The XMASS Experiment searching for Dark Matter at …...The XMASS Experiment searching for Dark...

The XMASS Experimentsearching for Dark Matter

at the Kamioka Observatory

Status and Previous Results

Kai Martens

Kavli IPMUThe University of Tokyo

January 17, 2014 Kai Martens, Kavli IPMU, @ University of Utah 2

Outline

- Kavli IPMU: a word about my institute

- Dark Matter: setting the stage

- Kamioka Observatory: the preeminent underground sciencelaboratory in Japan

- XMASS Experiment: - design

- results from first commissioning phase

- refurbishing

- the next step in the program


Kavli IPMU

one of the 5 original WPI centers:- founded Oct. 1, 2007- first institute in Todias Jan. 11, 2011- first Kavli institute in Japan May 10, 2012

best evaluations among WPI centers5 year “extension” to be decided soon...


The Kamioka Observatory:

KamLAND (Xe)

CANDLES

NEWAGEEGADS

Super-Kamiokande,(T2K)

XMASS

IPMULab

CLIO Legend:DM, , , GW

3km

to M

ozum

i

new: KAGRA

Tokyo

Nagoya

IPMU Lab:U, Th → HP-Ge ← few 100ppt

→ ICP-MS ← 1ppq*Kr → API-MS ← 1ppt*

* after concentration


The XMASS CollaborationKamioka Observatory, ICRR, University of Tokyo:

Y. Suzuki, M. Nakahata, S. Moriyama, M. Yamashita, Y. Kishimoto,

A. Takeda, K. Abe, H. Sekiya, H. Ogawa, K. Kobayashi, K. Hiraide, B. Yang,

K. Ichimura, O. Takachio, N. Oka, K. Nakagawa, M.Kobayashi

IPMU, University of Tokyo: K. Martens, J.Liu

Kobe University: Y. Takeuchi, K. Miuchi, K. Hosokawa, Y.Ohnishi

Tokai University: K. Nishijima

Gifu University: S. Tasaka

Yokohama National University: S. Nakamura, I. Murayama, K. Fujii

Miyagi University of Education: Y. Fukuda

STEL, Nagoya University: Y. Itow, K. Kobayashi, K. Masuda, H. Uchida, H. Takiya

Sejong University: Y.D. Kim, N.Y. Kim

KRISS: Y.H. Kim, M.K. Lee, K.B. Lee, J.S. Lee

40 collaborators, 10 institutions, 2 countries


Early Observations: Oort & Zwicky 1932: Jan Oort (Leiden, Netherlands):

“measures” matter in galactic disk near sun from motion of nearby stars: 1M

⊙/375ly3 > 2m

stars

→ dark matter

1933: Fritz Zwicky (Caltech, USA):

virial theorem: ΣT = ½ ΣU→ galaxies in Coma cluster too fast for cluster's gravitational potential

→ dark matter

Vera Rubin @ Carnegie Institutionof Washington:

rotation curves of galaxies1975 AAS meeting

1980 paper


New Scientist (2005/03/19): Michael Brooks: 13 things that do not make sense: Maybe we can't work out what dark matter is because it doesn't actually exist. That's certainly the way Rubin would like it to turn out. "If I could have my pick, I would like to learn that Newton's laws must be modified in order to correctly describe gravitational interactions at large distances," she says. "That's more appealing than a universe filled with a new kind of sub-nuclear particle."

@ Carnegie Institutionof Washington:

1975 AAS meeting 1980 paperA: expected curve

given luminous matterB: observed curve...

http://apod.nasa.gov/ apod/ap110219.html:inner part of NGC 2841 by HST

measured60 galaxies

viagas clouds

(H, He)[not stars!]

vf

Galactic Rotation Curves: Vera Rubin

http://apod.nasa.gov/


Dark Matter: It's out there …

http://apod.nasa.gov/apod/ap060824.html

http://cosmicweb.uchicago.edu/filaments.html

blue: DM, red: hot gas

≈


So What Are We Looking For?

- gravitating: the only thing we know for sure...

- cold: → massive ← if particle (cold DM...)

- collisionless: → no E/M interaction→ no strong interactionat most: weakly interacting ???

(Rubin as in: Vera Rubin)

New Scientist (2005/03/19): Michael Brooks: 13 things that do not make sense: Maybe we can't work out what dark matter is because it doesn't actually exist. That's certainly the way Rubin would like it to turn out. "If I could have my pick, I would like to learn that Newton's laws must be modified in order to correctly describe gravitational interactions at large distances," she says. "That's more appealing than a universe filled with a new kind of sub-nuclear particle."

favorites are: neutralino, axion, Kaluza-Klein, gravitino


Dark Matter and the WIMP Miracle

WIMP denomination → weak interaction ...

WIMP decoupling after BB:

velocity averaged annihilation cross-section for GeV-TeV mass WIMPs:

<σv> = 3×10-26cm3/s → correct DM density

TeV ~ SUSY partners ???

but: LHC... ☹


WIMP Dark Matter Searches:Direct Detection

from proceedings Tev06:Search for WIMPs with CDMS and XENON

two types of interaction:- spin dependent (odd isotopes), smaller cross section- spin independent; σ ~ A2 (coherent: whole nucleus)

important for verification:different target materials

→ consistency!

CDM → massive → nuclear recoil (NR) 50keV

NR → ~10keV

ee

calibration sources are γ-sources...(ee)


The Current Situation: (WIMP hypothesis:)

compatible:- spectra:

CoGeNT/CRESST-II- modulations:

DAMA/CoGeNT

tension:- Xe vs. all of the above- modulation amplitude(halo assumptions...)

two regions of experimental focus:- low threshold...- low background...

Fig. 5 of 1310.8214:LUX 1st results (90% CL)

Xenon100 225d, 100dCDMS II, ZEPLIN-III, Edelweiss II

allowed regions:CDMS II Si (95%)

DAMA/LIBRACRESST !!


The XMASS ExperimentXe detector for weakly

interacting MASSive particlesXe -MASS detector Xe MASSive -detector

Exploiting Xenon:- high mass number → SI cross section- no long-lived radioactive isotope- good scintillation yield: ~ 46ph/keV- high density (liquid): 3g/cm3

- 48% odd isotopes (natural) → SD cross section- ββ candidate: 136Xe → 136Ba+2e–+2.48MeV

Ultimately: 10t fiducial volume multi-purpose detector:- neutrino mass: ββ- Dark Matter: WIMP limited by:- solar neutrinos: pp, 7Be

(the water tank in Hall C is designed to hold this...)


XMASS Progression:

100kg reference:arXiv:0912.2405

the PAST:100kg prototype,

(3kg fiducial), 30cmproof of principle

the current incarnation:Phase I

835kg WIMP detector(100kg fiducial), 80cm Ø

detector technologyadvantages:

- simple- scalable- self-shielding

ultimately: 24t (10t fid.) 2.5m Ø

24t, + ⊙

(10t fiducial)2.5m

the (far) future:

the next step:5 ton total, > 1.5ton fiducial


XMASS Physics:

Phase I:100/835 kg

main purpose:DM direct detectionχ + Xe → χ + Xe

Phase II:10/25 t

multi purpose:DM direct detection:χ + Xe → χ + Xe

pp and 7Be solar neutrinos:ν + e– → ν + e–

neutrinoless double-β decay:136Xe → 136Ba + 2e–

intermediate proposal:XMASS 1.5

to compete in the next roundof DM direct detection: ~1t hep/ph 0008296

Y. Suzuki

expected“singal”

rates [dru]:

low background speak:

dru(differential rate unit):

events / day / kg / keV

2ν2e– (2.38x1021y)

pp7Be10–6

10–4

χ m=50GeV

χ m=100GeV


Background Requirements:

WIMPs:

10-4 dru → cross sections down to a few times 10-45 cm2

solar neutrinos:

better than 10-5 druexpected rate in 10t: 14/d for pp + 6/d for 7Be

→ important considerations:- go underground (shielding against cosmogenic activation)- carefully select and monitor your materials for radio isotopes- shield your detector (against ambient radiation: 222Rn, n, γ)


Shielding Against Ambient Radiation

fast Neutron MC:

Xe

Watertank

fast neutrons:water tank

11m high (10m water)10m Ø

active: -veto

also

:h

all l

inin

g R

n r

etar

din

gR

n-”

po

or”

ou

tsid

e ai

r su

pp

ly

gamma radiation:xenon →

self-shielding:

inner pink circle: fiducial volume


Hall C in Kamioka: XMASS

Hall C: 15m,21m,15m (h,d,w)

water in tank:10m high

10m Ø


XMASS from the Outside In:

Outer Detector (OD):72 x 20inch PMTs (=SK)

calibrationport

Inner Detector(ID):642 PMTs

1100 kg LXe(total)

835 kg(inside PMTs)

100 kg(fiducial)


Inner Detector Structure:2nd

LAYER

FILLER

LXe in detector:1100kg

in sensitive volume:835kg


Building the Inner Detector 1:

photocathodecoverage:

62.4%

xenon insidethe photocathodes:

835 kg


Building the Inner Detector 2:

Feb

2010


Low BG PMT Development:

630 hexagonal + 12 round) PMTs:

Hamamatsu R10789: QE 28-39%+

XMASS photocathode coverage: 62.4%

BG: biggest worry → PMT

great development:

U < 1.0Th< 0.94K < 9.68Co = 4.47±0.34


Electronics:

new: FADCs

CAEN V17518 ch/board

bandwidth 500MHz,sampling rate 1GS/s

resolution 10bit1.2 μs window

Super-Kamiokande ATM:

12 bit ADC/TDCintegration: -100ns – +300ns

1.2 μs window


Xe Distillation: (85)Kr Removal

K.Abe et al, Astroparticle Physics

31(2009), p290

capacity of XMASS Xe distillation system:6kg/h; did 1 ton in ~ 1 week → (3.3±1.1)ppt

otherwise:extensive materials validationbefore construction:

IPMU Lab:U, Th → HP-Ge ← few 100ppt

→ ICP-MS ← 1ppq*Kr → API-MS ← 1ppt** after concentration

hall C distillation system:(used before filling detector)

experience:detector stable w/o:- routine getter operation- active Rn removal


XMASS Calibration System:

sourceexchange

“internal” γ calibration for XMASS:- 57Co (122keV) ← main anchor of energy scale

- 241Am (59.5keV), 109Cd (88keV), 55Fe (5.9keV)

“external” (hose)- 60Co, 137Cs- 252Cf neutron calibration (needs FADC)


57Co Calibration:reconstruction:

data MC

@ center:data vs. MC

W (59.3 keV)

57C

o (1

22 k

eV)

57C

o (1

36 k

eV)

center position setsenergy scale:

→ 13.8 p.e./keVee

energy resolution: 4% rms

position resolution (rms):center: 1.4 cm±20cm: 1.0 cm


Energy Scale Evolution

photoelectron yield is continually improvingeven after we stopped forcing circulation through hot getters!


Detector Stability

det

ect

or

stu

dy

det

ecto

rst

ud

y

tracking intrinsic background signals with our 57Co energy scale:

ratio

w/a

vera

ge →

46 keV 0.47%variablity: 1.1MeV 0.60%

1.3 MeV 0.36%


Extrapolating the Energy Scale

XMASSanalysisthreshold

nuclear recoil scintillation efficiency Leff

follows Xenon100 (arXiv 1104.2549):

57Co

55Fe

109Cd

241Am

above linear fit is used to interpolatekeV

ee energies between calibration

points (from internal gamma sources)and extrapolate to lower energies

dominantsystematic..


XMASS Unexpected BG: The Culprit:

the culprit:Al seal for PMT window:

238U and 210Pb found in raw material(not in secular equilibrium)

unexpected BG traced tosurface between PMTs:

β may deposit most energyinside the material

→ low energy signal...

early suspect:Gore-Tex (light barrier) under PMT rim:

may contain up to 6% modern carbonbut: no signal when measured in XMASS

(no longer needed after refurbishment)


Main BG Source: PMT Al Seal...shape assumed inoriginal MC production:

(full volume)

shape derived fromPMT inspection:

(full volume)

Surface 210PbPMT Al 235U-231PaPMT Al 210PbPMT Al 232ThPMT Al 238U-230ThPMT gammaDots: XMASS data

E > 5 keVee

:

BG understood!


The Good News: BG (Low), LY high(still optimizing: reduce leakage of surface events into FV)

but:

graph (- XMASS...)taken from:

E. Aprile(Princton 2010)

high light yield (LY) → low threshold → go for light WIMPs

(heavy WIMPs → large A → large cross section → to come)

XMASS full volume !!!


Light WIMPs:Full Volume, Low Threshold

playing to our strength:

full volume = 835kg4 hit threshold = 6.75 days(now standard after refurbishment)

backgrounds to be considered: cuts applied:

- no OD activity → ID trigger only- no afterpulse activity → timing RMS < 100ns- Cherenkov (40K in photocathode) → q(20ns)/q(total) < 0.6- ringing, radioactivity → ± Δt > 10ms

systematics to account for:L

eff, energy resolution & scale, trigger efficiency, cuts

our limit:maximally allowed

cross section:MC prediction < data


Cherenkov CutCherenkov light emission in PMT glass (40K, …)

Cherenkov photons emitted “immediately”:→ PMT timing resolution (~3.5ns)

scintillation events:→ time constant τ ~ 25ns (low energy γ and nucl. recoil)

cut parameter:

charge(first 20ns) charge(total)

keep < 0.6

efficiency varies with p.e. range: 40% – 70%

data MC


Trigger Threshold & Data Reduction

70%

30%

signal efficiency(MC)

w/fit

(for low threshold data used inlow-mass WIMP and axion searches)

LED data

6.7d low thresholddata

signal efficiency after all cuts →


XMASS Low Mass WIMP Result:

systematic contributions:(in order of declining importance)

- Leff

uncertainty *(solely responsible for red shaded)

- energy resolution

- trigger efficiency

- energy scale

- Cherenkov cut(all fit into thick red line...)

* following arXiv 1203.1589 (Xe100)

Phys. Lett. B719 (2013) 78; arXiv 1211.5404

limit by:SG ≤ BG

data

σS

I,χ-n

cm

2


Solar Axions through Axio-Electric:production in the sun:- g

aN: nuclear de-exitation (57Fe, line @ 14.4keV)

- gaee

: Bremsstrahlung, Compton

- gaγγ

: Primakoff effect

XMASSsolar axion limit:g

aee< 4.5x10-11in

dire

ct b

ound

fro

m s

olar

ν fl

ux detectionin XMASS: axio-electric

samestraightforward

limitevaluation

Phys. Lett. B724 (2013) 46arXiv: 1212.6153


Annual Modulation Analysis

136.1 days (2010/12-2012/5)835kg → 0.31 tythreshold (analysis): 1 keV

ee

fit functiont0, ω fixed!


Annual Modulation ResultStandard HaloModel

(from: 1209.3339)

difference:summer-winter

solid: summerdashed: winter

JPS fall 2013:XMASS 90% CL

CDMS II 2009Xenon100 2012

DAMA/LIBRA 2008

Energy [keVee

] →

rate

dif

fere

nce

[co

un

ts/k

g/k

eV/d

] →

fits on data→

solid line:best fit

dashed:90% UL


XMASS Pre-Refurbishment Highlightsvery successful technology:

→ highest photoelectron yield in the community: 13.8 PE/keVee

→ very low analysis threshold (without fiducialization)

→ very stable long-term operation without active circulation, i.e. without recirculating through hot getters !!

→ eminently scalable: no drift length limitations

→ first results despite unexpected background

but: surface/materials backgrounds need to be better controlled...

→ detector refurbishment: results to come...


XMASS Detector Refurbishment:

done: we are taking new data

what was done:

- Cu surface cleanup → electropolish to “rm -rf U* Th*”

- Cu ring to cover PMT Al seal

- cover Cu rings with electropolished Cu plate for a flat inner Cu surface (no crevasses...)

duller (lighting aside) brighter


Chemically ElectropolishedEtched

duller (lighting aside) brighter


XMASS 1.5background target: ~10-2 /ton/day/keV

(we have a lot of experience now...)

again: two analyses: - low threshold, full volume:

→ cover all “allowed regions” - 5keV threshold, fiducial volume:

→ reach SI cross section of< 10-46 cm2

start construction in 2014 !tank for active water shield is ready...

improved photocathode geometry:to intercept light from surrounding surface...


ConclusionsXMASS 800kg completed first commissioning phase:

- exceptional light yield: 13.8 PE/keVee

- backgrounds largely understood - detector refurbishment completed:

- serious reduction in Rn exposure during work- inner surface cleaned- inter-PMT spaces covered

→ data taking was resumed with:– refurbished detector– full FADC system

Results (from “old” detector): - light WIMP limit- axio-electric cross section limit- annual modulation analysis

Next: XMASS 1.5 (1 ton fiducial, 5 ton total)

The XMASS Experiment searching for Dark Matter at …...The XMASS Experiment searching for Dark...

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