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The DAMIC (Dark Matter in CCD) Experiment at SNOLAB

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The DAMIC (Dark Matter in CCD) Experiment at SNOLAB Status of DAMIC Alvaro E Chavarria The DAMIC (Dark Matter UniversityIn of Chicago for the DAMIC Collaboration CCD) experiment at SNOLAB1 Mariangela Settimo Subatech, CNRS-IN2P3, Nantes (France) 1 DAMIC : DArk Matter In CCDDAMIC @ SNOLAB 675 Charge-Coupledm thick, 16 Mpix Device CCD, (CCD) 6 g Detection of point-like energy deposit from nuclear recoils ! induced by WIMPSWhy interactions Dark in the M bulkatter of CCDs. in CCDs ? • Detection of point-like energy deposits from nuclear recoils induced by WIMP interactions16 Mpix, 15 µm x15 µm, (10 keV675 Si µm ion thick, range 5.9 g 200mass A) 5) Unique spatial resolution: 3D position reconstruction and particle ID 3.7 eV to create e-h pair 1) High-resistivity (1011 donors/cm3) 2) Fully-depleted over several Sensitivity to DM masses 100s (typical CCDs few extremely pure silicon < 10 GeVµm (nuclear recoil) X-rays from ~ eVtens (electron of µm )recoil) 55Fe R&D program (2013-2015) 3D reconstruction (x, y, z) and unique 2 The chargespatial diffuses resolution towards the CCD40g detector commissioned in 2017 pixels gates, producing a “diffusion- 2 limited” cluster 15 a muon piercing Float-zonea 675 µm Si thick DAMIC CCD single muon track σ ≈ Z : fiducial volume definition and surface event rejection 18 Some results with the R&D detector Result with 10 g detector mass (data: 2015-2016) Hidden photon DM Selected results PRL 118, 141803 (2017) WIMP DM Hidden photon DM PRL 118, 141803 (2017) Selected resultsPRD 994, 082006 (2016) WIMP DM Exposure 0.6 kg d PRD 994, 082006 (2016) Exposure 11.5 g d radioactive bkg in the silicon bulk 2015 JINST 10 P08014 nuclear recoil calibration … and also: radioactive bkg in the silicon bulk 2015 JINST 10 P08014 • radioactive bkg in the silicon bulk, 2015 JINST 10electron P08014 recoil calibration nuclear recoil• nuclearcalibration recoil calibration, PRD 94, 082007 (2016) • electron recoil calibration, PRD 96, 042002 (2017)electron recoil calibration 3 PRD 94, 082007 (2016) PRD 94, 082007 (2016) PRD 96, 042002 (2017) 4 PRD 96, 042002 (2017) 4 DAMIC @ SNOLAB (2000 m underground lab) DAMIC @ SNOLAB 675 !m thick, 16 Mpix CCD, 6 g 24 DAMIC @ SNOLAB (2000 m underground lab) DAMIC @ SNOLAB 675 !m thick, 16 Mpix CCD, 6 g exposure ~ 1 min On surface 4) Unprecedented low energy threshold • Negligible noise contribution from dark current fluctuations (dark current < 0.001 e/ pixel/day with CCD cooled at 120 K). Readout noise dominant contribution. exposure8h blank (taken after • A readout noise of ≈ 2 e- is exposure) achieved by slow CCD readout (≈ 10 min / 16 Mpix image). 3.6 eV to produce 1 e-hole pair At Snolab + 1.2 eV band gap shielding 25 • Very long exposures (8 hours!) to minimize the n. of noise pixels above the energy threshold SNOLAB data • Lower threshold, higher WIMP recoil rate (exponential), small mass detector competitive • 17 Privitera Part B2 DAMIC-M 1280 1290 1300 1310 1320 1330 image 1330 a) 4180 c) Low-energy Electron 1320 Unique spatial resolution4190 candidates 1310 4200 image Particle identification and bulk / surface background50 pixels rejection 13004210 α b) Muon 12904220 On-going background measurements32 “Worms”Spatial straggling coincidence of beta decays ( Si) • 1280 4180 4190 4200 4210 4220 5 10 15 20 25 30 electrons 5 10 15 20 25 30 Energy measured by pixel [keV] 32 32Si (T1/2= 150 y, β ) ➔ 32P (T1/2= 14 days, β) • Cosmogenic Si • StraightFigure tracks: 5. a) DM detection minimum in a CCD; b) 3D position reconstruction; c) signatures of different ionizing particles in a CCD: a straight track (cosmic ray muon), large blob (alpha particle), “worm” (straggling electron) and small round clusters (low-energy X-ray, ionizingnuclear particles recoil, DM candidate) . On-going background measurements Search for spatially • MeV chargeexposure to air.blobs: This background rejection capability is particularly relevant for 32Si, a naturally occurring alphasisotope in silicon that is not removed by chemical purification, which is expected to be the dominant correlated beta decays. background• Cosmogenic contribution in the 32currentSi generation of silicon detectors32Si (cf.(T 1/2SuperCDMS= 150 [25y, ]).β DAMIC) ➔ 32 P (T1/2= 14 days, β) 32 32 Diffusion-limitedidentifies and rejects clusters: the decay sequence Si (T1/2= 150 y, β) → P (T1/2= 14 days, β) by spatial correlation, • reducing this background by at least a factor 100. Sensitivity with current low-energy X-rays, ThisSearch novel experimental for spatially technique has been demonstratedcandidate in a successful 32Si R&D - carried32P out at SNOLAB data is few Bq/kg nuclearduring recoils the last three years under PI Privitera leadership. Stringent limits on intrinsic radioactive contaminants incorrelated the silicon bulk of thebeta CCDs, incdecays.luding the first measurementin the of 32 Sinew activity indata ultra-pure set silicon using the spatial correlation method, show the power of DAMIC background identification and rejection methods [36]. TheSensitivity potential for DM withdetection currentis demonstrated in a search for low-mass WIMPs (Fig. 3.b; [35]) and for CCD spatial resolution 32 32 • hiddendata-ph otonis fewDM (Fig. Bq/kg 3.a; [34]), the latter establishing the world-best limits for ≈ eV DM masses evcandidateen Si - P • Bulk and surface radiogenicprovides backgroundswith minimal a unique exposure. handle The seven - CCD 40-g detector which will operate until 2018 at SNOLAB has achieved background rates as low as 5 dru, a level comparable to other low-mass dark matter experiments in the new data set to thelike understanding CRESST [15] and CDMSL of ite [23], and will provide further210 guidance – e.g. with a precise measurement ofDecay 32Si and tritium chain backgrounds, ofsee Section a single b.5 – for the design of PbDAMIC -M. the background• Bulk and surface radiogenic backgrounds Δt = 4 days Δt = 10 days 4 keV e 41 keV ppt e levels 4323 keV α Energy [keV] 3168 3168 3168 102 3166 3166 3166 3164 3164 sensitivity3164 to bulk ppt levels 3162 3162 3162 10 3160 3160 contamination3160 sensitivity to bulk 3158 193158 3158 y [pixel] y [pixel] y [pixel] 1 3156 3156 3156 contamination 3154 3154 3154 σ ≈ z : for fiducial volume definition 3152 3152 3152 10 1 5330 5332 5334 5336 5338 5340 5342 5344 5346 5348 5330 5332 5334 5336 5338 5340 5342 5344 5346 5348 5330 5332 5334 5336 5338 5340 5342 5344 5346 5348 and210 surface event rejection 210 210 x [pixel] x [pixel] x [pixel] Pb (T1/2= 22.3 y, β) ➔ Bi (T1/2=5.0 days, β) ➔ Po (T1/2= 138210 days, α) Figure210Pb 6. T he(T decay1/2= chain 22.3 of a y, single β) ➔Pb nucleus210Bi on(T the1/2 CCD=5.0 surface days,: a sequence β) ➔ of two210 betaPo and(T one1/2= alpha 138 particle days, separated α) in time by several days and detected in the same location (see text). • Cosmogenic tritium sensitivity•b.2 CosmogenicDAMIC-M at the Laboratoire tritium Souterrain sensitivity de Modane 2015 JINST 10 P08014, PRD 994, 0820006 (2016) The Laboratoire Souterrain de Modane offers several advantages when compared to SNOLAB. Its horizontal6 Sensitivity to activation rate downSensitivityaccess to through few the toFrejustens activation tunnel of greatly tritium simplifies rate the down atoms/kg/daylogistics toto perform few detector tens packaging, of tritium test and atoms/kg/day 11 assembly underground. Supply of radon-free air, crucial to minimize surface backgrounds, is already11 part of the laboratory infrastructure. While LSM is not as deep as SNOLAB, its rock overburden is sufficient to 5 Low background noise and energy threshold Comparing images (8h exposure) and “Blanks” (exposure = readout time) 1. Negligible dark current: < 0.001 e/pix/day (the lowest ever measured in a Si detector) - - σ = 1.6 e ~ 6 eV 2. read out noise: 1.6 e @140K ‣ Energy threshold ~ 40 eVee (~ 6σ above background, 3.77 eV Data 2017 for e-h pair in Si ) Linearity demonstrated for signals <10 e- ) 1 . 0 8 e e X-rays V 1 . 0 6 e k Optical photons 1 . 0 4 9 Linearity of the CCD response within . 5 1 . 0 2 ( k ± 5% down to 40 eVee 1 / 0 . 9 8 ) E ( 0 . 9 6 k 10−1 1 10 7 I o n i z a t i o n s i g n a l [ k e V ee] Response to electrons Tritium Al Energy loss in gates and SiO2 < 2 µm / 675 µm O σ ≈ 21 eV Si Current detector configuration Current status • Seven CCDs‣ 7 CCDs in stable in stable data data taking; taking since 2017 40 g detector(1 CCD sandwiched in ancient lead) One CCD ‣sandwiched40 g detector massin ancient lead • A data set (7.6 kg day) collected with full spatial resolution‣Operating (1x1 temperature binning), of optimized 140K for background‣ Exposure characterization for image : 8h and and 24h measurement(each image(32Si, acquisition 210Pb) is followed by a “blank” • A second datawhose setexposure being is the collected readout time) (so far 4.7 kg day‣) 7.6with kg bestday of energy data for thresholdbackground (1x100 binning)characterization ‣ So far, 4.6 kg day of data collected for binning: charge ofDM several search pixels (in 1x100 are addedhardware before binning) readout 3x3 1x1 some loss of spatial 8 resolution but improved signal to noise (same α-β event readout noise but more charge in a binned pixel) 5 Event clusters search strategy ‣ Pedestal and correlated noise subtraction (hot pixels among several images masked) ‣ LL fit of the signal in a moving window across the image ΔLL = ℒn - ℒs flat noise Gaus signal + flat noise data Example of one event Events blanks 102 Fit simulation Erec = 7.18 keV Data σ = 0.7 pix 10 1 −45 −40 −35 −30 −25 −20 −15 −10 −5 ∆ LL DLL cut : < 0.001 bkg events from exponential fit of the “blanks” distrib 9 Surface background
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