DARKSIDE-50 A test of novel techniques for the direct search of dark matter based on depleted argon with the DarkSide-50 TPC deployed in the refurbished CTF detector at Gran Sasso Commissione II Frascati 28 Novembre 2011 Gioacchino Ranucci On behalf of the DarkSide Collaboration Motivations and general concepts Detailed description of the proposed technical implementation Background, sensitivity capabilities and future perspectives Organization and management scheme, funding issues

Dark Matter New physics beyond Standard Model – Unambiguous evidence – Possibly connected with electroweak symmetry breaking, SUSY, and structure formation Multiple prospects for experimental observation – Astroparticle physics: direct and indirect searches – Particle physics: CMS and ATLAS at LHC – Cosmology: halo profiles, CMB, BBN The search of dark matter in Nature with direct techniques is surely one of the prominent subjects of experimental particle physics

M. Attisha e,  χ,n

Direct Detection Requirements Low energy nuclear recoils (< 100 keV) Low rate (~1 event/ton/yr for cm 2 ) Background is the main issue – Radiogenic and cosmogenic neutrons – Beta/gamma signals Detector designed to exploit as many tools as possible to prevent, control, suppress and tag background events

General program features Bring together techniques that ensure the best characterization and rejection of backgrounds in noble liquid detectors The ultimate goal is virtually zero-background with very large exposure i.e. many tons  years Choice of LAr dual phases TPC provides many handles on background DarkSide program introduces 3 innovative technologies crucial for achievement of zero background in very large detectors

Key aspects of the DarkSide-50 proposal Development and operation of a 50 Kg two-phase liquid argon TPC Will build on previous experiences, but adding three fundamental innovations Underground argon depleted in radioactive 39 Ar Low background, high-quantum-efficiency, low temperature photodetectors 3” Cryogenics PMTs 3” QUPID A compact high-efficiency external veto for neutrons -> CTF-RD These plus the powerful two-parameter background rejection features of argon unprecedented virtually background-free performances The technology of the project will pave the way towards the next generation of multi-ton, ultra low background argon detectors with ultimate Wimp sensitivity in the range  cm 2 Predicted sensitivity of DarkSide cm 2

field-shaping rings extraction and acceleration grids drift field (~1 kV/cm) multiplication field (~3 kV/cm) TPC in Action WIMP Scatter deposits energy in FV primary scintillation photons emitted and detected ionized electrons drifted to gas region secondary photons emitted by multiplication in gas region X liquid Ar gaseous Ar photodetectors (QUPIDS) transparent inner vessel fiducial volume boundary

Detailed scheme of the DarkSide-50 detector

Pulse shape discrimination – fast pulses (nuclear recoils) vs. slow pulses (  /  background events) depends on number of detected photons Scintillation to ionization ratio (S2/S1) – lower for nuclear recoils higher for  /  signals Position reconstruction – fiducial volume (rejection of surface contamination), in large detectors suitable to identify multiple interaction sites events from neutrons Background rejection techniques exploited within the TPC

2 Phase vs 1 Phase At cost of modest increase in complexity, 2-Phase adds two more discrimination criteria Fundamental to approach zero background Sharp position resolution crucial for surface background Use all background rejection tools available 2 phase detector compatible with 4  optical read- out and high light yield

Why is depleted argon from underground crucial? Radioactive 39 Ar produced by cosmic rays in atmosphere Beta decay Q=565 keV  /2=269 years In atmospheric argon 39 Ar/Ar ratio 8 x Specific activity 1Bq/Kg Limits size (and sensitivity) of Argon detectors to kg due to 39 Ar events pile-up

A Borexino-inspired solution for the 39 Ar challenge 39 Ar-depleted Argon available via centrifugation or thermal diffusion, but expensive at the ton scale Low background from 14 C crucial for observation of low energy neutrinos with organic liquid scintillator Hydrocarbons in deep underground reservoirs result in low cosmogenic 14 C (as witnessed by Borexino) Similarly 39 Ar production by cosmic rays strongly suppressed underground

Preliminary 39 Ar depleted argon search R&D NSF funded 2 promising sources identified in US Selected the Kinder Morgan Doe canyon complex, Cortez, Colorado Ar separated from the main stream of CO 2, 3% (balance He and N 2 )

Cryogenic Distillation Column Intended to separate the depleted Argon from accompanying Nitrogen and Helium Assembled and operated at Fermilab

Depletion factor >100 measured at Kimballton Kurf

Depleted Argon 75 of 110 kg collected, stable production at 1/2 kg/day Funding of expansion of extraction plant ($1.1M) expected FY12

3” Quartz photon intensifying detector (QUPID)  High Radiopurity (about 1 mBq per unit)  High quantum efficiency (30-40%)  High sensitive surface (30 cm 2 )  Good time and amplitude resolution (800 ps in timing, 21% SER width)

Excellent Multiphoton capability QUPIDS Performances

Cryogenics PMT’s Ultra-low radioactive photosensors will be used in a second phase of the DS-50 run In the first phase the TPC will be equipped with the current-version cryogenics metal-bulb PMT’s Low radioactivity-low temperature-high QE Hamamatsu R11065 – already purchased Photocathode material: Bialkali-LT optimized for low temperature operation At 420 nm (wavelength shifter) QE=35% Single pe spectrum

Background from photosensors The photosensors are the major contributors to the intrinsic radiogenic neutrons – QUPIDS a big breakthrough in this respect Hamamatsu is also developing ultra-low radioactive PMT’s, comparable to QUPIDS

Neutron veto  Idea: tag individual neutrons  Approach based on (n,  ) reaction on 10 B  Alpha particles have extremely short range  Alpha particles can be observed using borated liquid scintillator initial concept of BOREX (the precursor of Borexino)  99.8% efficiency for radiogenic neutrons with 1m thick shield is achievable (MC evaluation)  Evaluation of the efficiency via a neutron calibration source -> estimate of the residual untagged neutrons in the dataset

Neutron veto Elements of the neutron veto: boron loaded scintillator TMB+PC (50%/50%) with PPO 2g/l about 30 tons in total 110 low radioactivity 8” PMT’s Stainless steel supporting sphere - design completed and order submitted to the supplier Walter Tosto Serbatoi Delivery April 2012

Muon Veto: PMTs on the CTF walls 80 recovered among those already in use tyvek panels not shown Neutron Veto: Stainless steel sphere (BX) 4 m diameter 110 new 8” PMTs HQE and low radioactivity DarkSide-50: TPC within the cryostat filled with depleted argon DS-50 and CTF GLOBAL VIEW

Cryostat and TPC fully designed - beginning of construction imminent

Fused silica window coated with WLS and ITO Extraction grid Containment cylindrical wall of the Argon chamber diffusive PTFE coated with WLS Field cage copper rings “Quartz boundary” for the electroluminescent gas pocket WLS=TetraPhenylButadiene (TPB peak emission 420 nm) ITO=Indium Tin Oxide conductive layer

Assembly in a radon free clean room Following the Borexino experience for the Inner Vessel assembly in radon free clean room to ensure the ultimate surface cleanliness, the same strategy will be adopted for the TPC construction

CR1 – Radon Free Complete refurbishment of the clean room in front of Borexino, including the addition of the radon-free capability, for assembly of the TPC Essential also for future needs of rework and maintenance of the TPC

Top radon-free clean room as insertion “buffer” Contains all “connections” to the outside world

Cryogenic loop for continuous argon purification Already built at Fermilab Preliminary test at Fermilab or Gran Sasso

Electronics (tender in progress) 8” PMTs for neutron veto (orders submitted) Borated liquid scintillator (Psudocumene available, TMB not yet) Are crucial elements for the success of the program Already discussed at length in the September presentation of CTF-RD-DARKSIDE, thus not repeated here Other components

20 May DS-10 Summer September A benchmark to test many of the DarkSide-50 solutions Presented in September, not repeated here

Summary of the background suppression tools Radiogenic neutrons : neutron veto Cosmogenic neutrons: muon & neutron veto Electron recoil events: depleted argon & PSD & ionization to scintillation ratio (S2/S1) Surface background: cleanliness of surfaces & assembly in radon free clean room & position reconstruction (fiducial volume) All construction materials selected following a tight screening for radioactivity content

Background estimates (in parenthesis QUPIDS version) for three years of operation Total background from neutron and electron recoils <0.1 events sensitivity cm 2 for 100 GeV WIMPs

The techniques tested in DarkSide-50 can be effectively employed for a larger (5 tons) experiment with ultimate sensitivity  cm -2 Sensitivity

Minimal Dark Matter In a minimalistic approach look for a candidate with properties: weakly interacting, neutral, stable, massive Present direct searches and relic abundance predicts ~ 10 TeV mass scale candidate DLAr with 26 keV threshold

DarkSide Collaboration Augustana College – SD, USA Black Hills State University – SD, USA Fermilab – Il, USA IHEP – Beijing, China INFN Laboratori Nazionali del Gran Sasso – Assergi, Italy INFN and Università degli Studi Genova, Italy INFN and Università degli Studi Milano, Italy INFN and Università degli Studi Napoli, Italy INFN and Università degli Studi Perugia, Italy Joint Institute for Nuclear Research – Dubna, Russia Princeton University, USA RRC Kurchatov Institute – Moscow, Russia St. Petersburg Nuclear Physics Institute – Gatchina, Russia Temple University, USA University of Arkansas, USA University of California, Los Angeles, USA University of Houston, USA University of Massachusetts at Amherst, USA

Management Project engineer: P. Lombardi Project manager(s): An. Ianni, D. Montanari Project scientist: Hanguo Wang GLIMOS: S. Gazzana Steering Committee chair: Al. Ianni Operational Manager: A. Goretti Montecarlo coordinator(s):A. Cocco, A. Wright Analysis Coordinator: L. Grandi IB chairs and spokespersons: C. Galbiati, G. Ranucci

L2 managers – macro-areas L3 managers – working groups

Postponed to /3 of these 2 items s.j. to the DarkSide presentation Red: rejected from the Commissione Light blue: postponed or s.j. Green: not yet submitted, waiting for further developments US funding already fully awarded

Additional funding considerations If the DS-50 second phase will be realized with ultralow radioactivity PMT’s the green figures of the table will be changed, including the possible purchase of the devices The two radon free Clean Rooms are not only essential for the project, but also infrastructures of general interest which would be a valuable investment with persistent validity in the future also for other experimental applications. A global costing of both will be available upon the completion of their design (first months of 2012); at that time a cost sharing INFN/US funding could be discussed We strongly advocate a reconsideration of the decisions about the (non) funding of the items related to the TPC, acknowledging in this way the crucial role of the INFN groups also in this part of the detector, a role which we would like to see substantially growing in the future

INFN section Researchers (FTE) Technologists (FTE) Technicians (FTE) LNGS Milano Napoli1.2 Perugia INFN Staff involvement Plan for 2012 The commitments for the next years will be at least at the same level, very likely more than that Participation of the Genova group for the electronics (in view of the possible Borexino upgrade)

Conclusions The refurbishment of CTF and the deployment in its core of the DarkSide-50 TPC represent jointly a vey competitive program to test effectively a number of innovative solutions for the Dark Matter searches with argon liquid detectors The already interesting sensitivity of the 50 Kg sized TPC would find its natural evolution in a multi-ton scale detector (5 tons) incorporating the technological solutions of DarkSide-50, with sensitivity of cm 100 GeV - very competitive also in the high mass range favored by the current post-LHC (1/fb) global SUSY fits In the framework of the international collaboration that is vigorously pursuing the DarkSide-50 program the INFN groups have already a prominent role which could be further, drastically enhanced through the INFN support to a direct involvement in the TPC development, construction and operation

Back-up slides

Underground Argon Extraction Plant

HCR – Radon Free

DS-10 Transparent conductive windows for anode and cathode HHV field reversed through bottom transparent window Wavelength shifter coating side walls and windows Racetrack system is a flexible PCB surrounding One single hexagonal mesh for electrons extraction and multiplication ITO coated window TPB-coated reflecting foils Hexagonal Mesh

DarkSide-10 TPC 7 (top) + 7 (bottom) R1140 HQE Hamamatsu PMTs 20 cm × 20 cm

Light yield – Primary goal is the increase in light yield reflector, transparent windows, etc. – Study of long-term stability, reproducibility – Study of long term stability of PMTs Background rejection power – Study 3D localization algorithms – Characterize pulse shape discrimination as function of E drift – Characterize S2/S1 discrimination as function of E drift – Study rejection of background from lateral surfaces All the above studies required to inform design for DS-50 TPC Goals of DS-10 Run

LY=9.0 ± 0.1 null field, single phase LNGS: Light Yield in single phase mode

LY=8.9 ± 0.1 null field, two-phase LNGS: Light Yield in two-phase mode

LNGS: gamma-like null field - No Shield: ~69.9Hz in 25÷1600keV ee - Shield Closed: ~10.9Hz in 25÷1600keV ee 39 Ar beta-spectrum Trigger: majority of 4 PMTs with 0.5 p.e. threshold. With the shield closed trigger rate is ~ 17Hz. We acquire data without dead time.

LNGS: First set of operations with drift and extraction fields Demonstrated LAr purity level required for electron drift.

DS-50: WIMPs vs background rate WIMP: (10 GeV, cm 2 ) mBq/kg

DS-50: WIMPs vs background rate WIMP: (10 GeV, cm 2 ) mBq/kg 39 Ar and other backgrounds