Dark Matter Search with CsI ( Tl ) crystal scintillators : KIMS
A WIMP dark matter search with CsI ( Tl ) crystal
description
Transcript of A WIMP dark matter search with CsI ( Tl ) crystal
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A WIMP dark matter search with CsI(Tl) crystal
SeungCheon KimDepartment of Physics & Astronomy
Seoul National University
Seminar@Fermilab (2012-07-31)
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Outline
A brief introduction about dark matter
A Description about KIMS experiment
A study about the background
The recent results
The prospect
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Dark matter ?
Hubble image of coma cluster
1936 Zwicky first proposed the existence of dark matter, inferred from the motion of galaxies in coma cluster.
=> The total mass of the luminous matter is too small to explain the velocity of the galaxy in the cluster!
107-108KWhat is holding such hot gases ?
The recent x-ray measurement of the cluster also requires the invisible matter!
4Big Bang Nucleosynthesis
Gravitational Lensing Rotation curve for Galaxy
Bullet clustercosmic microwave
background
A pile of evidences for dark matter !!!
99 % of people believe the dark matter…
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What we know about dark matter
Dark (invisible) Þ no EM interaction rarely interacting very stable
MassiveÞ gravitationally evident non-relativistic (cold – most, not necessarily all of them) Influencing the structure formation of the Universe
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What we know about dark matter
Overwhelmingly abundantÞ the majority of the matter constituents in the universe ExoticÞ Not made of the known matter Need of the new theory other than SM (multifunctioning particle – not invented for DM – favored)
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WIMP (Weakly Interacting Massive Particle)- a good dark matter candidate
Ω : energy density of the universeΩ = ΩDM + ΩΛ + Ωbaryon = 1 for the flat universeΩDM = 0.222 ± 0.026 from WMAP
The weakly interacting relic particles can survivethe annihilation and so exist in this level of the density.
Supersymmetry and Univ-ersal extra dimensiontheories provide good candidates of this kind.Þ lightest neutralino lightest kluza-klein particle
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WIMP (Weakly Interacting Massive Particle)- a good dark matter candidate
And, it is testable with the experiment, even though very challenging.
It has been expected WIMP will deposit a few tens keVin the detector absorber.
The event rate will be very small.
Background reduction & discrimination is essential.
σ χ-nu-
cleon =10-4 pb
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KIMS (Korea Invisible Mass Search)
A WIMP search experiment using CsI(Tl) scintillators
It has been carried out at Yangyang underground lab(Y2L),South Korea.
Seoul National University: H.C.Bhang, J.H.Choi, K.W.Kim, S.C.Kim, S.K.Kim, J.H.Lee, J.I.Lee , J.K.Lee, M.J.Lee, S.J.Lee, J.Li, X.Li, S.S.Myung,S.L.Olsen, I.S.SeongSejong University: U.G.Kang, Y.D.KimKyungpook National University: H.J.Kim, J.H.So, S.C.YangYonsei University: M.J.Hwang, Y.J.KwonEwha Womans University: I.S.HahnKorea Research Institute of Standard Sciences:Y.H.Kim, K.B.Lee, M. LeeTsinghua University : Y.Li, Q.Yue, J. Li
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CsI (Tl) scintillator ?
Widely used, well-known detectorEasy handling
Large scintillation yield : 60,000/MeV
technique
nuclear recoil
electron recoilLow hygroscopicity
Pulse Shape DiscriminationÞ Distinguishing nuclear recoil event at the statistical basis
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CsI (Tl) scintillator ?
Large atomic number, Cs (133), I (127) Good for A2 scaling
High spin expectation value for proton Sensitive to SD interaction
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CsI (Tl) scintillator ?
However, it has some inherent background.
137Cs artificial radioisotope , half-life of 30 yearsbeta decay & delayed (2.5min) gamma of 662 keV
134CsProduced by the capture of neutron from the cosmicray muon, half-life of 2 years, beta decay & severalprompt gamma rays
87RbIt exists in the pollucite (ore for Cs).Natural abundance of 27.84 %, beta decay
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CsI (Tl) scintillator ?
It contains iodine (I) like DAMA/LIBRA experiment.Þ Cross-checking the WIMP-I interaction scenario for DAMA positive signal.
So, it’s very good idea to search for WIMP usingCsI (Tl) scintillators if one can manage its internal background .
One detector module : one CsI Crystal + 2 PMTs
PMT : 3” PMT (9269QA),RbCs photo cathode (green extended)
Crystal size: 8x8x30 cm3 (8.7 kg)(Beijing Hamamatsu Photon Techniques Inc.)Background level: 2-3 counts/keV/kg/day (cpd)
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Am241 calibra-tion
13.9keV Np L X-ray17.8keV Np L X-ray20.8keV Np L X-ray26.35keV gammaCs, I X –ray escape59.54 gamma...
Light yield:5-6 p.e/keVEth = 3 keV
- data- simulation (GEANT4.9.5)
CsI (Tl) detector for KIMS
Event window is 40µs.Digitized with 400MHz FADC
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Detector array
3 x 4 detector arrayThe total mass : 103.4 kg
Multiple hit eventsÞ Vetoed for DM search But, references for calibration & event selection
E of det10
Total E except det10
134Cs
134Ba
4+3+4+
2+
2+
0+
1969.871643.281400.551167.93
604.70
0.0
E [keV]β
-
27.3%2.50%70.1%0.033%0.10%
Decay for 134Cs (T1/2=2.06yr)Measurement: 2.053±0.145yr
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Neutron shield & Muon Veto(30 cm mineral oil)
Lead shield (15 cm)
Polyethylene(5 cm)
Copper shield(10 cm)
12 x CsI (Tl) crystal
N2 gas flow inside the Cu shield
KIMS Detector system
Yangyang Pumped Storage Power Plant
Experimental site:Yangyang Underground Laboratory (Y2L)
Minimum depth : 700 m / Access to the lab by car (~2km)
(Upper Dam)
(Lower Dam)
(Power Plant)
KIMS (DM Search)AMoRE (Double Beta Decay Experiment)
SeoulY2L
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The Experimental hall
Muon flux : 2.7 x 10-7/cm2/s (2000 m.w.e)
Neutron flux : 8 ×10−7/cm2/s for 1.5-6 MeV neutronsRn background in the air : 1-2 pCi/l
Clean room systemOnline-Monitoring of temperature, humidity, power stability, Rn background level
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Background study Multiple hit only
α spectrum
Crystal : 134,137Cs, 87RbPMT : 238U, 232Th, 40KSurface : progenies of 222Rn
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Background study
pmt0pmt1
Surface alpha (SA) background
Scintillation at PMT body
Clean acryl boxPMT PMT
These events makes troublein pulse shape discrimination.SA => Characterize its PDFPMT events Þ Find cuts to minimize its contamination.
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γ & β back-ground(electron recoil (ER) events)-main background
Background level (cpd) & their origin
Internal source ~ 1 (134,137Cs) ~ 0.1 (238U, 232Th)~ 0.3 ( 87Rb)
PMT ~ 0.5 (238U, 232Th, 40K)Crystal surface (progenies of 222Rn)
0.1 – 1 (?) (mainly, 210Pb)
Background at the low energy A few tens of recoil energy will be measured as a few keVbecause quenching.The current background level below 10 keV : 2.8 counts/keV/kg/day (cpd) + PMT dark current
Surface alpha (SA)
~ 0.1 Scintillation at PMT body
< 0.1
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Rejection of events produced from PMT itself=> PMT body scintillation & dark current (PMT events)
PMT generates events by itself.Þ PMT body scintillation + After pulse + random coincidence + etc.=> It can be problematic for the low energy, rare phe-nomena search.
After pulseÞ A delayed (~1 μs) pulse following the primary pulseÞ The atom in the PMT vacuum ionized by the accelerat-
ing photo-electron drifts to the photo- cathode producing a big signal (spike-like).
Signal size=> # of SPE
Time span btw neighboring SPEs(us)
pmt0 pmt1
Afterpulses in PMT Dummy Detector
Afterpulse by postive ion(H,He,N2 ...) in PMT
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Clean acryl boxPMT PMT
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PMT events rejection
Compton events
From the PMT dummy detector data,We have developed the event selection condition tominimize the PMT events contamination.The Efficiency for nuclear recoil (NR) events (WIMP candidate) : ~30%
tail events
Clean acryl boxPMT PMT
Pulse Shape Discrimination (PSD)
Discriminating the event type by the pulse shapeEnabling the estimation of NR event rates statistically
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, LMT10=logi ie
i
Att t
A
PSD parameter
Reference data for PSDSurface alpha (SA) : from the Radon-contaminated crystalNuclear Recoil (NR) : by irradiating the test crys-tal with Am-Be neutron source Electron Recoil (ER) : compton scattering
SA
NR
ER
at 3 keV
Opening the data…
Period: September 2009 – August 2010 => 317.15 live daysExposure: 24524.3 kg•days
After applying the PMT noise selection cut,Assuming the data are composed of SA, ER and NR eventsEstimating the fraction NR event rates with PSD.
Candidate events for WIMP : NR occurring in one detector
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The estimation of the NR events rate
Pdf = f0 x FNR + f1 x FSA + (1-f0-f1) x Fgamma
The posterior pdf for f0 & f1 is obtainedfrom Bayesian analysis method.
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The estimated NR event rates for 12 detectors
90 % limit68 % interval
SA
The positive signals at higher energy bins seem cor-related with SA contamination.
The PSD PDF of NR at higher energy bins might be contaminated with gamma ray caused by the inelasticscattering. => need of further study
For the conservative claim, in the combining all the detectors,the detectors with high SA contamination are ex-cluded.
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The overall NR events ratedet0, 8, 11 excludedTotal exposure: 24524.3 kg days
No meaningful excess of NR events is observed.
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KIMS NR event rates & DAMA/LIBRA annual modulation amplitude
The annual modulation amplitude from DAMAÞ At 2-4keV, 0.0183±0.0022 counts/day/kg/keV
Þ At the corresponding energy range (3.6-5.8 keV) in KIMS, (QF for DAMA =0.08 adopted) The 90 % C.L limit is 0.0098 counts/day/kg/keV This is well below the DAMA modulation amplitude. Any scenario with Iodine for DAMA is not consistent with KIMS results !!!
DAMA/LIBRA The spectrum of the amplitude of the annual modulation
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KIMS & DAMA in iDM model
For 70 GeV mass WIMP
CsI
Expected spectrum for iDM for I & Cs target=> M=70 GeV, δ=116keV, vesc=500 km/s, ve=235km/s, σ=0.93e-3 pb
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The new exclusion limits from KIMS
Spin-Independent interaction Spin-dependent proton interaction
Published in PRL 108, 181301 (2012)
Annual modulation study in KIMS
The analysis of 2.5 years data is almost completed.PSD was not taken into account. There’s time-varying background from 134Cs (2year half-life).The results will be reported very soon.
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Sep. 2009 ~ Feb. 2012
3-6 keV
Prospect of KIMS experiments
New PMTs are under test.Þ ~10 p.e/keV ? Decreasing the energy thresholdÞ Reduced 238U, 232Th, 40K Decreasing current background level by ~ 1 cpd
The treatment of the surface backgroundÞ Polishing or surrounding with active veto layer ?
Adding low background NaI (Tl) scintillators is be-ing planed.
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Summary
12 CsI (Tl) scintillators (103.4kg) have beenrunning at Yangyang underground lab.
Most of backgrounds of the detector has been understood.
The improved limit of WIMP-nucleon interaction hasbeen derived based on 1 year data with PSD.
Annual modulation study based on 2.5 years was almost done and will be reported very soon.
New upgrade is being planed.
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Back up
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Event rate of WIMP in the detector
number of target nuclei
number density of WIMP
WIMP nucleon cross sectionCoherent enhancement along nucleus
nuclear form factor
The effect by the velocity Distribution of WIMP
If σn = 10-5 pb, for 100 GeV WIMP and Iodine target,The expected event rate is 1-2 counts/keV/kg/day,considering quenching.
CsI (Tl) detector for KIMS
One detector module : one CsI Crystal + 2 PMTs
PMT : 3” PMT (9269QA), Electron tubeRbCs photo cathode(green extended)
Crystal size: 8x8x30 cm3 (8.7 kg) (Beijing Hamamatsu Photon Techniques Inc.)Background level: 2-3 counts/keV/kg/day (cpd)
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Am241 calibra-tion
13.9keV Np L X-ray17.8keV Np L X-ray20.8keV Np L X-ray26.35keV gammaCs, I X –ray escape59.54 gamma
5-6 p.e /keV
Neutron shield(30cm mineral oil)
Lead shield (15cm)
Polyethylene(5cm)
Copper (10cm)
CsI(Tl) ScintillatorNeutron detectorMuon detector(Neutron sheild)
KIMS Detector system
N2 gas flow inside the Cu shield
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Annual Modulation Studies (w/o PSD)
Sep. 2009 ~ Feb. 2012
Total DAQ rate is under 6Hz . 2.5 year data to see annual modulation ; 75.53 ton∙daysThe temperature of detector array is 20 - 21.6 OC depending
on the position, and it is maintained stably with a maximum fluctuation of around 0.2 OC.
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3-6 keV0
0 02cos ( ), 2.980 , t 153(June 2)365
t t
peak peakR N e b A t t y
Annual modulation amplitude is ob-tained including the exponential de-cay of 134Cs.
Annual modulation amplitude is consistent with null.
The 90% upper limit of the ampli-tude is comparable to DAMA’s an-nual modulation signal (0.0189 cpd/kg/keV)
We are trying to give final numbers shortly in this summer.