LAL, Paris, April 18th, 2017 The DAMIC experiment: searching for WIMPs and beyond with CCDs Paolo Privitera (Photo image: particle tracks in a DAMIC CCD )
Dark Matter WIMPs 101 1) Mass and energy in the Universe ρ DM ≈ 0.3 GeV/cm 3 Astrophysical evidence for DM: Galaxy rotation curve, lensing , CMB 2) WIMP “miracle”: a Weakly Interacting Massive Particle in thermal equilibrium with SM particles freeze-out in the early universe. To give the observed DM density, interactions and masses must be close to weak-scale < 𝛕 v > ≈ 3x10 -26 cm 3 /s ≈ 1 / (20 TeV) 2
3) Milky Way motion 4) WIMP kinetic energy in the Earth (detector) frame ½ m χ v o 2 ≈ 30 keV (m χ = 100 GeV ) Low energy interaction with matter v o ≈ 10 -3 c Silicon 5) Coherent elastic scattering 6) WIMP E χ ‘ E χ v o m χ λ χ ≈ 10 F WIMP escapes detector (weakly interacting) nucleus m N Detection of E R v R nucleus recoil
Experimental challenges • Massive target-detector • Ultra-pure target (radioactive contaminants) • Low energy threshold (tens of keV vs MeV in neutrino physics) • Low background (deep underground; material screening and selection) Xenon 1T Cryostat support in the Veto water tank Cryostat 4
7) Nuclear recoil ionization efficiency (quenching factor) Nucleus recoil electron E R E e E det E det R • e • • • • • • • •• •• • • • • • • • • Take a nucleus and an electron of the same energy (E R = E e ). R e In general, E det < E det (the nucleus dissipates its energy through mechanisms other than ionization) “Lindhard theory” For a given detector Xe (“electron”) energy threshold, the nuclear recoil energy threshold depends on the QF. Essential to measure. 5
WIMP exclusion plot Detector energy threshold, resolution and QF E R ≈ E χ • r E χ = 0.5 m χ v 2 m χ ≈ m N E R ≈ 0.5 m N v 2 r = 6
Next generation WIMP frontier Kg mass – low threshold > Tons mass – Xenon 7
Beyond the WIMP paradigm • “Dark QED” models kinetically mixed hidden photon A’ A’ 𝜗 x kinematic terms ≈ 𝛿 • A rich, unexplored DM phenomenology : A’ massive or light, 𝜓 (elastic) scalar, Dirac , p , p fermion; (inelastic) scalar, Majorana fermion • Nuclear recoils • Electron recoils: the e - (not 𝜓 ) sets the typical momentum transfer m χ ≈ few GeV v o (outer shell electron) 2 E R = ½ m χ v o ≈ 1 keV nr (+ quenching factor) (does not depend on m χ , can explore MeV DM masses!) 8
ultra-light A’ light A’ ( m A’ ≈ m χ ) freeze-in from thermal SM bath 9
Charge-Coupled-Devices Dark Energy Survey Camera NA32 CERN 1984-86 250 µm thick CCDs with enhanced IR sensitivity developed at LBNL 10
DAMIC enabled by 11
How a CCD works Metal-Oxide-Semiconductor capacitor +V - - - Metal gate Si oxide (insulator) n-type Si (buried channel) - - - p-type Si + + + electron-hole pairs generated by a photon or ionizing particle (3.6 eV ee / e-hole pair) A CCD is an array of MOS capacitors
“horizontal clocks” (faster) Output Charge motion serial register amplifier “vertical clocks” Charge motion Charge transfer 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3 CCD in action
CCD pixel 10-100 µs pixel charge readout V time Reset pulse (inject noise charge) Correlated Double Sampling (CDS) (signal – pedestal) cancels the reset noise (and also other correlated noise) pedestal signal Performed analogically in standard CCD readouts Summing well pulse (inject pixel charge)
Why Dark Matter in CCDs ? • Detection of point-like energy deposits from nuclear recoils induced by WIMP interactions (10 keV Si ion range 200 A) 2) Fully-depleted over several 1) High-resistivity (10 11 donors/cm 3 ) 100s µm (typical CCDs few extremely pure silicon tens of µm ) 15 Float-zone Si
6 cm Copper frame CCD Clocks, Bias, Wire bonds 2k x 4k and Signal cable 3) Sizable mass First DAMIC CCDs from DECam! a DAMIC CCD 6 cm x 6 cm, 16 Mpixel (15 µm x 15 µm) has a record thickness of 675 µm and 5.9 g mass DAMIC100 currently taking data at the SNOLAB underground laboratory 16
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. blank (taken after exposure • 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 1.2 eV band gap • 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
5) Unique spatial resolution: 3D position reconstruction and particle ID X-rays from 55 Fe The charge diffuses towards the CCD pixels gates, producing a “diffusion- limited” cluster a muon piercing a 675 µm thick DAMIC CCD single muon track σ ≈ Z : fiducial volume definition and surface event rejection 18
• “Worms” straggling electrons • Straight tracks: minimum ionizing particles • MeV charge blobs: alphas • Diffusion-limited clusters: low-energy X-rays, nuclear recoils • CCD spatial resolution provides a unique handle to the understanding of the background 19
SNOLAB Creighton Mine #9 Sudbury, Canada Nickel-Copper active mine out for a nice walk … in the cage, dropping at 50 km/h 2 km underground BBC documentary, Dancing in the Dark: the end of Physics 20
Abandon all hope, ye who enter he r e Inferno, Canto III, Dante 21
entering the lab nice dress! get ready for a shower coffee …… 22
DAMIC at SNOLAB DAMIC R&D program in the J-Drift hall started in early 2013 CAB, FIUNA, Fermilab, LPNHE, SNOLAB, U Chicago, U Michigan, U Zürich, UFRJ, UNAM 23
DAMIC @ SNOLAB 24
Linearity demonstrated for signals <10 e - 1.08 ) ee X-rays k( E) / k( 5. 9 keV 1.06 Optical photons 1.04 1.02 1 0.98 0.96 1 − 10 1 10 Ionization signal [keV ] ee Response to electrons Tritium Al Energy loss in gates and SiO 2 O < 2 µm / 675 µm σ ≈ 21 eV Si
Gamma-rays Very large Single-scatter Si K-shell dynamic range Compton spectrum Si L-shell 57 Co source Fluorescence 122 keV photo-electric absorption Compton edges 39.5 keV 47.5 keV 136 keV
Nuclear recoil calibration 24 keV neutrons from 9 Be( γ ,n) reaction Sb-Be MCNP simulation 3 GeV WIMP UChicago E e ≈ 0.2 E r (Lindhard)
Nuclear recoil spectrum • “Neutron-on” with BeO (n+ γ ) “neutron-off” with Al (only γ ) Clear signal from neutron- induced nuclear recoils bkg-subtracted • Nuclear recoil ionization efficiency from adjusting MC E r to E e spectrum • single recoil spectrum • systematic uncertainties are small, dominated by 3.2 keV r 9% uncertainty on total predicted rate 60 eV ee
Nuclear-recoil ionization efficiency in silicon deviation from Lindhard theory observed – crucial for low-mass WIMP searches with silicon detectors
Background, background, background • Lead shielding to stop Spanish galleon environmental γ rays (Chicago) Inner 2” shielding made of ancient lead to avoid bremmstrahlung γ s from 210 Pb β -decay (22 yrs half-life) <0.02 Bq/kg Roman ship (Modane, France) 50 Bq/kg • Material selection and cleaning: copper machining, “secret” recipe etching (surface bkg) Radioactive!
Background tour-de-force • Since 2013 background reduced by >10 3 • ≈ 5 dru achieved before DAMIC100 installation (similar to competitors) In the last year: - Seven interventions at SNOLAB. - Nitrogen purge installation (Radon). - Improvements in treatment of copper surfaces. - Suppression of background from thermal neutron captures in copper. - Mitigation of • Background rate may be smaller in background from DAMIC100: new CCD box and packages, condensation e.g. 3 H. roman lead
DAMIC background characterization E = 6.8 MeV E = 8.8 MeV E = 5.4 MeV RUNID= 491, EXTID= 6, cluster_id= 1388 RUNID= 345, EXTID= 6, cluster_id= 1801 RUNID= 490, EXTID= 6, cluster_id= 1345 506 500 504 500 50 504 504 450 450 502 45 502 400 400 502 40 500 350 350 500 35 500 498 300 300 30 498 498 496 250 250 25 496 496 494 200 200 20 494 494 150 150 492 15 492 492 100 100 10 490 490 490 500 500 50 488 488 0 0 488 0 6048605060526054605660586060606260646066 6050 6052 60546056 6058 6060 6062 60646066 6050 6052 6054 6056 6058 6060 6062 6064 6066 Δt = 17.8 d 2 Δt = 5.5 h 3 1 Three α at the same location! Powerful method to measure U/Th bkg Example α + β in the bulk – ppt limits of 2015 JINST 10 P08014 2 Bragg peak 1 3 Not seen 32
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