
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 46
Ring likelihood
Old New
sub-GeV sub-GeV
multi-GeV multi-GeV
Phys. Rev. D 71, 112005 (2005)
Reduced systematic uncertainty: XX% (old) XX% (new)Better separation of single vs. multi-ring events
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 47
Better data/MC agreement for PC and FC datasets
Ring Counting
New
Old
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 49
L/E and Zenith Angle: SK-I + SK-II
Stronger constraint on Δm2 with high resolution sub-sample of data
H.Sekiya, Jul 30th 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)
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 53
L/E Systematic Errors
NormalizationMulti-mu / PC relative normFlux: anti numu/numu ratioFlux: up/downFlux: horiz/verticalNeutrino flight lengthenergy spectrumSample-by-sample multi-GeVQE cross sectionSingle-pi cross sectionDIS cross sectionCoherent pi cross sectionNC/CCFC reductionPC reductionRing countingPID 1-ringEnergy calibrationUp-down asym of E calibrationNuclear effectsNon-nu BG (cosmic ray mu)PC stop/thru separationQE and Single-pi MADIS (Bodek-Yang correction)Solar activitySingle-meson pi0/pi+pi-PC stop/thru bottomPC stop/thru barrel
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
• Search for proton decays via pe+p0 and pm+p0 in Super-Kamiokande
54
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
3 flavor oscillation using SK-1
• Assume One mass scale dominance Δm212 = 0
• Earth matter effect play an important role.• Normal mass hierarchy (90%CL)
– sin2θ13 < 0.14
– 0.37< sin2θ23 < 0.65
• Inverted mass hierarchy(90%CL)– sin2θ13 < 0.27
– 0.37< sin2θ23 < 0.69
62
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
Super-K: Expected number of events
~7,300 ne+p events~300 n+e events~360 16O NC g events ~100 16O 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))
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008 S.Ando, NNN05
Supernova Relic NeutrinosS.Ando, Astrophys.J.607:20-31,2004.
H.Sekiya, Jul 30th 2008 , Philadelphia, ICHEP2008
0
0.5
1
1.5
2
2.5
3
3.5
4
Constant SN rate
(Totani et al. 1996)
Totani et al. 1997
Malaney et al. 1997)
Hartmann et al. 1997)
Kaplinghat et al. 2004
Ando et al. 2005
Fukugita et al. 2003
Lunardini et al. 2006
SK-II limit = 3.68 /cm2/sec
SK-I limit = 1.25 /cm2/sec Combined limit = 1.08 /cm2/sec
preliminary(E>18MeV)
Super-K results so farFlux limit VS predicted flux
T.iida, poster #90.5
ne can be identified by delayed coincidence.
Neutron tagging in water Cherenkov detector
ne
e+
pn
g
g
Positron and gamma ray vertices are within ~50cm.
2.2MeV g-ray
DT = ~ 200 msec
Another possibility
n+p→d + g
Number of hit PMT is about 6 in SK-III
p
Gd
n+Gd →~8MeV gDT = ~30 msec
GADZOOKS!
(J.Beacom and M.Vagins) Phys.Rev.Lett.93:171101,2004
Add 0.2% GdCl3 in water
This apparatus deployed in the SK tank.
BGO
13 cm
18 cm 1
8 cm
5 c
m
BGO
0.2 % GdCl3Solution
Am/Be
GdCl3 test vesselTest neutron tagging at Super-K
α + 9Be → 12C* + n 12C* → 12C + g(4.4 MeV) n + p → …… → n + Gd → Gd + g (totally 8 MeV)
BGO signal (prompt signal (large and long time pulse))
Look for Cherenkov signal (delayed signal)
H.Watanaba, poster #94
Cherenkov signal of Gd gamma rays
τ = 23.7±1.7μs
[msec]
Num
ber
of E
vent
s
0
Time from prompt
100ms
Vertex position92% within 2m
dR [cm]
Energy spectrum
Measured time, vertex and energy distributions are as expected from the MC simulation.
Recon. Energy [MeV]
Nu
mb
er
of E
ven
ts
Black: DataRed: MC
H.Watanaba, poster #94
10-4
10-3
10-2
10-1
1
10
102
103
104
6 8 10 12 14 16 18 20
SK-I final data sample
SSM(BP2004) * 0.4(efficiencies are considered)
Selection criteria of delayed signal:(1) Vertex position within 2m(2) Energy of delayed signal > 3MeV(3) Time after the prompt within 60msec.(4) Ring pattern cutsSelection 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 8B)
H.Watanaba poster #94
Possibility of SRN detectionRelic 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(Evis =10-30 MeV)
SK10 years (e=67%)
Assuming 67% detection efficiency.
0123456789
10
10 15 20 25 30 35 40 45 50
relic+B.G.(inv.mu 1/5)
B.G. inv.mu(1/5)
atmsph.ν–
e
Visible energy (MeV)
even
ts/1
0yea
rs/2
MeV
Items to be studied before introducing gadolinium to SK
Effect to water transparencyWater transparency should be long enough to do various physics at SK.Water purification systemCurrent water purification system remove ions. So, it must be modified to purify water without removing gadolinium.Material effectsCorrosion by gadolinium solution should be checked.How to introduce/removeHow to mix gadolinium uniformly in the tank. How quickly/economically/completely can the Gd be removed? Ambient neutron level in the tankDoes 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.