the tokyo dark matter experiment ndm03 13 jun. 2003, nara hiroyuki sekiya university of tokyo
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The Tokyo Dark Matter Experiment
NDM03 13 Jun. 2003 , Nara
Hiroyuki SekiyaUniversity of Tokyo
Tokyo Group Dark Matter Search Projects
LiF / NaF bolometer for SPIN DEPENDENT interacting neutralino search.
Present collaborator
M. Minowa, Y. Inoue, H. Sekiya, Y. Shimizu, W. Suganuma (University of Tokyo)
Recoil direction sensitive organic crystal scintillator for WIMP wind detection.
1 LiF / NaF Bolometer
K. Miuchi et al., Astropart. Phys. 19 (2003) 135A. Takeda et al., Submitted to Phys. Lett. B
MotivationNeutralino-nucleus cross section
NNFN CG 224 F
N
NN
G
MM
MM
Reduced mass
Fermi coupling constant
Isotope unpaired abundance λ2J(J+1)19F p 100% 0.6477Li p 92.5% 0.41123Na p 100% 0.04173Ge n 7.8% 0.065127I p 100% 0.023
)1(2 JJC SDN
)( 2AC SIN
Enhancement factorSIN
SDNN CCC
If only either proton or neutron contribution is considered, SD interaction term .
λ:Landé factor J :Total spin in the nuclei
19F has the largest λ2J(J+1)
Odd group model
MotivationMore precisely
If contributions of both proton and neutron are considered,
J
JSaSaC
NnN
12
nppSDN
ap’ an : χ-nucleon coupling
<Sp>N nucleon spin<Sn>N in the nucleus
19F has opposite sign of <Sp>N/<Sn>NN compared to 22Na, 73Ge,127I.
2 parameters should be determined.
Isotope J <SP>N <Sn>N
19F 1/2 0.441 -0.1097Li 3/2 0.497 0.00423Na 3/2 0.248 0.02073Ge 9/2 0.009 0.372127I 5/2 0.309 0.075
19F fills the unique role for ap, an determination.
D.R.Tovey et.al., PLB 488 (2000) 17
Detector
NTD Ge thermistor is glue to the LiF / NaF crystal which is cooled down 10mK.
The thermistor is biased through load register, and the voltage change across the thermistor is fed to the voltage sensitive amplifier placed at the 4K cold stage.
Detector
NTD thermistorOFHC copper
NaF Crystal
Bolometer array consists of 8 crystals and NTD thermistors. Each crystal is 2cm×2cm×2cm.
1 of 2 stages
Total mass LiF array 168g NaF array 176g
Shields
Old lead*
Old lead 200 years old for LiF run 400 years old for NaF run
Depth 2700 m.w.e (1000 m)Muon flux ~ 10-7 cm-2s-1
Measurement at Kamioka
Dark Matter
Measurement at Kamioka
Installation started in Nov. 1999LiF run: 22 Nov. 2001- 12 Jan. 2002NaF run: 23 Dec. 2002 – 24 Jan. 2003
Obtained Energy Spectra
Total exposure : 4.1 kg days (LiF) 3.4 kg days (NaF)
The spectra look quite similar and the BG rate is about 10-30 counts/keV/day/kg in this energy region.
It is quite likely that the remaining background is due to radioactive impurities in the OFHC copper holder or the NTD thermistor.
Results – SD σχ-p limits
ρDM 0.3 GeV cm-3
v0 220 km s-1
vesc 650 km s-1
vEarth 217 km s-1
Astrophysical parameters
Best limits (90% C.L.) SD: 23pb for 40GeV
Assuming all events are χ-proton events. Best bin method.
Results – SD limits in the ap-an plane
J
JSaSa
NNp
12
nnpSD
N-WIMP
We derived the ap-an limits from the cross section limits.
We compared with the UKDMC because they provided the limits from Na and I respectively.
Although BG rate is still high, we killed more than 2/3 of the parameter space allowed by UKDMC for 50 GeV WIMP because of 19F.
2. Organic Crystal Scintillator
Y. Shimizu et al., Nucl. Instr. Meth. A 469 (2003) 347H. Sekiya et al., Submitted to Phys. Lett. B
MotivationRealistic distinctive WIMP signals arise from the earth’s motion in the galactic halo.Annual modulation of event rate is one of the possible WIMP signal caused by earth’s revolution around the sun (30km/s).
The earth’s velocity through the galactic halo (230km/s) provides more convincing signature of the WIMP.
WIMP signature would be identified by direction sensitive detector.
WIMP wind is blowing on the earth!
Sun230km/s
isothermal halo
Detector
Scintillation efficiency of organic single crystal depends on direction of nuclear recoils with respect to crystallographic axes.Study of anthracene as a WIMP detector was reported by DAMA. Nuovo Ciment C15(1992) 475We had measured this property of the stilbene crystal because of its availability.
stilbene
monoclinic system
Light yield : 30% of NaI
λmax : 410 nm
Decay time : 5 ns
Detector
2cm x 2cm x 2cm
55 Fe 5.9 keV spectrum using PMT (HAMAMATSU R329)
Actual light output : 2 p.e/keV
Calibration with neutrons
Carbon recoils would be effective in detecting WIMPs with interesting mass region.
We had measured the recoil angle/energy dependency of carbon recoils in the stilbene single crystal with neutrons.
target crystal
θN
θn
Recoil energy ER and Recoil angle θN are uniquely determined by kinematics.Evisible
ER
Methods
Relative efficiency (ER, θN) =
incident neutron energy En
scattered neutron energy En’
with fixed scattered angle θn
neutron counter
Calibration with neutrons
7Li(p,n)7Be using 3.2 MV Pelletron accelerator
Two neutron sources are employed.
Low energy region precisely (40 keV < ER <140keV)
Wide energy region (30 keV < ER < 1MeV)
252Cf
@ Tokyo Institute of Technology
Results
The efficiency depends on the angle with respect to c’ axis, and the variation is about 7 %.
We cannot see any dependence on the angle with respect to other crystallographic axes.
There is a clear increase at low energy
Application to Dark Matter Detector
WIMP signature appears in the light output spectra.
VE
WIMP wind
Nuclear Recoil
Scattered WIMP
γ
Recoil directions show a peak at γ= 180°
Light outputs should be
minimum at c’ // VE.
maximum at c’ ┴ VE.
Expected Spectra
Assuming spherical isothermal halo.
ρ0=0.3 GeV/cm, v0=220 km/s, vE=232km/s
The measurements can be achieved more robustly.
Difference of the spectra similar to that of annual modulation is predicted.
Our Plans
42°to the earth’s axis 42°
38°
84°to c’
0°to c’
VE
WIMP windThe earth moves towardconstellation Cygnus. (R.A. 21h12.0’, Dec +48.19°)
WIMP incident angle with respect to c’ axis modulates 84°over 12 sidereal hours at N38°(KAMIOKA)
If the c’ axis is inclined 4°to the North pole,
Status
Preparing Stilbene 5cmφ x 5cm (100g) x 3Low background PMTs. (HAMAMATSU R8778MOD)With the shields of the bolometer, pilot run will be performed at Kamioka underground laboratory.
Preparing naphthalene crystal which is expected more anisotropic response. (50%? Z. Phys 165 1961 1)
naphthalene
Prospects octafluolonaphthalen, dodecafluoloanthracene for SD interacting WIMPs.
Summary
Tokyo group carries out two dark matter search projects.SD interacting WIMP search was performed using LiF/NaF bolometer at Kamioka Observatory.We derived the limits on the ap-an plane (WIMP-nucleon couplings) . Although BG rate was still high, we excluded large parameter space allowed by UKDMC.
Recoil direction sensitive detector with organic single crystal is developed.The anisotropic scintillation response of carbon recoils in stilbene crystals are confirmed and the sensitivity for dark matter was investigated.We are preparing the dark matter search experiment with organic single crystals at Kamioka.