Recent results from XMASS K. Ichimura Kamioka observatory, ICRR, the University of Tokyo Kavli IPMU for the XMASS collaboration Revealing the history of the universe with underground particle and nuclear research, May 11-13, 2016 1
Contents • XMASS experiment • Recent results from XMASS • direct dark matter search by annual modulation • two neutrino double electron capture on 124 Xe • Toward next phase : XMASS-1.5 2
0νββ solar neutrino Dark Matter 3 The XMASS experiment ★ Multi purpose low-background experiment with LXe • Xenon MASSive detector for solar neutrino (pp/ 7 Be neutrino) • Xenon neutrino MASS detector (ββ decay) • Xenon detector for Weakly Interacting MASSive Particles (DM)
K.Fushimi Yokohama National University: Tokushima University: Y. H. Kim, M. K. Lee, K. B. Lee, J. S. Lee KRISS: N. Y. Kim, Y. D. Kim IBS: Y. Itow, R. Kegasa, K. Kobayashi, K. Masuda, H. Takiya ISEE, Nagoya University: Y. Fukuda Miyagi University of Education: S. Nakamura K. Nishijima The XMASS collaboration: Tokai University: Takeuchi R. Fujita, K. Hosokawa, K. Miuchi, Y. Ohnishi, N. Oka, Y. Kobe University: K.Martens, Y. Suzuki, X. Benda Kavli IPMU, the University of Tokyo: , O. Takachio, A. Takeda, M. Yamashita, B. Yang Ogawa, H. Sekiya, S. Tasaka M. Kobayashi, S. Moriyama, M. Nakahata, T. Norita, H. K. Abe, K. Hiraide, K. Ichimura, Y. Kishimoto, K. Kobayashi, Kamioka Observatory, ICRR, the University of Tokyo: 10 institutes ~40 researchers. Masaki Yamashita
5 1~3 ton (FV)/ 6 ton pp-solar neutrinos: 10 cpd DM search σ SI < 10 -48 cm 2 Multi purpose : ~a few events/day pp solar neutrinos σ SI < 10 -46 cm 2 DM search DM search >10 ton (FV)/ 24 ton 1.5mφ next phase The XMASS experiment XMASS 1.5 XMASS-II 80cmφ 100kg (FV)/832 kg Current phase XMASS-I XMASS-I aims at the search for dark matter • Phasing Approach • Phasing Approach : double-beta decay of 136 Xe
Detector and its characteristic(1) • 70 20-inch PMTs for muon veto Φ10m x 10m ultra pure water shield PMT R10789 6 and so on, as well as “Standard” WIMPs Able to detect Axion Like Particles (ALP), hidden photon, inelastic scattering • High sensitivity for e/γ events as well as nuclear recoil • 2 keV for fiducial volume analysis Achieved 0.3 keV in XMASS-I (full volume) • • High light Yield (~15 p.e. / keV ) and Low energy threshold • 642 low background 2inch PMTs : 62% photo-cathodes coverage • its scalability for further detector upgrade • Single phase liquid xenon detector with 832 kg LXe sensitive volume. • Located in the Kamioka mine in Japan (~2700 m.w.e.) 0.8m Φ
7 capture etc. separation of nuclear recoils from e/γ Background rate in the fiducial volume before threshold. with a large mass and low energy energy, XMASS has good sensitivity Even modest background at low • WIMPs, 2ν double electron Detector and its characteristic(2) scattering, bosonic super- Sensitive to WIMP inelastic • day/kg/keV ee at a few 10’s keV XMASS achieved O(10 -4 ) events/ • Lowest BG rate at a few 10’s keV • Added to D.C.Malling thesis (2014) Fig.1.5
History of XMASS-I Data Taking resumed After RFB commissioning run data counts/day/kg/keV After RFB Before RFB Now Nov. 2013 May 2012 Dec. 2010 Refurbishment • commissioning run construction 8 The longest running time among LXe detectors! • Now, the 3rd year continuity operation is ongoing. • After refurbishment, event ~5 keV is reduced to ~1/10. • PMT Al seal were covered by copper ring and plate to reduce BG as detector refurbishment ~1/10
Diversity of physics in XMASS arXiv : coherent elastic arXiv:1604.01218 Supernova Solar axion Double electron capture annual modulation arXiv : 1511.04807 724 (2013) 46 Phys. Lett. B 1510.00754 Editor’s Suggestion 9 113 (2014) 121301 Phys. Rev. Lett. bosonic super-WIMPs PTEP 2014, 063C01 Inelastic scattering 719 (2013) 78 Phys. Lett. B Light mass WIMP ν-nucleus scattering χ ¡+ ¡ 129 Xe ¡ � χ ¡ + ¡129Xe* ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ 129 Xe* ¡ � 129 Xe ¡+ ¡ γ ¡
10 Direct dark matter search by annual modulation in XMASS-I
annual modulation • 11 Low analysis threshold ( 1.1 keV ee ) without particle ID • 1 year exposure ( = 0.83 ton・year ) is comparable exposure time. • XMASS-I annual modulation analysis • No particle ID (including electron signals) Modulation amplitude : (0.0112±0.0012) cpd/kg/keV for 2-6 keV • • 9.3σ significance • Total exposure : 1.33 ton・year, 14 cycles. • Annual modulation claimed by DAMA/LIBRA • to relative motion of the Earth around the Sun Event rate of dark matter signal is expected to modulate annually due R. Bernabei et al., Eur. Phys. J. C (2013) 73:2648
Stability Check by Detector calibration Gate ~15PE/keV 57 Co number of PEs be removed Top PMT can Moter Stepping ~5m valve 12 • light extracted from the data/MC comparison absorption and scattering length for the scintillation Intrinsic light yield of the liquid xenon scintillator, • from Z=-40cm to +40cm 57 Co 122 keV calibration data taken every (bi-)week, The scintillation light yield response was traced by • and 137 Cs Inner Calibration sources : 55 Fe, 109 Cd, 241 Am, 57 Co @122keV
13 • (1) p.e. yield Stability Check by Detector calibration account. Uncertainties due to this instability is taken into • ±0.6% Relative intrinsic light yield : stayed within • Scattering length : remains stable at 52cm (2) (3) Absorption length change : 4m ~ 11m From the 57 Co calibration data, We observed changes the absorption length. We can trace observed p.e. yield change as a • 3)we continuously circulate the gas purification 2)It recovered after purification work in gas phase 1)sudden drop at the power failure p.e. yield changes : • • 2013. Dec 31 2014. Jul 02 2014. Dec 31 NPE/122 16 14 Abs.(m) 10 5 Relative deduced scintillation light yield 1.02 1.00 0.98 0 100 200 300 400 Day from 2014. Jan. 1
Data set & event selection 14 1.ID trigger event (≧4 hit), no outer detector hits. 2.Veto 10ms after the events 3.RMS of time hits < 100 ns 4.Remove Cherenkov events (orig. in glass) 20ns > 60% of total hits. 5.Remove events in front of PMT ratio Nov. 20, 2013 - Mar. 29, 2015 504.2 calendar days 359.2 live days for analysis 0.82 ton・year exposure Efficiency • remove events which have num. of hits in earlier • remove events which have higher maxPE/totalPE
Modulation analysis method Systematic error due to time dependence of • The data in each time-bins were further divided into energy-bin (bin width = 0.5 keV ee ) • Two fitting methods were performed. Both of them fit all energy/time bins simultaneously • light yield was treated by following two method The data set was divided into 40 time-bin as a relative efficiency difference 15 Method 1 : pull term Method 2 : covariance matrix R data : observed data,R ex : expected rate, Nbins:Ebins x tbins R data : observed data,R ex : expected rate σ(stat) : statistical error, σ(sys) : systematic error K ij : 1σ correlated syst. error on the expected event rate (roughly 10 days livetime each) based on the relative cut effciency • 7 ¡GeV ¡WIMPs ¡w/ ¡2 ¡x ¡10 -‑40 ¡cm 2 ¡ 8 ¡GeV ¡WIMPs ¡w/ ¡2 ¡x ¡10 -‑40 ¡cm 2 ] ee Rate [events/day/kg/keV 1.1-1.6 keV (4.8 - 6.8 keVnr) 1.05 ee 1 0.95 0.9 stat ¡error ¡ systema0c error 0.85 0 100 200 300 400 500 Day from 2014.Jan.1 E bins t bins ! N bins ( R data − R ex i,j − α K i,j ) 2 χ 2 = χ 2 = X i,j k )( V stat + V sys ) − 1 X X ( R data − R ex kl ( R data − R ex + α 2 , l ) , k l σ (stat) 2 i,j + σ (sys) 2 i,j k,l i j
WIMP case V esc : 544 km/s gives < 5.4x10 -41 cm 2 A i : Amplitude C i : Constant σ χ : WIMP-nucleus cross section m χ : WIMP mass t 0 : 152.5 day T : 1 year V 0 : 220 km/s V esc : 650 km/s ρ dm = 0.3 GeV/cm3 ±1 σ expected • ±2 σ expected XMASS XENON100(2012) LUX(2014) XENON10-S2 (2011) CDMS-Si (2014) CoGeNT (2013) DAMA/LIBRA(2009 Savage) time variation data was fitted by: XMASS(2013) DAMA/LIBRA region is mostly 16 No significant signal, derived < 4.3x10 -41 cm 2 at 8 GeV (90% C.L.) • Fitted in 1.1-15 keV ee energy excluded by annual modulation search 2D fitting (time and energy bin), range • function of the WIMP mass Modulation amplitude becomes a • WIMP case : • Z t j + 1 2 ∆ t j C i + σ χ n · A i ( m χ ) cos 2 π ( t − t 0 ) � R ex � i,j = dt, T t j − 1 2 ∆ t j
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