Direct dark matter searchwith XMASS
K. Abe for the XMASS collaboration

Outline• XMASS experiment.
• Single phase liquid xenon detector• Many targets were searched with XMASS.
• WIMP search• fiducialized volume.
• Modulation signal search• Inelastic scattering by WIMP

• Solar axion• Bosonic super WIMP
• Solar KK axion

The XMASS collaboration:• Institute for Cosmic Ray Research, theUniversity of Tokyo• Kavli Institute for the Physics andMathematics of the Universe, theUniversity of Tokyo• Kobe University• Tokai University• Yokohama National University

10 institutes 35 researchers.

• Miyagi Educational University• STE lab., Nagoya University• Tokushima University• Center for Underground Physics, Institutefor Basic Science• KRISS3

SiteJapan

Kamioka Mine

KamLAND

Super-KCANDLESXMASS(Lab-C)

Kamioka MineEGADSNEWAGETokyo~1000m underneath
Mt. Ikenoyama.
(2700 m.w.e.)
Kamioka mine
Lab-C

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XMASS I detector• Single phase liquid xenon detector with 835kg of liquid xenon, 80 cm diameter.

• 642 low RI PMTs.
• 62% inner surface is covered with photocathode area
• High light yield (14.7 pe/keV)• Low threshold (0.3keVee)• Detector is inside 10m x 10m water tank with 70 PMTs (20 inch ) for muon veto and shieldfor neutron/gamma from the outside.
• Stable data taking is ongoing from end of 2013.

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XMASS single phase liquid xenon detector for Dark matter seachSingle phase :– Only liquid phase, only scintillation signal.
– DM signal (uniformly distributed) areidentified by fiducial volume analysis.
– BG events are stopped outside volume.

– Also PSD can reduce background.
• Scintillation decay time difference between
nuclear recoil (WIMP indueced) and electronrecoil can be used.

Two phase :– Liquid phase and gas phase, scintillation
signal(S1) and ioinization signal(s2).
– DM signal (nuclear recoil) are identified by
S2/S1 ratio and fiducial volume.
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Dark matter search by XMASS
1. WIMP1. Search in fiducialized volume
2. Modulation
3. Inelastic

2. Solar axion
3. Bosonic super WIMP
4. KK solar axion

WIMP search by fiducialized volume
Traces of U-chain gamma-rays from PMTs

 Self-shielding of g-ray background Owing to high atomic number(Z=54),
external gamma-ray background(mainly coming from PMTs) can beshielded by liquid xenon itself.
 By selecting events occurred in therestricted inner volume (fiducialvolume)
low background can be achieved.

Fiducialvolume8

Vertex reconstruction (based on photo electron, R(PE))
 Position reconstruction(1) Making acceptance map: Many grid points are defined inside whole detector volume including
detector surface. Events are generated at each grid point and photo-electrons (pe) expected ineach PMT are calculated by our MC.
(2) From measured pe and scaled acceptance map (m) in (1), position is calculated
where following likelihood is maximum.
Γ(x): Gamma function(Γ(n) = (n-1)!, n>0)

Reconstructed position distribution of 57Co events (122 keV)

PMTsz
z = +40cm
z = +20cm
z = 0cmz = -20cm
z = -40cm

detector
calibration

Source position
along verticalaxis (z-axis)

9Background understanding without fiducialization
 RI from the detector material is the BG
source.
 The each RI activity was evaluated by
the observed spectrum fitting for > 400pe (~30 keV) by the MC expectations
with constraints from the screening
results by HPGe.
ex. RI screening results for PMT with HPGe detector.
• PMT + base(whole measurement)
mBq/PMT
232Th

1.80±0.31

238U

2.26±0.28

210Pb

200±100

60Co

2.92±0.1640K

9.10±2.15

observed data

event/day/kg/keVee

WIMP search region (2-15keVee)+ Data-- BG prediction■ 1s systematic error

Energy spectrum
after fiducialization• 706 live days taken in Nov. 2013 –Mar. 2016
• Fiducial mass 97kg (R<20cm)[/day/kg/keVee]

Blue histogram : all MC with statistical error

0.005 MC only

Pb210 Ring bulk

EnergyInternal(keVee)

PMT Al seal

Pb210 Surface

Pb210 Plate bulk

0.004
0.003

N.R.

• Main BG in the WIMP search region
 Not internal but detector surface
events (miss-reconstructedevents).
• Internal RIs dominate above 15keVee

PMT gamma• Neutrons, alpha-rays are negligible

15• Dominant systematic uncertainty is
condition of detector inner surface(gap size, surface roughness)

0.002
0.001
0510202530

Reconstruction energy [keVee]

11WIMP search results
 706 live days data
 Energy spectrum of data was fittedwith background MC and WIMP
signal MC in the energy range of
2–15 keVee considering systematicerror in both background and WIMPMC.
 Best fit result is consistent with no
WIMP case, then 90% C.L. upper
limit on the WIMP-nucleon crosssection was derived.
 Our preliminary limit is 2.2×10-44 cm2for 60 GeV WIMP mass.

+ Data・ BGMC + WIMP(90% C.L.)■ 1s error for best fit

WIMP
60 GeV, 2.2e-44 cm2(90%CL)

DAMA/LIBRA(Na)CoGeNT(2013)

CDMS-Si (2014)

Miss-reconstructed surface events
Structure around PMT
 Majority of remaining BG
 Certain amount of surface events are wrongly
reconstructed as the event inside fiducial volume.
 Surface of copper plate Light leakage from the gap around boundary of surface
Large
structures.
miss-recon.
Gap It is difficult to observe photon by the near side PMTs.
Small
Light leakage from gap

miss-recon.

Copper plate

quartz window

Gap~100 um
CopperringPMT

Position where scintillation happens
Scintillation photons

Search for annual modulation
 Due to Earth’s rotation around the Sun,annual modulation of event rate of dark
matter signal is expected.
 DAMA/LIBRA claims modulation at 9.3s
 Total exposure of 1.33 ton year (14 cycles) Modulation amplitude of (0.0112+/-0.0012) cpd/kg/keV for 2-6 keV

 Annual modulation search in XMASS
 800 live days x 832 kg (=1.8 ton year), not fiducialized.
 Look for event rate modulation not only for nuclear recoil but also for e/g events
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Modulation search with 2.7 years dataPE yield@122keV (PE/keV)• Large mass (832 kg)• Data set 2013/11/20 2016/07/20 (800.0 live days)• XMASS (1.82 ton x year) ⇔DAMA/LIBRA (1.33 ton x year)=>Current statistics is larger
than that of DAMA/LIBRA (2013)• Stable detector operation
especially during run2.
run1 +/-2.4%, run2 +/-0.5%• Low energy threshold: 1.0 keVee(4.8 keVnr) (15PE/keV)• Likelihood analysis
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Modulation analysis: WIMP results• Assuming WIMP (standardhalo model)
 T= 1year, t0=152.5 day (fixed) v0=232km/s, vesc=544km/s
 rDM=0.3GeV/cm3• WIMP mass range 6 to 20GeV/c2• Exclude DAMA/LIBRA allowed
region by modulation search<1.9 x 10-41cm2 (90% CL) @ 8GeV

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Model independent case•Without assuming any specific model
except for T=1 year, t0=152.5 day•

Includes both NR and e/g signals•

Null hypothesis p-value is 0.11 (1.6 s)•

Most stringent amplitude for modulation search.

•

When models assumed, the relation btw NaI and Xe will bechanged.

Search for 129Xe inelastic scattering by WIMPs

By inelastic scattering 129Xe can be excited thenemit gamma.
c + 129Xe  c + 129Xe*129Xe*  129Xe + g (39.6keV)

Red: XMASS (90% C.L.
stat. only)
Pink band: XMASS(w/ sys. error)
Black: DAMA LXe
2000 (90% C.L.)

Search peak signal.
Natural abundance of 129Xe: 26.4%
Updated results will be published soon.
Signal MC for 50GeV WIMP

Observed data (165.9 days)(1)= pre-selection(2)= (1) & radius cut(3)= (2) & timing cut(4)= (3) & band cut

Published in PTEP 063C01 (2014)18

Search for solar axion• Axion can be generated by Primakoffeffect(g+Za+Z), Compton scattering(e+ge+a) andbremsstrahlung (e+Ze+a+Z).
• In XMASS, electron signal which can be caused byaxio-electric effect (a+Ze+Z) was searched. gaee

Bremsstrahlung andCompton effectgaee

Axio-electric effectgaee
Expected energy
spectrum of solar axion(ma=0, 1, 2, 4, 8, 16
keV/c2)19

Search for solar axions
 6.7 days data taken at 2012 was used.
 90% confidence level limit were set as expected signal
dose not exceed observed data

Bremsstrahlung andCompton effectObserved data

gaee
expected signal

Axio-electric effectgaee

Published in Phys. Lett. B 724
2046 (2013)

Search for bosonic super-WIMPs (1/2)
 Lighter and more weekly interacting thanWIMPs
 Candidate for warm dark matter
 It can be pseudoscaler(ALP) or vectorboson(Hidden photon).
 For vector boson, this analysis was the firstexperimental constraint.
 It can be detected by absorption of the particle,
which is similar to the photoelectric effect.
 Search for mono-energetic peak at the mass ofthe particle
v or aPublished in Phys. Rev. Lett. 113, 121301 (2014)21

Search for bosonic super-WIMPs (2/2)
 For vector boson case (Hidden photon) The first direct search in the 40–120 keV range.
 The limit excludes the possibility that suchparticles constitute all of dark matter.

Vector boson

 For pseudoscaler case (ALP) The most stringent direct constraint on gaee.

 Updated results will be published soon.
Pseudoscalar

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Solar KK axion search• In the large extra dimension model, motions of a particle canbe seen as mass state of Kaluza-Klein (KK) excitations.
• Axions may be able to propagate and acquire KK excitations,acquires an infinite tower of KK modes where the lowest KK
state is the normal PQ axions.

Expected KK axion number ofdensity vs distance from sun.

• KK axion would be produced in the Sun via the Primakoff
effect (γZ→aZ) and a photon coalescence mechanism(γγ→a).
• A small fraction is trapped by the gravity of the Sun.
• Such solar/stellar KK axion can explain the solar corona
problem by massive axion decay. High temperature of thesolar corona ~10^6 K.
• Search KK axion decay into two photons inside the detector.

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Solar KK axion search

Result of search• Caused by the change of distance from the Sun,signal modulation is expected.
• Searched modulation signal.
• First experimental constraint for KK solar axion.
• Published in PTEP
Volume 2017, Issue 10,
1 October 2017, 103C01,

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Summary• XMASS• Single phase liquid xenon detector.
• Stable data taking is ongoing from 2013

• Many targets are searched with XMASS.
• WIMP• Search with fiducialized volume.
• 705.88 live days data
• 2.2×10-44 cm2 for 60 GeV WIMP mass

• Modulation.
• 2.7 years of data. (800 live days x 832 kg)• No significant modulation.

• Inelastic search.

• Solar axion.
• Bosonic super WIMPs.
• Solar KK axion