暗黒物質（ダークマター）の探し方

東京大学大学院理学系研究科
物理学教室
蓑輪 眞

暗黒物質の“歴史”
1933

F. Zwicky かみのけ座銀河団の速度分散
Virial 定理 << 赤方偏移の光学観測

Helv. Phys. Acta. 6 (1933) 110

Dark Matter の存在

1965

A. Penzias & R. Wilson 宇宙マイクロ波背景輻射 (CMB) の発見
電波観測の発達1973

J. Ostriker & P. Peebles
ApJ. 186 (1973) 467
多体系のシミュレーションによる回転銀河の安定条件<< 我々の銀河の回転速度 220km/s
Dark Matter の存在1970 年代後半

電波（ HI ガス 21cm 輝線）による銀河回転曲線の観測
Dark Matter の存在の立証

1992 COBE

宇宙マイクロ波背景輻射のゆらぎの発見
インフレーションモデル、 Ωtot= 1 を示唆1998 Super Kamiokande ニュートリノ質量の発見
HDMCDMWMAP の援護射撃

Pictures: NASA/WMAP Science Team;
http://map.gsfc.nasa.gov/

近傍にある証拠

halodiscbulge

DAMA's <discovery>by annual modulation
R. Bernabei et al., Riv. Nuovo Cim. 26(2003)1.)

The story

1998 イタリア DAMA
“ Annual Modulation を確認”
2000 アメリカ CDMS
2002 フランス EDELWEISS“すべてのイベントを暗黒物質”とする conventional な方法でDAMA region を Exclude

Spin Independent Interaction

NaIGe(Phys. Lett. B 545 (2002) 43)

Current exclusion limits inSDSI −n and  −pSISD

Dark Matter Candidates Axion ( KSVZ, DFSZ )Neutralino
Lightest Supersymmetric Particle
R-parity の保存から安定

宇宙初期に生成され現在まで存在し続ける gauginosと higgsinos の線形結合の基底状態
 coupling SUSY モデル依存
.39GeV ＜ｍ χSearches, direct and indirect
INDIRECT searches  WW , ZZ ,   e  , p  , d  ,  , ,...

BESS, GLAST, AMS, SK, AMANDA, MACRO, . . .

DIRECT searches
Nuclear recoil detection

Direct detection,relevant parameters●Event rateSUSY 
R~ −N ×n 〈v 〉∝ −N ××∫ v f v  dv
MIsothermal Halo ModelCross section●   N depends fundamentally on the χ-quark interaction strength.
  N

Spin-independent (SI):● Spin-dependent (SD):●H, h, squark exchangeZ, squark exchangeSpin Independent and Spin Dependent cross sections●Cross Section −N =●SI interaction

●SI−N≃A

22
−N
2
−p



2CSI
N  ASD interaction





,
λ : Lande factor

SD
−NSI
−nA : mass number : reduced mass2
−N2
−p J  J 1 =0.75

2SD
−NSI
−NSD
−pJ : total spin of the nucleus

●SD Cross Section●conventional approximation
using the odd-group model in which
the contribution of either proton or neutron isconsidered. 22SD
−N J  J 1 −N SD= −p20.75
−pIsotope

unpairedabundance

λ2J(J+1)

Li

p92.5%

0.411

F

p100%

0.647

23Na

p100%

0.041

73Ge

n

7.8%

0.065

I

p100%0.023

719127(odd-group model)

LiF for SD

Spin Dependent (SD)andSpin Independent (SI)SD
− pSI
−nSD

can be as large as

310 ~10SIFig. 10 of J. I. Collar et al, New J. of Phys. 2 (2000) 14.1

4

●Expected energy spectra
R : countratedRR
 c1 0 e  c2 ER / E 0r [count/keV /kg/day]
dERE0 rE R : recoil energy
c1 ,c 2 : const361     N 
D
 v0

R0 
 [count/kg/ day]M  M N  1pb  0.3GeVcm -3  230km/s v0 : DM velocity

E0 : kinetic energy of DM
M  : DM mass
M N : target massr

4M  M NM  M 2N

 D : DM density

low threshold andlow BG are required1E 0 = M  v 02
2r=

4 MM N 2

 M  M N

E 0 r=10 keV

for M=50GeV and A=19

Mean recoil energy E o r vs target massNuclear form factor

Running experiments and projects(not complete)
Bolometers
CDMS(Ge/Si)● Tokyo(LiF/NaF)● CRESST(Al2O3)●Scintillators/semiconductors● DAMA(NaI, CaF2, LXe)
● UK(NaI)● HDMS(Ge), GENIUS(Ge)● Osaka(CaF2)● XMASS(LXe)Quenching factor●E visible
q=ERq < 1 for scintillators and semiconductors
0.3 (Na), 0.09 (I)0.25 (Ge)0.46 (LXe)→ low effective threshold●q = 1 for bolometers

Techniques for DM detection
Pulse shape discrimination/analysis PSD/PSA(e/γ- nuclear recoil separation)● Bolometry (q = 1)
● Phonon-ionization simultaneous
measurement (e/γ- nuclear recoil separation)●Annual modulation● Direction sensitive detection● Bubble chamber (!)● Background shields●

PSD/PSA (NaI(Tl))
B. Ahmed, et al., Astropart. Phys. 19 (2003) 691.

nuclear recoil

γ6-8 keVStatistical subtraction
B. Ahmed, et al., Astropart. Phys. 19 (2003)
691.
6 – 7 keV
7 – 8 kev

γ-calibration

real data

Bolometer
Specific heat: TC∝
D3

D : Debye temperaturePhonon – ionization
simultaneous measurement
Same response to e/γbut
to nuclear recoil,
q=1for phonon
q = 0.25 for ioninzatione/γ- nuclear recoil
separation

Annual modulation
公転30 km/s
6月

Only a few % effect

12月

Direction sensitive detection
Strong DM wind

暗黒物質の風太陽
DARK HALO(Maxwell 分布)
Ｖ０ ＝220km/s

VＥ
暗黒物質の風

散乱さ れ た暗黒物質

γ
反跳原子核

Heavy liquid bubble chamber
J. I. Collar/U. Chicago
CF3Br / CF3I heavy liquid bubble chamber
Total insensitivity to Minimum Ionizing Particles, yet sensitive to lowenergy nuclear recoils (tunable dE/dx and E-threshold)Background shields
Oxygen Free High Conductivity Copper(OFHC)● Old-age(archaeological) lead
●Pure water● Active self shielding

T 1/ 2 210

Pb ~20 years●

fiducial volume●Radon purge

Tokyo Group (Minowa)●

Bolometer experiments (SD-sensitive)●●●Pilot run LiF
Nokogiriyama(~15m.w.e.) ~1999
Results LiF/NaF Kamioka(~2700m.w.e.)~2003

R&D, direction sensitive scintillators●Pilot run stilbene

Kamioka

2003~

Bolombeter SD limits in ap and an
2
−N2
−p

 J  J 1 =0.75
2SD−NCSDN



SD
−p2
F2
−N=4 G J 1
 (ap  S p(N)   an  S n(N)  )J2

ap , an :  - nucleon couplings

 S p(N) ,  S n(N)  : expectation values of p, n spin in NCSDN

<Sp(N)> and <Sn(N)>For ap , andetermination,
 19F is complementary toNa, 73Ge, 127I
due to opposite sign of23

 S p(N)  /  S n(N)  NaF is more sensitive to an

than LiF.

SD limits in ap and an  S p(N)  /  S n(N)   0  S p(N)  /  S n(N)   0e.g. N i = 23Na, 127I COMPLEMENTARY e.g. N i = 19F

ananapap

Detector
 Bolometer

Wide choice of target● High resolution●Low threshold● No quenching●(Evisible = Erecoil) NTD Ge thermistor●Neutron Transmutation
Doped Germanium

Schematic drawing of a bolometer●Set Up–

Outer Shields

 set at Kamioka Observatory( ~2700m.w.e. )●Inner Shield●old lead (>400yrs.)
from a wrecked ship.
21 Pb < 2.0 Bq/kg
0suspending itwith kevlar cords.
vibration noise
reduction

NaF crystal

●NaF Bolometer Array–

8pcs×22g NaF crystals(2×2×2 cm3)

NaF crystal

Expected sensitivity in the ap–a n plane

NTD Ge thermistor
Assuming the same spectrum as that of LiF target

●Result (SI and SD σχ-p limit)–Assumption : All events are dark matter eventsSI Best limits( for 103 GeV)SDSI : 0.036 pb
SD : 47 pb

●Result (SD limits in the ap–an plane)
50 GeVc-2

anan

an

10 GeVc-2

ap

ap
100 GeVc-2

Kamioka LiF

apan

UK 2000 NaI

predicted
In MSSM

This work NaF
LiF NaF combined

aptan   2, 810 GeV  M 2  1 TeV
10 GeV    1 TeV

 We improved LiF limits in a part of the parameter space.
 Combined limits with the results of LiF are more stringent
than UK2000 limits for 10, 50 and 100 GeV-2.

Direction sensitive scintillators
R&D
銀河中での地球の動き
Cygnus公転 30km/s

232 km/s

太陽DARK HALO
(Maxwell 分布 )Ｖ０＝ 220km/s

地球の自転

DM の入射方向の日変化

地球の自転動きが暗黒物質のシグナルを生むDM wind is fairy strongスチルベンシンチレーター
重荷電粒子の入射方向によって発光量が異なる Z. Phys. 162 (1961) 122
反跳原子核の方向に対する感度が期待される
trans-stilbene

発光量: 30% of NaI

発光波長 : 410 nm
減衰時間 : 即発成分 5 ns

単斜晶系

単結晶

結晶軸 c’ に対する角度により光量異なるmχo(10GeV) の暗黒物質に感度のある低エネルギー (100keV 以下 )の炭素反跳について測定を行った

中性子による発光効率測定
炭素反跳測定の原理

θＮ

中性子散乱角 θn を固定して (θn = 120°)
TOF で入射エネルギー En と散乱エネルギー En’を測定
反跳原子核
反跳エネルギー ER は一意に決まる
反跳角 θNkinematics
中性子↔ＷＩＭＰ

求めるべきもの
発光効率の c’ 軸に対する反跳角 θ 依存性θスチルベン結晶

５ｃｍ φ× ５ｃｍ １１６ｇ

Cryos Beta Ltd.,

Ukraine結果θ炭素反跳の発光効率は c’ axis に関して 7 % 変化
ほかの軸に対する（ φ ）依存性は見られなかった

暗黒物質探索への利用

VE

VＥ

暗黒物質の風

散乱された暗黒物質γ
反跳原子核

スチルベンの発光量は
c’ // VE のとき最小c’ ┴ VE のとき最大暗黒物質にシグナルは光出力スペクトルに現れるcosθ= c’ の方向と反跳方向の内積

暗黒物質探索への利用
Cygnus. の方向
RA21 ｈ 12 ｍ ,dec.+48.19°
神岡（北緯 36 度 25 分）で実験する

c’ 軸を北極に向けておけば地球の自転により風向きとｃ’軸の角度が約 80° 変化する。
スペクトルの変化

２００３．１２．１

変化量

風向きの変化
測定期間中の地球の進行方向と結晶軸ｃ’のなす角 α の変化

α問題点
実験を制限しているもの
1. バックグラウンドレート
PMT 中の放射性不純物（ダイノード構造のセラミック、ステム部分のガラス）
結晶の周りを古い鉛で囲う
光検出器の改善（量子効率の意味でも）Avalanche Photodiode, Prism PMT など
2. スチルベン結晶

Anisotropy not sufficient
A=12 is not large enough for SI detection



SI
−N2

∝A 

より異方性のある、アントラセン・ナフタレンの使用
Spin に依存した相互作用に有利な 19F を含む結晶の製造
Octafluoronaphtalene, dodecafluoroanthracene

END
Tokyo Group ResultsPublications
H. Sekiya, M. Minowa, Y. Shimizu, Y. Inoue, W. Suganuma:
Measurements of anisotropic scintillation efficiency for carbon recoils in a stilbene crystal for dark matter
detection,
astro-ph/0307384, Physics Letters B571 (2003) 132-138.
A.Takeda, M.Minowa, K.Miuchi, H.Sekiya, Y.Shimizu, Y.Inoue, W.Ootani, Y.Ootuka:
Limits on the WIMP-Nucleon Coupling Coefficients from Dark Matter Search Experiment with NaF Bolometer,
astro-ph/0306365, Physics Letters B572 (2003) 145-151.
Y. Shimizu, M. Minowa, H. Sekiya, Y. Inoue:Directional scintillation detector for the detection of the wind of WIMPs,astro-ph/0207529, Nuclear Instruments and Methods in Physics Research Section A 496 (2003) 347-352.
K. Miuchi, M. Minowa, A. Takeda, H. Sekiya, Y. Shimizu, Y. Inoue, W. Ootani, and Y. Ootuka:
First results from dark matter search experiment with LiF bolometer at Kamioka Underground Laboratory,
astro-ph/0204411, Astroparticle Physics 19 (2003) 135-144.

Another challenging DM search
XMASS experiment●Goals☆ Direct detection of Dark Matter→ Discovery of Dark Matter☆ Real time observation of low energy solar  (pp, 7Be)
→ Precise determination of  oscillation parameters☆ Observation of 0 decay→ Majorana property and absolute mass of 
XMASS = Multipurpose Ultra low-background detector
with liquid Xe

●Key idea
Self shielding for  ray background by liquid Xe (Z=54)

Reconstruct the vertex of events from PMTs information→  ray backgrounds are absorbed in outer volume→ Dark matter can go into fiducial volume

●Strategy of the scale-up

100kg prototype

800kg detector～ 10ton detector～ 30cm～ 80cm

～ 2.5mR&D
Dark matter search

Now

Multipurpose detector

●Sensitivity of 800kg detector for DM (SI)

Cross section for nucleon (pb)Spin independent interaction
10-4

Discover!
DAMA (NaI)

Exclude10-6

Edelweiss (Ge)ZEPLIN (Xe)
CDMS (Ge)10-8

10-10 1
10103
102
WIMP Mass (GeV)

XMASS 800kg detector (Xe)
(5keV threshold, 5year)
Annual modulation (3 discovery)
Raw spectrum (3 discovery)>102 improvement of sensitivity for existing experiments

END
XMASS Project

Sensitivity necessary for thediscovery of DM (SI)SI
−n−12≥2×10pb ?

●How can one reach●Current typical limit with NaI(Tl) – PSA by UKgroup is:dR−1≤4×10 events/ kg /day / keV @ 4−5 keVd E visible

SI
−n−5

≤2×10 pb@ M ~100 GeV

Well experienced experimentalist
would not say that
One has to go down todR−8≤4×10 events/ kg /day / keV @ 4−5 keVd E visible●
with 6 p.e. /keV (NaI(Tl))
5 p.e. /keV (LXe)● even lower p.e.'s for larger volume detectors●

To go further,Huge mass conventional detectors never help
even with PSA.
➔ exposure[kg ・ days]
more than-1
several× {background rate[/kg/day]}is not necessary● Bolometr background rate is too high.
● Annual modulation effect is too small.
● Innovation needed➔ Direction sensitive (and, hopefully, SDsensitive) detector➔ Self-shielding ultra pure material(e.g. LXe)
➔ Even cleverer detection technique to bediscovered (e. g. Bubble chamber)●