DarkSide: recent results with underground argon and outlook Davide Franco APC on behalf of the DarkSide Collaboration Prospects in Low Mass Dark Matter 30 th November and 1 st December 2015 MPI-P Munich
DarkSide • WIMP dark matter search using direct detection • Dual-phase Liquid Argon Time Projection Chamber (LAr TPC) • Ultra low background • Deep underground (LNGS) • Low-background materials, including Ar target • Active neutron and muon vetoes • in situ background measurement • Powerful background rejection • Pulse shape discrimination (PSD) • Ionization to scintillation ratio (S2/S1) • Surface rejection using 3D position reconstruction • Multiple scatter event rejection 2 Davide Franco – APC Paris
Dual-Phase Liquid Argon TPC 3 Davide Franco – APC Paris
Discrimination Power S2/S1 Amplitude ER S1 S2 µ s NR S1 S2 Rejection 10 2 -10 3 Factors Benetti et al. (WARP) 2006 4 Davide Franco – APC Paris
Discrimination Power S2/S1 Pulse Shape Discrimination Amplitude ER ER S1 S1 S2 µ s NR NR S1 τ singlet ~ 7 ns τ triplet ~ 1600 ns S1 S2 Rejection 10 7 -10 8 10 2 -10 3 Factors DarkSide Phys. Lett. B 743 (2015) Benetti et al. (WARP) 2006 5 Davide Franco – APC Paris
The DarkSide Program DarkSide-50 ARGO DarkSide-20k DarkSide-10 2013-2016 2025-2030 2020-2025 2011-2013 150 kg 30 ton 150 ton 10 kg 50 kg FM 20 ton FM 100 ton FM ~10 -47 cm 2 “neutrino floor” ~10 -45 cm 2 ~10 -48 cm 2 6 Davide Franco – APC Paris
DarkSide-50 The TPC • 50 kg active mass of UAr • 19 top + 19 bottom R11065 HQE 3’’ PMTs • 36 cm height, 36 cm diameter • Low field of 0.2 kV/cm drift • Lateral walls covered by high reflectivity PTFE • Cold pre-amps • 2.8 kV extraction field 7 Davide Franco – APC Paris
DarkSide-50 Liquid Scintillator Veto (against Radon-free Clean Room Rn < 5 mBq/m 3 neutrons) • 4 m diameter sphere • 30 tonne of boron-loaded scintillator (PC+PPO+TMB) • 110 8” PMTs • LY ~ 500 pe/MeV Cherenkov Water Detector • 11 m diam. x 10 m • 1000 tonne water Cherenkov detector • 80 PMTs 8 Davide Franco – APC Paris
The Detectors 9 Davide Franco – APC Paris
The 39 Ar Issue Before DS50 39 Ar β decay Atmospheric Ar: Q-value: 565 keV Half life : 269 y 39 Ar/ 40 Ar = 8 x 10 − 16 Rate ~ 1 Bq/kg Underground Ar: 39 Ar < 6.5 mBq/kg Atmospheric Argon Underground Argon (arXiv:1204.6011) Total Rate Estimated BG Rate BG Subtracted Rate [mBq /100 keV] [mBq/100keV] [mBq/100keV] Atmospheric Argon 108.8 ± 0.4 1.5 ± 0.2 107.2 ± 1.9 Underground Argon 1.87 ± 0.06 1.5 ± 0.2 0.32 ± 0.23 39 Ar Suppression 58.2 ± 1.9 > 153 (95%) Factor arXiv:1204.60111 Depletion Factor > 150 10 Davide Franco – APC Paris
Internal Calibrations Atmospheric argon: ~1 Bq / kg of 39 Ar 83m Kr AAr @ E drift = 0 V/cm 1.5 x 10 7 events in 47.1 days 41.5 keV ee Q-value = 565 keV Injected gaseous 83m Kr E = 41.5 keV 39 Ar Q = 565 keV ee Average Light Yield : ~7.9 pe/keV at null field ~ 7.0 pe/keV at 200 V/cm Electron lifetime : ~ 10 ms Maximum drift time in the TPC: 375 µ s at 200 V/cm Drift velocity 0.93 mm/ µ s 11 Davide Franco – APC Paris
Nuclear Recoil Energy Scale SCENE Scintillation Efficiency of Nuclear Recoils in Noble Elements Neutron calibration in large detectors affected by neutron multiple scatterings SCENE has collected extremely pure samples of single nuclear recoils in a small scale TPC 12 Davide Franco – APC Paris
External Calibrations Neutron source: AmBe w/ and w/o collimator 13 Davide Franco – APC Paris
External Calibrations Gamma sources: 57 Co (122 keV), 133 Ba (356 keV), 137 Cs (663 keV) Full MC description of all the DS detectors, including: Custom-made ionization, excitation, and recombination • model Full photon generation and tracking • Optical tuning of the TPC: accuracy at a few percent level • Nuclear quenching model • 14 Davide Franco – APC Paris
The Neutron Veto AmBe Calibration Run 15 Davide Franco – APC Paris
The Neutron Veto 7 Li (1015 keV) + α (1775 keV) (6.4%) n + 10 B 7 Li * (839 keV) + α (1471 keV) (93.7%) 7 Li + γ (478 keV) Neutron Veto Efficiency Efficiency from capture signal alone at > 99% (from calibrations and simulations) AmBe Calibration Run α + γ ~0.6% of lost neutrons because of escaping • proton capture gamma ~0.05% of neutrons leave no signal in LSV at all • α Larger total efficiency due to thermalization signal Cut at 1 PE threshold: ~0.9% acceptance loss 16 Davide Franco – APC Paris
DS50 Timeline • Oct 2013 : LArTPC, Neutron Veto and Muon Veto commissioned TPC filled with atmospheric argon (AAr) • • Up to June 2014 : data taken with high 14 C content in LSV 47.1 live days (1422 kg day fiducial) for the first physics result • TMB ( 14 C) was removed to reduce the 14 C rate • • Oct to Dec 2014 : Calibration of TPC with radioactive sources • Jan 2015 : Add radiopure TMB at 5% concentration • Mar to Apr 2015 : filling with underground argon (UAr) and re- commissioning the detector • Apr to Aug 2015 : Accumulate data with UAr for dark matter search 17 Davide Franco – APC Paris
Atmospheric Argon Run: the PSD Phys. Lett. B 743 (2015) 456. No S2/S1 ~38 keV r ~206 keV r WIMP Expected Region 0 event observed ~1.5 x 10 7 events of 39 Ar 47.1 days livetime with AAr 1422 ± 67 kg day 18 Davide Franco – APC Paris
Underground Argon 1. Extraction at Colorado (CO 2 Well) Extract a crude argon gas mixture (Ar, N 2 , and He) 2. Purification at Fermilab Separate Ar from He and N 2 3. Ship to LNGS Ready to fill into DS-50 19 Davide Franco – APC Paris
The Underground Argon Run AAr UAr 20 Davide Franco – APC Paris
The Underground Argon Run Field-Off Three-dimensional likelihood fit based on MC pdf’s using: • S1 field-off • S1 field-on • Drift time Atmospheric Ar Underground Ar: Underground Ar Naturally shielded against cosmic rays Field-On Rate ~ 0.7 mBq/kg 39 Ar 85 Kr Depletion factor ~ 1400 Davide Franco – APC Paris
The 85 Kr Contamination 99.57% BR via β (687 keV) 0.43% BR via β (173 keV) + delayed γ (514 keV) Coincidence mean time: ~ 1.46 µ s Deriving the small BR rate from the main branch: 35.3 ± 2.2 cpd 33.1 ± 0.9 cpd Davide Franco – APC Paris
Underground Argon Run: the PSD arXiv:1510.00702 No S2/S1 Energy [keV ] nr 20 40 60 80 100 120 140 160 180 200 1 90 WIMP Expected Region f 0.9 250 0 event observed 0.8 50% 65% 80% 90% 95% 99% 0.7 200 0.6 150 0.5 0.4 100 0.3 0.2 50 0.1 0 0 50 100 150 200 250 300 350 400 450 S1 [PE] 70.9 days livetime with UAr 23 Davide Franco – APC Paris
Underground Argon Run: the PSD + S2/S1 arXiv:1510.00702 with S2/S1 Energy [keV ] nr 20 40 60 80 100 120 140 160 180 200 1 90 WIMP Expected Region f 0.9 50 0 event observed 0.8 50% 65% 80% 90% 95% 99% 0.7 40 0.6 30 0.5 0.4 20 0.3 0.2 10 0.1 0 0 50 100 150 200 250 300 350 400 450 S1 [PE] 70.9 days livetime with UAr 24 Davide Franco – APC Paris
WIMP Limits 41 − 10 ] 2 [cm WARP (2007) DarkSide-50 (AAr, 2014) DarkSide-50 (UAr, 2015) 42 − 10 σ DarkSide-50 (combined) ) 4 1 0 2 ( I - X a d n a 43 P − 10 PICO (2015) CDMS (2015) − 44 ) 10 2 1 0 2 ( 0 0 1 N O N E X LUX (2013) − 45 10 46 − 10 2 3 4 10 10 10 10 2 M [GeV/c ] χ DarkSide arXiv:1510.00702 25 Davide Franco – APC Paris
Towards DarkSide-20k Depleted Ar : the URANIA and ARIA projects Plant expansion Replacement of the Ar extraction plant in Colorado to URANIA Replacement of the Ar extraction plant in Colorado to reach capacity of 100 kg/day of UAr m UAr ARIA ence between 39Ar/ eduction factor of 10 per pass at the rate Very tall distillation column in Seruci mine (Sardinia) for chemical and isotopic purification of UAr Exploits finite vapor pressure difference between 39 Ar/ 40 Ar: 39 Ar reduction factor of 10 per pass at the rate of 100 kg/day Cecco 16 26 Davide Franco – APC Paris
DarkSide-20k Octagonal TPC (~3 m diamater – ~3 m height) • Target mass: 23 ton UAr • Equipped with 15 m 2 of SiPM: more radio-pure, smaller total mass, higher • efficiency Veto similar to the DS50 one • LoI submitted in April • Technical proposal under submission • − 41 10 ] 2 WARP (2007) [cm PICO (2015) − 42 10 PandaX-I (2014) DarkSide-50 (2015) σ − 43 10 DarkSide-50 (3 yr proj.) CDMS XENON100 (2012) − 44 10 (2015) LUX (2013) 45 − 10 46 − 10 DarkSide-20k (100 t yr proj.) 47 − 10 Argo (1000 t yr proj.) 48 − 10 49 − 10 Coherent neutrino-nucleon scattering floor − 50 10 2 3 4 10 10 10 10 2 M [GeV/c ] χ all projected sensitivities are evaluated by assuming 0 bg 27 Davide Franco – APC Paris
ARGO: the Solar Neutrino Channel Elastic Scattering Goals: In 400 tonne year in the RoI: 7 Be : ~ 4,400 events Observation of CNO neutrinos in 400 • pep : ~ 1,600 events CNO : ~ 1,100 events tonne year exposure (accuracy at ~15%) Metallicity solar models discrimination • 7 Be accuracy at 2% • Pep accuracy at ~10% • arXiv:1510.04196 28 Davide Franco – APC Paris
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