XMASS experimentS. Moriyama, spokesperson of XMASS experimentMay 15, 2019 @ external review committee, ICRR

1

XMASS experiment

Solar neutrino Dark matter

Double beta

●XMASS◎ Xenon MASSive detector for Solar neutrino (pp/7Be)◎ Xenon neutrino MASS detector (double beta decay)◎ Xenon detector for Weakly Interacting MASSive Particles (DM)

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History of XMASS-I

With 1.5 yrs commissioning data and 5 yrs data, searches for various types of dark matter and unobserved phenomena in nuclear physics and astrophysics were conducted. After the completion of data taking, data analysis is continuing.

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

Const-ruction

Data taking(completed)

Commis-sioning

data taking

Detectorrefurbish

ment

Dec. May Nov.

• Light WIMPs• WIMP-129Xe inelastic scattering• Bosonic super-WIMPs• Solar axion• 124Xe 2n double electron capture

• Annual modulation• Hidden photons/ALPs DM• 124Xe 2n double electron capture• Solar Kaluza-Klein axion• WIMPs search by fiducialization

Mar.

Dataanalysis

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The XMASS collaborationKamioka Observatory, ICRR, Univ. of Tokyo：M. Nakahata, S. Moriyama, M. Yamashita,A. Takeda, K. Abe, H. Sekiya, K. Kobayashi, K. HiraideK. Ichimura, S. Tasaka, T. Suzuki, N. Kato, S. Imaizumi, Y. Chen

IPMU, University of Tokyo： Y. Suzuki, K. Martens, A. MasonKobe University: Y. Takeuchi, K. MiuchiTokai University: K. NishijimaYokohama National University： S. NakamuraMiyagi University of Education： Y. FukudaISEE, Nagoya University： Y. Itow, K. SatoTokushima University: K. FushimiNihon University: H. OgawaTohoku University： Y. KishimotoKRISS: M.K.Lee, K.B. Lee,CUP, IBS： Y.D. Kim, Y.N. Kim, Y.H.KimCAP, IBS: B.S.Yang, Tsinghua University: B.D. Xu

34 collaborators,14 institutes

Spokesperson:S. Moriyamafrom Dec. 2016

Y. Suzukitill Dec. 2016

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XMASS-I: single-phase LXe detector• BG red. by fiducial volume (FV) cut

– Very large photoelectron yield~ 14.7 p.e./keV ó Super-K ~6 hits/MeV

– Event reconstruction based on observed hit pattern ~ a few keV.

– 832 kg in total, 100 kg in r<20 cm FV.– Target of a WIMP search ~ 2x10-45cm2.– Good to search for e/g events as well.

• Larger det. has better performance.– T info useful (scintil. const. 30-40 ns)– Better self-shielding for e/g/n– Attenuation >10 m for scintillation light

80cm

Self shielding for g injection (XMASS-I)

5

Refurbishment work• Radioactivity inside the PMTs’ aluminum seal between

quartz window and metal body caused background.• Copper plates covering that seal were installed. • Achieved ~1/10 BG reduction in all volume, ~1/100 in FV.

6

DM search in XMASS-I• In the past six years, our experimental reach was

extended to low mass, high mass, and non- WIMPs. E. Aprile et al., PRL 109, 181301 (2012)

10GeV 100GeV

Cros

s sec

tion

(cm

2 )

E

Rate

Low mass

Rate

E

High mass

10-45

10-39

Non WIMPs

1. Low mass WIMPs• DAMA/LIBRA indication

• Annual modulation• Low E threshold required

• w/o reconstruction• Bremsstrahlung/Migdal effect

2. High mass WIMPs• Low background required

• Reconstruction = fiducialization useful

• Understanding BG necessary

3. Non WIMPs: we pioneered!• Dark Photon, axion-like

particles: kinetic mixing, axioelectric effect. 7

1. Low mass: annual modulation• In isothermal halo models, vDM~200-

300 km/s and WIMP nuclear recoils have ~ a few keV, low E.

• We are moving relative to their rest frame, the galaxy.

• Stable observation of a WIMP signal over many years should trace this variation.

• Direct check of DAMA/LIBRA “results”

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1. Low mass: annual modulation• By looking at modulation

amplitudes in individual energy bins, XMASS would see this modulation regardless of the nature of the recoil (nuclear/electron).

DM mass[GeV]1−10×2 1−10×3 1 2 3 4 5 6 7 8 9 10 20

]2D

M-n

ucle

on S

I cro

ss s

ectio

n [c

m

45−10

44−10

43−10

42−10

41−10

40−10

39−10

38−10

37−10

36−10

35−10

34−10

33−10

32−10

31−10

30−10

29−10

DAMA NaI (2017)

CDMS Si(2013)

CRESST-II(2016)

CRESST Surface(2017)

CDMSLite(2015)

SuperCDMS(2014)

XMASS modulation (2018)

LUX(2017)

XENON1T(2018)

PandaX-II(2017)

DarkSide-50 (2018)

Collar (2018)

expectedσ 1 ±

expectedσ 2 ±

XMASS modulation(Bremsstrahlung)

XMASS modulation(nuclear recoil)

Figure 5: (Colour online) Summary of the search results. The red line is the result of the bremsstrahlung analysis for 0.32–1 GeV DM. For comparison, data fromthe CRESST sapphire surface detector [11] and CRESST-II [33], which are searching for the elastic nuclear recoil signals, are shown in each colour. The black lineshows the result of the nuclear recoil search at 4–20 GeV. For comparison, results from CDMS-Si [34], CDMSLite [10], SuperCDMS [35], LUX [3], XENON1T[2], PandaX-II [4], DAMA/LIBRA [36, 37], and XMASS-I [18], DarkSide-50 [38], and the liquid scintillator experiment by Collar [39] are shown for each colour.The green and yellow bands for each result show the ± 1 σ and ± 2 σ expected sensitivity of 90% CL upper limits for the null-amplitude case, respectively.

several data-taking tests, we were able to record stable data withthe three-PMT hit triggers.

The primary uncertainty in the low-threshold data came froma weak light emission of the PMTs with a one PE. From themeasurement for several PMTs in room temperature, the proba-bility of the emission per a one PE was ∼0.3 - 1.0%. Given thatthe light emission occurs even after dark hits, changes in thedark hits for each PMT directly change the event rate aroundthe threshold. Thus, an additional condition for the run selec-tion was applied to suppress this uncertainty; periods where thedark-hit rates for individual PMTs as well as the total dark-hitrate among all the PMTs changed more than 500 Hz from thenominal values were removed from the analysis. Furthermore,the event with this light emission has characteristic timing andangular distributions of hit PMTs; the time difference betweenthe PMT emitting the light and other PMTs receiving the lightafter emission distributed more than 35 ns and the latter PMTswere located within 50 degrees from the former PMT. There-fore, if any pair of hits in the events agrees with these condi-tions, the event was eliminated from the analysis. This eventselection, referred to as a flasher cut, was applied only for threePMT hit events, and the uncertainty due to the weak flash effectafter this cut is 0.4% at maximum.

The χ2 and expected event rate functions for the time varia-

tion fitting are the same as those in the sub-GeV DM analysisexcept for the energy range. Most of the uncertainty for elasticnuclear recoil signal is discussed in [18], only the uncertainty ofthe xenon scintillation efficiency for nuclear recoil is different.As discussed above in section 4, the measurements for energybelow 3 keVnr in [27] are considered.

From the multi-GeV DM analysis, we obtained the best-fitcross section between 4 and 20 GeV DM mass. The best-fitcross section is -3.8+2.0

−4.5 × 10−42 cm2 at 8 GeV, and no significantsignal was found in this analysis including other mass. Becauseof this, a 90% CL upper limit on the DM-nucleon cross sectionwas determined. The 90% CL sensitivity at 8 GeV was 5.4+2.7

−1.7× 10−42 cm2, and the upper limit was 2.9 × 10−42 cm2 (p-value:0.11). The result of the DM search via the nuclear recoil signalis plotted in the multi-GeV region of Fig. 5. The upper limitsand allowed regions determined by other experiments are alsoshown.

Compared with the result from the previous analysis ofXMASS data [18], the result of the present analysis is ap-proximately 6.7 times better at 8 GeV. Because both the low-threshold data and the new scintillation efficiency below 3keVnrin [27] improve the sensitivity. The search for DM massbelow 3 GeV was not performed via nuclear recoil. This is be-cause the maximum recoil energy is below 1 keVnr, which is

6

• 1.82 ton y (XMASS) ó1.33 ton y (DAMA)

• Brems. Effect: search for sub-GeV WIMPs

• First direct check using the modulation effect. 9

2. High mass: fiducialization• Understanding background is most crucial.• Background in the whole 832 kg mass & calib data.

Detailed background modeling& calibration based on • Ex-situ measurements of detector parts (Ge, ICPMS)• In-situ measurements ofsurface activity (a rays, Rn, etc.)• Inner calibration sources.

Modeling >30 keV (no DMsignal expected) succeededby fitting energy spectrumincluding systematic errorson individual activity etc.

2 MeV

1 MeV

~30 keV

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2. High mass: fiducialization• Fiducial mass 97 kg and 706 days data was used to search

for an excess above the background energy spectrum.• Observed energy spectrum was consistent with expected

background within systematic errors. Upper limit on cross section: < 2.2x10-44 cm2 @60 GeV (90% C.L.)

Most stringent limit among resultsfrom single-phase liquid xenon detectors.

This work

Cu plate LXe inside

PMT20 cm FV

20 cm FV

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3. Non WIMPs: Bosonic Super-WIMPs• WIMP candidates have not been

observed in accelerators. • Cold DM has some issues.• Lighter DM particles with much

weaker int. are good candidates.• Dark photon, axion like particles

are leading candidates.• Astrophysical constraints weak.• A peak in E spec@mDM expected.• Our PRL paper got Editors’

Suggestion.

XMASS Collaboration / Physics Letters B 787 (2018) 153–158 157

Fig. 4. Best fit NPEcor distributions. The upper scale is translated into the corre-sponding γ -ray energies. The black dots represent the data. The stacked histograms show the BG MC for RIs in/on the detector components (green), RIs in the liquid xenon (red), and xenon isotopes activated by neutrons (light blue). The dark hatched blue area shows the estimated contribution from dead PMTs. The magenta part of the histogram shows the best fit HP signal for a HP mass of mH P = 85 keV/c2.

χ2f it

(mH P ,α′/α

)≡

Nbin∑

i

(Ri

obs − RiBGtot − Ri

H P (mH P ,α′/α))2

(δRi

obs

)2 +(δRi

BGtot

)2 +(δRi

H P

)2

+χ2sys, (6)

where Riobs, R

iBGtot , and Ri

H P are the event rate in the i-th bin for data, BG MC, and HP MC, respectively, and the δRi

obs , δRiBGtot , and

δRiH P are their respective statistical errors. R BGtot is the sum of the

different BG MC event rates, i.e.:

R BGtot =∑

j:RI types

p j R j-th BG, (7)

where the summation is taken for all the RIs of the three BG cate-gories described in Sec. 3.3. The R j-th BG is the expected event rate from the j-th RI and p j is its scale parameter whose initial value is unity. The χ2

sys is a penalty term to handle systematic uncertain-ties. It is defined as:

χ2sys =

j= 14C,39 Ar∑

j:RI types

(1 − p j

δp j

)2

+5∑

m= 1

(%Cm

δCm

)2

, (8)

where the δp j are the 1σ uncertainties of the BG estimates de-scribed in Sec. 3.3. The first summation on the right side constrains the parameters p j around ± 1σ from unity during the fit. Because the amounts of 14C and 39Ar in xenon are obtained directly from the fit to the data, these RIs are not included in this summation while they are included in the summation in Eq. (7). The second summation is a penalty term related to the five special MC correc-tions C1–C5 described in Sec. 3.4. The m-th correction factor Cm

listed in Table 1 is modified by %Cm in the fit, and its uncertainty δCm constrains this modification. R j-th BG and R H P are functions of the %Cm , the model function type fmodel described in Sec. 3.4, and a global energy scale ϵE :

R j-th BG = R j-th BG (ϵE ,%C1, ...,%C5; fmodel) , (9)

R H P = R H P (mH P , g Ae;ϵE ,%C1, ...,%C5; fmodel) . (10)

The χ2f it was minimized separately for every 2.5 keV/c2 step in

HP mass between 40 and 120 keV/c2 and for each step of α′/α in

Fig. 5. Constraints on g Ae of the ALPs (top) and α′/α of the HP (bottom). The red line shows the 90%CL constraint presented in this paper. The black line shows our previous result [5]. The blue, magenta, green, and orange lines are limits reported by the XENON100 [18], the Majorana Demonstrator [19], the LUX [20], and the PandaX-II [21]. The dotted, dashed, and dash-dotted lines in light blue color are constraints from indirect searches derived from red giant stars (RG), diffuse γ -ray flux, and horizontal branch stars (HB), respectively [6].

the α′/α > 0 region, by fitting the p j , %Cm , ϵE , and fmodel . The optimization of these parameters except for fmodel was done using ROOT TMinuit [16]. As for fmodel , each fmodel was tested separately, and the one giving the smallest χ2

f it was chosen for each mass and each α′/α. This method corresponds to handling the model function shape as one of the fitting parameters [17]. The sensitivity obtained with this method is conservative compared to sticking with one model. We thus obtained the χ2

f it profile as a function of α′/α for each mass. The minimum of the profile is the most probable α′/α parameter for that mass.

4. Result

The best fit result for HP with mH P = 85 keV/c2 is shown in Fig. 4, where the minimum χ2

f it /NDF = 131/122 with α′/α =1.1 × 10− 26. No significant signal was found at any HP mass. The difference between the minimum χ2

f it and the χ2f it with no sig-

nal was at most 1.62. We thus set the 90% confidence level (CL) constraint on α′/α from the relation:

∫ a900 exp

(− χ2

f it/2)

da∫ ∞

0 exp(− χ2

f it/2)

da= 0.9, (11)

where a and a90 denote α′/α and its constraint, respectively. The constraint for each mass is shown in Fig. 5. Compared to our pre-vious work, the constraints improved by a factor of 10–50. The constraints from other direct and indirect searches are also shown in the figure. Our result gives the most stringent limit in the mass range from 40 to 120 keV/c2. The indirect limits for HP around 90 keV/c2 are α′/α < O (10− 24), relatively weak compared to the higher and lower mass regions (α′/α < O (10− 27) for mH P ≥ 200

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XMASS

XMASS

XMASS

XMASS

Development of low BG techniques• Surface alpha ray counter: world best

– Surface contamination (Rn daughters) are common 00000000 BG source among DM detectors.

• New low BG photosensors: future applications– Lower RI– Better

First time to operate underground~ 1x10-5 a/cm2/hr achieved w/ our efforts óCommon a-ray counter: ~1x10-3 a/cm2/hrEven bulk contamination can be measured.

XIALLC

UltraLo‐1800AlphaParticleCounterSiteRequirements&Planning

 

Version: 0.4 

Friday, February 15, 2013  

 

 

 

   

 

XIA catalogue

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XMASS-I PMTNew PMT

U Th 40K 60Co

shape to catchphotons

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Publications (DM, Nucl. Phys., Tech.)“Distillation of Liquid Xenon to Remove Krypton,” Astopart. Phys. 31 (2009) 290.“Scintillation-only Based Pulse Shape Discrimination for Nuclear and Electron Recoils in Liquid Xenon,” Nucl. Instrum. Meth. A659 (2011) 161. “Radon removal from gaseous xenon with activated charcoal,” Nucl. Instrum. Meth. A661 (2012) 50. “Light WIMP search in XMASS,” Phys. Lett. B719 (2013) 78. “Search for solar axions in XMASS, a large liquid-xenon detector,” Phys. Lett. B724 (2013) 46.“XMASS detector,” Nucl. Instrum. Meth. A716 (2013) 78. “Search for inelastic WIMP nucleus scattering on 129Xe in data from the XMASS-I experiment,” PTEP 2014 (2014) 063C01. “Search for bosonic superweakly interacting massive dark matter particles with the XMASS-I detector,” Phys. Rev. Lett. 113 (2014) 121301. “Micro-source development for XMASS experiment,” Nucl. Instrum. Meth. A784 (2015) 499. “Search for two-neutrino double electron capture on 124Xe with the XMASS-I detector,” Phys. Lett. B759 (2016) 64. “Direct dark matter search by annual modulation in XMASS-I,” Phys. Lett. B759 (2016) 272.“A measurement of the time profile of scintillation induced by low energy gamma-rays in liquid xenon with the XMASS-I detector,” Nucl. Instrum. Meth. A834 (2016) 192. “Detectability of galactic supernova neutrinos coherently scattered on xenon nuclei in XMASS,” Astropart. Phys. 89 (2017) 51. “Search for solar Kaluza-Klein axion by annual modulation with the XMASS-I detector,” PTEP 2017 (2017) 103C01. “Identification of 210Pb and 210Po in the bulk of copper samples with a low-background alpha particle counter,” Nucl. Instrum. Meth. A884 (2018) 157. “Improved search for two-neutrino double electron capture on 124Xe and 126Xe using particle identification in XMASS-I,” PTEP 2018 (2018) 053D03.“Direct dark matter search by annual modulation with 2.7 years of XMASS-I data,” Phys. Rev. D97 (2018) 102006. “A direct dark matter search in XMASS-I,” arXiv:1804.02180 [astro-ph.CO], accepted for publication in Phys. Lett. B.“Search for dark matter in the form of hidden photons and axion-like particles in the XMASS detector,” Phys. Lett. B787 (2018) 153.“Development of low radioactivity photomultiplier tubes for the XMASS-I detector,” arXiv:1808.03617 [physics.ins-det].“Search for sub-GeV dark matter by annual modulation using XMASS-I detector,” arXiv:1808.06177 [astro-ph.CO].“Search for WIMP-129Xe inelastic scattering with particle identification in XMASS-I,” arXiv:1809.05358 [astro-ph.CO].

22 papers published so far.Still increasing.

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Activity towardsa future direct DM search

• We planned to construct a larger detector, XMASS-1.5. However, we concluded that it was not competitive anymore.– The primary reason was that the background due to e scattering by low E solar n is difficult to distinguish from a WIMP signal.

– Dual phase detectors were already approved for construction.

• We aim for a future, more sensitive, third generation detector (G3) and in the meantime to collaborate with a competitor building a multi-ton G2 detector.

• This plan was submitted to the future project committee in ICRR.• The committee agreed that XMASS-1.5 was not competitive with

other contemporary projects and accepted this change of our plan for the future. It also recognized participation in one of the G2 experiments, in particular the XENONnT experiment, as appropriate.15

Summary of the past six years• We have completed XMASS-I data taking.• Using >5 yrs data, various types of DM candidates

were searched for. Heavy WIMPs sSI< 2.2x10-44cm2. • Nuclear physics, axions, and neutrino physics were

studied using the XMASS-I dark matter detector.• Cutting edge low BG tech. development continues.• New activity towards a discovery of dark matter

particles was initiated and is ongoing.• XMASS data analysis continues. Hints of new

physics are waiting to be discovered.16