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.