DarkSide: latest results and the future liquid argon dark ... · High WIMP mass search Roberto...

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

Roberto Santorelli - SUSY July 2018 2

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)


• 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


DarkSide-50 design


� ∅ 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


• 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


AmC / AmBefor neutron veto efficiency

(standard isothermal WIMP model)


� 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


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)


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


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


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


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



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

16

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)



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


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.


Summary

Roberto Santorelli - UB Feb2018 22

• 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

Roberto Santorelli - UB Feb2018 23

• 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)


Background budget comparison


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


ICP-MS analysis: Hg dominant contamination (~ 10 ppb)

• 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


DarkSide: latest results and the future liquid argon dark ... · High WIMP mass search Roberto...

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