darkside: latest results and the future liquid argon dark ... · high wimp mass search roberto...
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SUSY18 – Barcelona 24-July-18
Roberto Santorelli
CIEMAT – Madrid
DarkSide: latest results and the future liquid argon dark matter program
Outline
• LAr for DM direct searches: recent results
• Technology breakthroughs
• The global Ar community
• Activity in Spain
• Prospects and conclusions
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Experimental signature• WIMPs are still excellent candidates for particle DM• WIMP masses: 0.001 - 10 TeVand cross sections:
10-40 - 10-50 cm2
• Nuclear recoils ~ 10s keV• Featureless recoil spectrum (no bump) • Single scatters (uniform throughout the detector)
Motion of the Earth respect to the halo:
Annual modulation (~7%)→ Additional signature
• Rate variation (June – December ~3%)• Direction asymmetry (Daily rotation)
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• Large Exposure (Mass × Time) : 100s ton × year
• Low Energy Threshold : ~10 keVNR
• Event topology : γ/β, α, µ
• 3D Event Reconstruction : neutrons
• Material radiopurity : < mBq/kg
• Discrimination between Signal and Background
: > 99.99…%• Low Background Rate : <0.1 evt in the exposure
Challenging tasks
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DarkSide-50 design
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� ∅ 36.5 cm x 36.5 cm TPC
� 46.4 kg of Active LAr (150 kg of UAr)
� 38 3” Hamamatsu R11065 PMTs
� 4 m(d) sphere veto: 30 tonne boron loaded liquid scintillator (120 PMTs, eff > 99.8 %)
� 1 kt ultrapure water - Cherenkov Veto (80 PMTs, eff > 99.8 %)
� Installed @ LNGS (3400 m.w.e)
High WIMP mass search
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• Studying the backgrounds with – The first 71 days of data - PRD 93, 081101 (2016)– Monte Carlo Simulations with G4DS - JINST 12, P10015 (2017)
• identify and reduce background sources• Goal: blinded-box with < 0.1 bg events in the exposure (dashed line)• Unblinding
f90: fraction of S1 light collected in the first 90 ns
S1 [PE]
f 90
Type of bkgEstimated ectspassing the cut
Surface α 0.001
Cosmogenic n <0.0003
Radiogenic n <0.005
ER 0.008
Total 0.09±0.04
High WIMP mass search• No WIMP-like event in the
search box → 90% CL exclusion limit
1.1 × 10-44 cm2 @ 100 GeV/c2
3.8 × 10-44 cm2 @ 1 TeV/c2
3.4 × 10-43 cm2 @ 10 TeV/c2
”DarkSide50 532 day Dark Matter Search with Low Radioactivity Argon”
ArXiv:1802.07198
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AmC / AmBefor neutron veto efficiency
(standard isothermal WIMP model)
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� Light WIMPs (~2 GeV) might produce low energy NR
� Other lighter DM candidates (~50 MeV) might induce low energy ER
� Low-energy interactions → S1 too small (Eth ∼13keVnr)
� S2 charge signal sensitive to a single e-
Hardware trigger efficiency 100% above 30 PEEth<0.6 keVnr sensible to low mass WIMPS2 yield = 23 (1) PE/e-,
� electrons captured 0.5 x 10-5 e-/e- ER Analysis threshold set to Ne-= 3
� Low E phenomena from the TPC walls (Fiducialization: volume under inner 7 PMTs)
� Large S1 with S2 in the ROI
Low WIMP mass search (S2-only)
Background:
Low WIMP mass: above 1.8 GeV
• At low energy excess of events (between Ne-= 4 and Ne-= 7)
� ER energy scale: 37Ar� NR energy scale: AmBe and AmC
• Ne- spectra expected for recoils induced by dark matter particles with 2.5, 5 and 10 GeV/c2 for 10−40cm2 xsec
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Competitive results at Mχ > 1.8 GeV/c2
arXiv: 1802.06994
Fits performed using two thresholds Ne- = 7 (down to ~3 GeV) and Ne- = 4
Two models with and w/o fluctuations in the energy quenching (Eth=0.6 keVnr)
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Low WIMP mass: sub-GeV❖ ER analysis
❖ Binned profile likelihood method - threshold @ Ne- = 3
❖ Two extreme cases considered, light mediator & heavy mediator
→ Constraints improved for 30 MeV < Mχ < 100 MeV and heavy mediator (FDM(q) = 1)
arX
iv: 1
802.
0699
8
� Price� ER discrimination
� Radioactive isotopes39Ar → 1.01 Bq/kg
� Density � ~50% odd isotopes ( 129Xe, 131Xe )
for spin dependent interactions� No long-lived radioactive isotopes
� Available in large quantity� ER background discrimination
LXe:
LAr:
LAr vs LXe: why LAr?
“Measurement of the specific activity of ar-39 in natural Argon”NIM-A 574 (2007) 83–88
“Backgrounds and pulse shape discriminationin the ArDM liquid argon TPC”
arXiv:1712.01932
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Technology breakthrough 1: radiopure Ar• URANIA project:
Procurement of 50 ton of UArextracted from the CO2 wells at Kinder Morgan facility in Cortez, Colorado (∼100 kg/d)
• ARIA project:• Seruci 1: chemical purification of
the UAr by cryogenic distillation (reduction factor 1000 per pass, 150 kg/d)
• Seruci 2: Active Ar-39 depletion via isotope distillation (factor 10/pass)
28 modules,12 m each, 360 m in total under test at CERN
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Technology breakthrough 2: radiopure cryogenic SiPM• Radiopure ~2 mBq/PDM dominated by the
substrate(for SiPM and front-end) • Working in LAr• High PDE (>40%)
6 SiPMs→ channel4 channels → one tile (PDM)25 tiles → one Motherboard
> 8200 PDMs
~20 m2 SiPM, LY above 8 PE/keVA packaging facility (NOA) at
LNGS funded by INFN
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5x5 cm PDM
• AAr in ProtoDune style large cryostat to provide shielding and active VETO
• About 8x8x8 m3 inner volume, 750 t of LAr in each one
➡Fully scalable design for future larger size detector (300 ton)
DarkSide-20k:Baseline design• New collaboration with groups from DS-50,
ArDM and DEAP-3600.
• More than 70 institutions, 350 researchers, 12 countries (still growing)
• Data taking from 2022
• 100s ton×yr exposure with < 0.1 evt bkg
• ∼50 ton UAr (baseline)
• ∼ 20 m2 SiPM, LY above 8 PE/keV
• Sensitivity ~1047 for 1 TeV/c2 WIMP mass
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DarkSide-20k in the hall C @ LNGS
3 m
DarkSide-20k:A 50-tonnes argon detector 100s tonne×year bkg-free search for
dark matter
Global Argon Dark Matter Coll: A 300-tonnes depleted argon
detector 1000s tonne×year bkg-free search for dark matter
Future program
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DS-20k 100-200 t×yr (proj)300 tonne 1-3 kt×yr (proj)
• Collaboration between CIEMAT, ETH and LSC (RE18)
• Low background setup (LSC underground lab)
ArDM: tonne-scale LAr-TPC at LSC
• Double-phase EL-TPC • 850 kg active volume (≈2 t total)• Cryogenic low radioactive PMT arrays (R5912 2×12)• 50 cm passive neutron shield (Poly, 20 ton)• Control of the cryocoolers and cryogenic system fully integrated in the existing PLC• 4x ADC V1720, 8ch, 12bit, 2Vpp, 250MS/s (up to 2.2 kHz trigger rate (289 MB/s – 8 µs digitalization)
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Main Lab under mount Tobazo~850 m rock~2500 mwe1400 m2
• Scintillation pulse shape discriminator: f90
• Rejection power: statistical separation
108 ER Rejection50% NR Acceptance
252Cf calibration
Signal region
ER/NR discrimination
JCAP 2017.03 (2017) Roberto Santorelli - SUSY July 2018 19
• “Measurement of the attenuation length of argon scintillation light in the ArDM LAr TPC” ArDMCollaboration Nov 8, 2016. 11 pp. Astropart.Phys. 97 (2018) 186-196
• “Commissioning of the ArDM experiment at the Canfranc underground laboratory: first steps towards a tonne-scale liquid argon time projection chamber for Dark Matter searches” ArDM Collaboration Dec 19, 2016. 30 pp. JCAP 1703 (2017) no.03, 003
• “Backgrounds and pulse shape discrimination in the ArDM liquid argon TPC” ArDM Collaboration Dec 2, 2017. 18 pp. e-Print: arXiv:1712.01932
More results
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ArDM to DART : UAr studies (SiPM tests at LSC)
• Design and MC studies on-going
• Vessel under construction
@CIEMAT-Madrid
• OFHC copper vessel ~6.9 kg.• PMMA cylindrical support structure (two
halves cylinder + two plates 6 mm thickness) ~200 g.
• Lateral (outer) 3M foil.• 2 SiPM tiles (top+bottom).• Maximum internal volume ~2.6 L. • LAr volume ~0.8 L.
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Summary
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• New competitive results with LAr, DS-50: ■ High mass WIMPs
■ Low mass WIMPs
■ Sub-GeV Dark Matter
• Exciting technological developments on several fronts: underground Ar, SiPMs, veto…etc
• PSD+ UAr+ SiPM+Radiopurity makes Ar a good candidate to lead the path towards the neutrino floor: DarkSide-20k and more….
Backup
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• There is a remarkable and unique opportunity of having largeexposures(~100 kg⋅yr) in background free mode with an argon TPC:the futurescientific program of the DM direct search community must include adedicated worldwide effort based on this technology!
• At the moment, the dual-phaseLAr TPC is the only technology able toexceed the current experimental limits, reaching the neutrino floor
• A DM search with xenon requires a “background-free” exposure of300ton×yr to reach the neutrino floor
• Considering aβ/γ rejection power in xenon of the order of 99.5% and 300ton×yr. Sensitivity limited by
DD with LAr @ 10-100 ton tonne-scale
o pp neutrinos (20 events/ton×yr in the 0-10 keVER)
o 222Rn (15 events/tonne×yr for every 1 mBq of 222Rn)
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Background budget comparison
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Assessment of the gamma bkg
ER spectrum
• Good data/MC agreement • ER background
– Ar-39: the dominant background, 0.95 Bq/kg
– γ ray: amount to 26% ER background
• ICPMS analysis:– Hg dominant contamination
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ICP-MS analysis: Hg dominant contamination (~ 10 ppb)
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• ultra low background (radiopure and clean)
• background rejection(pid through pulse shape, light/charge, dE/dx)
• low energy threshold (high scintillation/ionization yield)
• large mass (available in large quantity/affordable)
Element Z(A) Boiling point (Tb)@1bar [k]
Liquid density
@Tb [g/cm3]
Energy loss
dE/dx (MeV/cm)
Radiation lengthX0(cm)
Collision length λ(cm)
Ionization [e-/keV]
Scintillation [γ/keV]
Cost
Ne 10(20) 27.1 1.21 1.4 24 80 46 7
Ar 18(40) 87.3 1.40 2.1 14 80 42 40
Kr 36(84) 119.8 2.41 3.0 4.9 29 49 25
Xe 54(131) 165.0 3.06 3.8 2.8 34 64 46
Noble liquids technology for particle physics
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