Search for WIMPs with background evaluation in a fiducial volume by - PowerPoint PPT Presentation

Search for WIMPs with background evaluation in a fiducial volume by XMASS-I. 35th International Cosmic Ray Conference (ICRC2017) July 12-20, 2017 BEXCO, BUSAN, KOREA Institute for Cosmic Ray Research, Univ. of Tokyo Hiroshi Ogawa for XMASS collaboration

Introduction Dark Matter Deposit Energy （ WIMP ） • WIMPs elastically scatter off nuclei in targets, producing nuclear recoils. Xenon is a popular target for WIMP searches. • In this presentation, high precision WIMP search in XMASS is processed using fiducial volume cut. • XMASS concept : FV cut and “Self shielding” in LXe reduces the external background. • Background in fiducial volume is evaluated with systematic error. XMASS concept • Then, expected background and WIMPs signal are compared with the reduced data. WIMPs cross section limit is estimated for each WIMP mass.

The XMASS detector • Single phase (scintillation only) liquid Xenon detector. • 630 hexagonal & 12 round PMTs with 28-39% Q.E. • photocathode coverage: ~ 62% • Each PMT signal is recorded by 10-bit 1GS/s waveform Copper ring covers the aluminum seal digitizers ( → huge background origin!) of PMT. • The inner detector surface is covered by copper plate to reduce gaps in this surface. • 832 kg Lxe sensitive volume (0.8 m f ). Self-shielding + reconstruction can reduce BG from external and detector surface. 1.2m diameter

Background evaluation in XMASS-I • RI screening of detector materials was implemented using HPGe detector. • RI activity are estimated by spectrum fitting for > 400pe (~30keV) with constraints from related assays screening results.

Background evaluation in XMASS-I (Cont.) • Pb210 in copper surface & bulk. • Alpha-ray events are identified using Po210 scintillation decay time. Copper bulk Po210 • Copper surface/bulk 210Pb concentration are Copper surface estimated by alpha-ray from 210Po decay. • ※ Alpha from bulk makes continues spectrum by energy loss in copper • Contamination with 210Pb (~20 mBq/kg) p.e. in the bulk of the oxygen-free copper 0 30 60 90 120 x 10 3 was measured by a low background alpha-particle counter (XIA Ultra-Lo-1800) • RI in liquid xenon. • Coincidence analysis is used for Rn222 as Bi214- Po214 (164μs) decay, Kr85 as beta -gamma (1.015μs, 0.434%). • Concentration of C14 and Ar39 in Lxe are estimated from spectrum fits.

Event reduction • (1) pre-selection : remove electronic noise events, Cherenkov events and so on. • To remove surface events, two kind of vertex reconstruction are used. • (2) Timing based vertex reconstruction R(T) : First hit timing of each PMT is used. Position is fitted by likelihood. Events are kept if R(T)<38cm. • (3) NPE based vertex reconstruction R(PE) : an acceptance map is made for both volume and surface events with the XMASS MC. position is calculated by likelihood. Also energy is estimated. Events are selected if R(PE) <20cm yielding a ~100kg fiducial mass.

• Dataset : 2013Nov 20 - 2016Mar 29 • >2yr of stable operation. • Livetime :705.879days. • In pre-selection, Reduction of Cherenkov event is effective. • PMT hit timing is used. • Main origin of Cherenkov : K40 beta in PMT photo-cathode. • After pre-selection applying R(T) + Event/day/kg/keVee Pre-selection + R(T)<38cm+R(PE)<20cm R(PE) gives another O(10 -3 ) reduction. • After applying all cuts (pre-selection + R(T) cut + R(PE) cut), event rate becomes ~4 × 10 -3 /day/kg/keVee @ 5-5.5keV. 7 Reconstruction energy [keVee]

Background MC • Background MC is generated using XMASS MC for each RI’s Pre-selection + R(T)<38cm+R(PE)<20cm decay mode and its activity. Blue histogram : all MC with statistical error Event/day/kg/keVee • Optical parameters of LXe are Pb210 Plate bulk N.R. traced with our Co57 and Co60 Pb210 Ring bulk Internal regular (weekly) calibration. PMT Al seal Pb210 Surface PMT gamma • Same event reduction is applied to background MC, which has the same livetime as the dataset. Reconstruction energy [keVee] • ~90% of remaining BG is of detector surface origin (not internal BG). => mis- reconstructed events.

Pb210 surface MC Mis-reconstructed events : reconstE=2-5keV Plate surface Surface of • Remaining events most likely Window surface detector occurred in places with no Fiducial R=20cm volume direct path for light to hit a region nearby photocathode and thus get reconstructed in fiducial volume. Possibilities are : • Surface events directly on the inner surface of copper plates. • Light leakage from a gap. : An event which is generated Cu ring ⇔ ～ 100μm inside a gap often has a special pattern. But PMT holder reconstruction put it into the “Butt” of plate fiducial volume. Data ： may happen in gap reconR=16.4cm

Systematic error for BG Contents Systematic error (each event position and RI) 2-15keVee 15-30keVee • The systematic error evaluation (1) Plate gap +6.2/-22.8% +1.9/-6.9% for mis-reconstruction requires an dependence (2) Ring roughness +6.6/-7.0% +2.0/-2.1% understanding of surface (3) Cu ref dependence +5.2/-0.0% +2.5/-0.0% condition uncertainly. (4) Plate floating +0.0/-4.6% +0.0/-1.4% • It can be estimated considering (5) Al seal dependence +0.7/-0.7% +0.0/-0.0% (3) Cu reflection (4)Plate floating the specific condition of each (6) Reconstruction +3.0/-6.2% +0.0/-0.0% dependence relevant detector part. (1) Plate gap geometry (7) Timing (decaytime, +4.6/-2.9% +0.4/-5.3% TTS) Event/day/kg/keVee (8) Timing (response in +0.0/-8.0% +0.0/-0.0% Sum of remaining BGMC detector surface) Surface condition validity (9) Absorption & +0.7/-6.7% +1.5/-1.1% scattering (2)Ring roughness (10) Dead tube origin +10.3/-0.0% +45.2/-0.0% Microscope obs. for (11) N.R. +0.7/-0.7% +0.0/-0.0% ring side Reconstruction energy [keVee]

Systematic error for BG (cont) Contents Systematic error (each event position and RI) 2-15keVee 15-30keVee • The validity of reconstruction, (1) Plate gap +6.2/-22.8% +1.9/-6.9% dependence dependence on optical properties (2) Ring roughness +6.6/-7.0% +2.0/-2.1% of Lxe, detector response at (3) Cu ref dependence +5.2/-0.0% +2.5/-0.0% detector surface and so on are (4) Plate floating +0.0/-4.6% +0.0/-1.4% treated as systematic errors. (5) Al seal dependence +0.7/-0.7% +0.0/-0.0% (6) Reconstruction +3.0/-6.2% +0.0/-0.0% WIMP search region (2-15keVee) (7) Timing (decaytime, +4.6/-2.9% +0.4/-5.3% ※ histogram color TTS) Data, BGMC Event/day/kg/keVee (8) Timing (response in +0.0/-8.0% +0.0/-0.0% ※ bar color detector surface) Systematic error (9) Absorption & +0.7/-6.7% +1.5/-1.1% RI activity error scattering MC stat error (10) Dead tube origin +10.3/-0.0% +45.2/-0.0% (11) N.R. +0.7/-0.7% +0.0/-0.0% Reconstruction energy [keVee]

Search for WIMPs with + Data ・ BGMC + WIMP(90% C.L.) background evaluation in ■ 1 s error for best fit the fiducial volume. WIMP • We performed a WIMP dark matter 60 GeV, 2.2e-44 cm 2 (90%CL) search with events of the final reduction sample with consideration for background events predicted by background MC as preliminar result. • The energy spectrum of the data was fitted with background MC plus WIMP MC in the energy range of 2-15keVee. The systematic error is included both background MC and WIMP MC. • A 90% C.L. upper limit on the WIMP- nucleon cross section was derived. • Our preliminary exclusion limit is 2.2x10 -44 cm 2 for 60GeV WIMPs mass.

Conclusion • A high sensitivity WIMP search are conducted using 706 days of data. • Fiducial volume cuts are applied to both data and background MC. • The systematic error is evaluated for background MC. • A WIMP dark matter search in the events of the final reduction sample from the data considering known backgrounds simulated with our background MC is derived as preliminar result. A preliminary exclusion limit is 2.2x10 -44 cm 2 for 60GeV WIMPs.

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Light leakage from Alseal • Low energy spectrum has large Data (w/o FV cut ） Event/day/kg/keV w/o FV cut dependence of Alseal geometry. BGMC(Al seal ） (1)Observed geometry (room temp) • We observed the Alseal (2) (1) + Low temp shrink.=> center model geometry by PMT cutting. (3) (2) + shrink more BGMC(others) • We evaluated 3 kind of model and estimate the spectrum shape. • These model dependence is evaluated as systematic error of background. NPE scale (Co57 122keV) energy [keV] （１） Observed geometry in （２） (1) + Low (3) (2) + shrink room temp shrink. more temperature (center model)

To improve the sensitivity : • Increase the fiducial volume • Select ultra low BG detector material ➜ material screening Leakage from detector for RI measurement. material/surface RI • Discriminate against BG events, Internal RI in LXe especially surface BG. ➜ XMASS-I Developed a new PMT. • Reduce inner detector RI ➜ filtering(222Rn), Distillation (85Kr) and screening for Rn emanation. D.C.Mailing Ph.D (2014) Fig 1.5

New PMTs for future XMASS 3inch dome shape 2inch hex shape new PMT current PMT R13111 R10789 (for XMASS-I) • Surface events can be identified and rejected very effectively by new dome- shaped PMTs, which have high and uniform collection efficiency for whole area. • Performance test was carried out using the first batch of the new PMTs. • Reduction of radioactivity in PMT parts was done. 17