神岡地下実験室における 暗黒物質探索実験 RESCEU 研究会 2003 年 9 月 2 日 ＠鴨川 関谷洋之 東京大学 みのわ研究室

Composition of the Universe Ω Results of WMAP

2/12 Asahi.com

Neutralino as Dark Matter  DM Axion ( KSVZ, DFSZ )  SUSY WIMPs ( LSP : neutralino ) Non Baryonic Cold Dark Matter Candidate Leading candidate Lowest mass, linear superposition of Photino, zino, higgsino Various implementation of MSSM leads to various Mass window : GeV < < TeV Neutralino

DM Neutralino Search Looking for antiprotons, positrons, gamma rays ・・・ produced by WIMP annihilation in the GALACTIC HALO. BESS, GLAST, AMS, ISS, … Looking for high energy neutrinos as final products of WIMP annihilation in the celestial bodies (EARTH, SUN). SK, AMANDA, MACRO, … Measuring the nuclear recoil produced by the elastic scattering of WIMP by the DETECTOR nuclei. DAMA, CDMS, UKDMC, TOKYO, … Ｄｉｒｅｃｔ Ｓｅａｒｃｈ

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) 135 A. Takeda et al., Submitted to Phys. Lett. B

Motivation Neutralino-nucleus cross section Reduced mass Fermi coupling constant Isotopeunpairedabundanceλ 2 J(J+1) 19 Fp100% Lip92.5% Nap100% Gen7.8% Ip100%0.023 Enhancement factor If only either proton or neutron contribution is considered, SD interaction term. λ:Landé factor J :Total spin in the nuclei 19 F has the largest λ 2 J(J+1) Odd group model

Motivation More precisely If contributions of both proton and neutron are considered, a p ’ a n : χ-nucleon coupling N nucleon spin N in the nucleus N 19 F has opposite sign of N / N compared to 22 Na, 73 Ge, 127 I. 2 parameters should be determined. IsotopeJ N 19 F1/ Li3/ Na3/ Ge9/ I5/ F fills the unique role for a p, a n 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 thermistor OFHC 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 ～ cm -2 s -1 Measurement at Kamioka Dark Matter

Measurement at Kamioka Installation started in Nov LiF run: 22 Nov Jan NaF run: 23 Dec – 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 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 σ χ-p limits ρ DM 0.3 GeV cm -3 v0v0 220 km s -1 v esc 650 km s -1 v Earth 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 a p -a n 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 19 F.

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

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 convincing signature of the WIMP. WIMP signature would be identified by direction sensitive detector.  WIMP wind is blowing on the earth! Sun 230km/s isothermal halo

Detector Scintillation efficiency of organic single crystal depends on direction of nuclear recoils with respect to crystallographic axes. We 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 θNθN θnθn Recoil energy E R and Recoil angle θ N are uniquely determined by kinematics. E visible ERER Methods  Relative efficiency (E R, θ N ) = incident neutron energy E n scattered neutron energy E n ’ with fixed scattered angle θ n neutron counter

Calibration with neutrons  7 Li(p,n) 7 Be using 3.2 MV Pelletron accelerator Two neutron sources are employed. Low energy region precisely (40 keV < E R <140keV) Wide energy region (30 keV < E R < 1MeV)  252 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. VEVE WIMP wind Nuclear Recoil Scattered WIMP γ Recoil directions show a peak at γ= 180° Light outputs should be minimum at c’ // V E. maximum at c’ ┴ V E.

Our Plans 42°to the earth ’ s axis 42° 38° ～ 80°to c ’ ～ 0°to c ’ VEVE WIMP wind The solar system moves toward constellation Cygnus. (R.A. 21h12.0’, Dec °) WIMP incident angle with respect to c’ axis modulates about 80°over 12 sidereal hours at N38°(KAMIOKA) If the crystal is placed with c ’ axis pointing to the North pole,

本日の風向き予報 北 東西 ９：００ １５：００ 暗黒物質は北風！ C ’ 軸との角度 水平に北に向けたとき

Expected Spectra Assuming spherical isothermal halo. ρ 0 =0.3 GeV/cm, v 0 =220 km/s, v E =232km/s  The measurements can be achieved more robustly.  Difference of the spectra similar to that of annual modulation is predicted.

Status Stilbene 5cmφ x 5cm (100g) x 3 Low BG PMTs. (HAMAMATSU R8778MOD) Low BG NaI for active shields.(Horiba) With the shields of the bolometer, pilot runs are performed at Kamioka underground laboratory. Preparing naphthalene crystal which is expected more anisotropic response. (50%? Z. Phys ) 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 is 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.