Direct search for WIMP Dark Matter particles with the LUX-ZEPLIN - PowerPoint PPT Presentation

Direct search for WIMP Dark Matter particles 
 with the LUX-ZEPLIN (LZ) detector Kirill Pushkin University of Michigan on behalf of the LZ collaboration 14 th Pisa meeting on Advanced Detectors, May 27 – June 2, 2018

The LZ collaboration 
 April, 2018 
 (250 scientists, engineers and technicians; 37 institutions ) 2

LZ = LUX + ZEPLIN LUX (USA, SURF) ZEPLIN-III (UK, Boulby) 100 kg FV ZEPLIN pioneered WIMP-search with 1.1 × 10 -46 cm 2 6 kg fiducial volume (FV) two-phase Xe at 50 GeV/c 2 3.9 × 10 -44 cm 2 (decommissioned in early 2017) Scale-up using demonstrated technology and experience for LZ low-risk but aggressive program: 5,600 kg FV • Very low internal background strategy • Infrastructure inherited from LUX • LZ expected sensitivity: 1.6 × 10 -48 cm 2 in 1000 days D. Akerib et al., “Projected WIMP sensitivity of the LUX-ZEPLIN (LZ) dark matter experiment”, 3 arXiv:1802.06039v1 [astro-ph.IM]

Sanford Underground Research Facility 
 (Lead, South Dakota) 4

Why LXe is suitable for Dark Matter search Properties of Xenon Atomic Number (Z): 54 Mass number (A): 131.30 Number of electrons per energy level: 2, 8, 18, 18, 8 Density STP: 5.894 g/L MelKng point: 161.4 K Boiling point: 165.1 K Triple point: 161.405 K • Dense liquid (3 g/cm 3 ) for a massive WIMP target at modest cost (~2000 USD/kg) and scale. • No intrinsic radioactivity other than 85 Kr and 222 Rn which both can be significantly removed using certain techniques (cryogenic distillation and radon reduction using gas chromatography). • High sensitivity to spin-independent (SI) WIMP interactions due to its high atomic mass (acts coherently on the entire nucleus and scales as A 2 ). • For spin-dependent coupling, the cross-section depends on the nuclear spin factor. Does not scale with nuclear size ( 129 Xe and 131 Xe). 5

Two phase time-projection chamber S1 – primary S2 – scintillation electroluminescence 6

LZ (LUX-ZEPLIN) • LXe TPC: 50 times larger than LUX • 1.6 km underground (4300 m.w.e.), SURF, Davis Campus • Underground installation will start in fall 2018 LZ • Physics data taking will start in 2020 LZ LUX Total mass: 10 T WIMP active mass: 7 T WIMP fiducial mass: 5.6 T 7

Engineering model of the LZ detector • 494 Hamamatsu PMTs, R11410-22, 3” (low radioactive) • TPC walls are covered with highly VUV light reflective PTFE • Nominal cathode operating voltage ≈ 50 kV, E ≈ 310 V/cm • ~2 T of LXe in the skin veto region (93 Hamamatsu, R8520 PMTs and further 38 Hamamatsu R8778 PMTs) • The second veto system contains liquid scintillator – Gadolinium (17.3 T) to tag neutrons. • 120 Hamamatsu R5912 PMTs mounted in water tank 8

Radioactive background strategy u Xenon purification from 85 Kr and 39 Ar • Distillation system at SLAC based on LUX R&D • Final 84 Kr/Xe ~ 0.015 ppt (g/g) u Extensive radioactive assay of detector materials • Gamma screening with inductively coupled plasma mass-spectrometry (ICP-MS), neutron activation analysis (NAA) • Comprehensive radon emanation measurements u Strict surface cleanliness protocols • Detector assembly in 222 Rn reduced clean rooms • Dust control, < 500 ng/cm 2 on all LXe wetted surfaces • Rn-daughters plate on TPC walls <0.5 mBq/m 2 9

222 Rn reduction system for LZ 
 (designed and constructed at the University of Michigan) 222 Rn input: 8.3 mBq Xe flow rate: 0.5 slpm Charcoal trap temperature: 190 K 1 mBq threshold, LZ’s goal 222 Rn input: 20 mBq Vacuum-jacketed cryostat with 11 kg Xe flow rate: 0.5 slpm of HNO 3 etched Saratech adsorbent Charcoal trap temperature: 190 K 222 Rn emanation from some charcoals (the list is not complete, read the article) • Carboact: (0.23±0.19) mBq/kg Total 222 Rn concentration reduction output from the • Regular Saratech: (1.71±0.20) mBq/kg • LZ detector vs mass of adsorbent (Xe flow rate 0.5 SLPM) HNO 3 etched Saratech: (0.51±0.09) mBq/kg K. Pushkin et al., “Study of radon reduction in gases for rare event search experiments”, 10 submitted to NIM A and arXiv:1805.11306v1 [physics.ins-det].

Veto system performance • WIMP-like nuclear recoil backgrounds in 6-30 keV region of interest • Before and after application of outer detector plus skin veto Before veto After veto 10 2 10 2 140 140 120 120 10 1 10 1 100 counts/tonne/year 100 counts/tonne/year ≈10 events/5.6 ton ≈1 event/5.6 ton z [cm] z [cm] 80 80 in FV in FV 60 60 10 0 10 0 40 40 20 20 10 − 1 10 − 1 0 0 0 2 20 2 30 2 40 2 50 2 60 2 70 2 0 2 20 2 30 2 40 2 50 2 60 2 70 2 r 2 [cm 2 ] r 2 [cm 2 ] 11

Projected background rates • Counts/kg/day/keV in 5.6 ton fiducial volume • Signal scatter events with no veto signal Nuclear recoils Electron recoils 10 − 3 10 − 4 10 − 4 Rate [counts/kg/day/keV] Total Rate [counts/kg/day/keV] 10 − 5 Xe 6 3 1 222 Rn 8 B 10 − 6 10 − 5 Total 10 − 7 Solar ν Det. + Sur. + Env. hep 220 Rn 10 − 8 Det. + Sur. + Env. 10 − 9 85 Kr 10 − 6 10 − 10 DSN A t m 10 − 11 0 50 100 150 200 0 20 40 60 80 100 Nuclear recoil energy [keV] Electronic recoil energy [keV] 12

Counts/1000 days: WIMP search region-of-interest (ROI) LZ 1000 day exposure; Counts for a 40 GeV/c 2 WIMP ROI Total sum of ER and NR with 99.5% ER discrimination and 50% NR efficiency: 6.49 events 13

Projected LZ sensitivity, spin-independent, 
 (5.6 ton FV, 1000 live-days) 42 − 10 LZ sensitivity (1000 live days) LUX (2017) Projected limit (90% CL one-sided) XENON1T (2017) 43 − ] 10 2 1 expected ± σ SI WIMP-nucleon cross section [cm PandaX-II (2017) +2 σ expected 44 − 10 45 − 10 pMSSM11 (MasterCode, 2017) 46 − 10 47 − 10 t n e v e o n i r t u e n 1 ) S 48 N − ν 10 E C ( t i m i l y r e v o c s d i o n i r t u e N 49 − 10 10 100 1000 1.6 × 10 -48 cm 2 at 40 GeV/c 2 2 WIMP mass [GeV/c ] D. Akerib et al., “Projected WIMP sensitivity of the LUX-ZEPLIN (LZ) dark matter experiment”, 14 arXiv:1802.06039v1 [astro-ph.IM]

LZ schedule • Critical decision, step 1 – (CD1) Review – March 2015 • CD2 Review – April 2016 • CD3 Review – February 2017 construction can start in earnest • Cryostat fabrication has recently been completed • PMT array assembly began in March of 2018 • Xenon handling installation and commissioning starts this fall • TPC installation will start in Spring-Summer of 2019 • Xe liquefaction will start in winter of 2019 • First physics data are expected in Spring of 2020 15

Summary • The LZ detector will be the largest dual-phase Xe detector in the world with an active mass of 7 tons optimized for a potential discovery of WIMPs. • The detector’s components are carefully selected and meticulously assayed for the presence of radioactive background. • The active veto system will help to suppress NR background. • The LZ detector will have an order of magnitude sensitivity improvement compared to the currently running LXe experiments. • The underground installation will begin this fall and data taking will start in 2020. 16

Backups 17

LZ sensitivity vs 222 Rn level − 48 10 × 3.5 ] 2 [cm LZ sensitivity (1000 live days) 2 WIMP-nucleon cross section at 40 GeV/c Projected limit (90% CL one-sided) 3 low estimate projected high estimate highest estimate 2.5 2 1.5 1 0 2 4 6 8 10 222 Rn specific activity [ Bq/kg] µ 18

17 T gadolinium loaded liquid scintillator GdLS 19

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