Dark Matter Experiments: LiF/NaF Bolometer & Organic Crystal Scintillator

1. The Tokyo Dark MatterExperiment NDM03 13 Jun. 2003 , Nara Hiroyuki Sekiya University of Tokyo

2. Tokyo Group Dark Matter Search Projects • LiF / NaF bolometer for SPIN DEPENDENT interacting neutralino search. • Recoil direction sensitive organic crystal scintillator • for WIMP wind detection. Present collaborator M. Minowa, Y. Inoue, H. Sekiya, Y. Shimizu, W. Suganuma (University of Tokyo)

3. 1 LiF / NaF Bolometer • K. Miuchi et al., Astropart. Phys. 19 (2003) 135 • A. Takeda et al., Submitted to Phys. Lett. B

4. Motivation Neutralino-nucleus cross section Reduced mass Fermi coupling constant • Enhancement factor • If only either proton or neutron contribution is considered, SD interaction term . Odd group model λ:Landé factor J :Total spin in the nuclei 19F has the largest λ2J(J+1)

5. Motivation More precisely D.R.Tovey et.al., PLB 488 (2000) 17 • If contributions of both proton and neutron are considered, <Sp>N nucleon spin <Sn>N in the nucleus ap’an : χ-nucleon coupling 2 parameters should be determined. • 19F has opposite sign of <Sp>N/<Sn>N compared to 22Na, 73Ge,127I. 19F fills the unique role for ap, an determination.

6. 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.

7. Detector NaF Crystal • Bolometer array consists of 8 crystals and NTD thermistors. • Each crystal is • 2cm×2cm×2cm. Total mass • LiF array 168g • NaF array 176g OFHC copper NTD thermistor 1 of 2 stages

8. Shields Old lead* Old lead 200 years old for LiF run 400 years old for NaF run

9. Measurement at Kamioka • Depth 2700 m.w.e (1000 m) • Muon flux ～10-7 cm-2s-1 Dark Matter

10. Measurement at Kamioka • Installation started in Nov. 1999 • LiF run: 22 Nov. 2001- 12 Jan. 2002 • NaF run: 23 Dec. 2002 – 24 Jan. 2003

11. 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.

12. Results – SD σχ-p limits • Assuming all events are χ-proton events. • Best bin method. Best limits (90% C.L.) SD: 23pb for 40GeV Astrophysical parameters

13. Results–SD limits in the ap-an plane • 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.

14. 2. Organic Crystal Scintillator • Y. Shimizu et al., Nucl. Instr. Meth. A 469 (2003) 347 • H. Sekiya et al., Submitted to Phys. Lett. B

15. Motivation • Realistic 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 convincingsignature of the WIMP. • WIMP wind is blowing on the earth! • WIMP signature would be identified by direction sensitive detector. 230km/s Sun isothermal halo

16. 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) 475 • We had measured this property of the stilbene crystal because of its availability. stilbene • Light yield : 30% of NaI • λmax : 410 nm • Decay time : 5 ns monoclinic system

17. Detector • 2cm x 2cm x 2cm • 55 Fe 5.9 keV spectrum using PMT (HAMAMATSU R329) • Actual light output : 2 p.e/keV

18. 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. Methods θN incident neutron energy En scattered neutron energy En’ with fixed scattered angle θn θn target crystal • Recoil energy ER and Recoil angle θN are uniquely determined by kinematics. neutron counter Evisible • Relative efficiency (ER, θN) = ER

19. Calibration with neutrons • Two neutron sources are employed. • Low energy region precisely (40 keV < ER <140keV) • 7Li(p,n)7Be using 3.2 MV Pelletron accelerator @ Tokyo Institute of Technology • Wide energy region (30 keV < ER < 1MeV) • 252Cf

20. 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

21. Application to Dark Matter Detector Scattered WIMP VE Recoil directions show a peak at γ= 180° WIMP wind γ Nuclear Recoil Light outputs should be • minimum at c’ // VE. • maximum at c’┴ VE. WIMP signature appears in the light output spectra.

22. Expected Spectra Assuming spherical isothermal halo. • Difference of the spectra similar to that of annual modulation is predicted. • The measurements can be achieved more robustly. ρ0=0.3 GeV/cm, v0=220 km/s, vE=232km/s

23. Our Plans The earth moves toward constellation Cygnus. (R.A. 21h12.0’, Dec +48.19°) WIMP wind 0°to c’ 38° 42°to the earth’s axis VE 42° 84°to c’ If the c’ axis is inclined 4°to the North pole, WIMP incident angle with respect to c’ axis modulates 84°over 12 sidereal hours at N38°(KAMIOKA)

24. Status • Preparing Stilbene 5cmφ x 5cm (100g) x 3 • Low background PMTs. (HAMAMATSU R8778MOD) • With the shields of the bolometer, pilot run will be performedatKamioka 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.

25. 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 still high, we exclude 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.