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8-10 December, 2019 Madison, WI USA Neutrinoless double beta decay with Andrea Pocar University of Massachusetts, Amherst (on behalf of the nEXO Collaboration) Playbill Why double beta decay? Why tonne scale? nEXO


  1. 8-10 December, 2019 — Madison, WI USA Neutrinoless double beta decay with Andrea Pocar University of Massachusetts, Amherst (on behalf of the nEXO Collaboration)

  2. Playbill β • Why double beta decay? • Why tonne scale? • nEXO • EXO-200 progenitor • R&D progress β Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 2

  3. 0 νββ decay = new physics L = -1 observation of 0 νββ decay • massive, Majorana neutrinos • lepton number violation ( Δ L = 2) • new mass creation mechanism • new mass scale 0 νββ rate • absolute neutrino mass 
 (model dependent) L = +1 [Schechter and Valle, 1982] possible probe for understanding the matter dominance in the universe through leptogenesis (via Δ (B-L)) Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 3

  4. 0 νββ decay rate transition particle physics probability of the ‘black box’ phase space nuclear factor: matrix element For virtual exchange of light Majorana neutrinos, the decay rate depends on an effective neutrino mass η ∼ < m ββ > Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 4

  5. Current state of the art 1 / 2 (10 25 y r ) 1 / 2 (10 25 y r ) T 0 ν T 0 ν isotope experiment year status (sensitivity) (lower limit) 5.6 
 >10.7 
 KamLAND-Zen (phase I+II) 
 2016 
 completed 
 Xe-136 (8.0) (>4) (KL-Z 800) (2019) (running) 11 >9 Ge-76 Gerda (phase I+II) 2018 running 4.8 >2.7 Ge-76 Majorana Demonstrator 2018 running 5.0 >3.5 Xe-136 EXO-200 (phase I+II) 2019 completed 1.5 >2.3 Te-130 Cuore (w/ Cuoricino) 2019 running 0.5 >0.35 Se-82 Cupid-0 2019 completed Te-130 SNO+ commissioning Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 5

  6. 0 νββ decay and neutrino mass (See-Saw I mechanism) current experiments 
 (~100 kg, T 1/2 ~10 26 y) “tonne-scale” 
 ( T 1/2 ~10 28 y ) 0 νββ rate • absolute neutrino mass 
 (model dependent) 1 = G 0 ν ( Q, Z ) | M 0 ν | 2 < m ββ > 2 T 0 ν 1 / 2 Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 6

  7. The history of 0 νββ decay experiments in one slide Tonne scale detectors ~16 orders of magnitude T 1/2 limit (mostly 90%CL) and 80 years Discovery of ν oscillations Age of the universe Year Slide courtesy of G. Gratta Data courtesy of S.Elliott and the PDG. Not all results are necessarily shown. Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 7

  8. Neutrino-less double beta decay Nucleon binding energy (MeV) observable when single β -decay is forbidden 2 νββ or disfavored Atomic number (Z) predicted and calculated in 1935 by Maria Göppert-Meyer new 2 νββ 0 νββ physics proposed in 1937 by Racah + Furry Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 8

  9. How does one look for a faint (at best) peak? Source mass • observe as many nuclei as possible • isotopic enrichment Energy resolution • spurious events from other processes • separate 2 νββ decay events Radioactive background control • eliminate other events (go underground, shielding, materials selection) Background discrimination • measure residual background as precisely as possible and extrapolate it to the energy+volume region of interest A note for the pessimist: How well one can achieve the above goals determines the physics that can be done in the absence of a signal Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 9

  10. Dura lex, sed lex N A = 6 . 022 × 10 23 Amedeo Avogadro • DBD candidate isotopes: 48 → 150 grams/mole • 10 28 nuclei = 16,600 moles → 800—2,500 kg • Add-in real-life non-idealities: 
 detection efficiency, isotopic fraction, backgrounds, detector live time, …. Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 10

  11. the Enriched Xenon Observatory (EXO) program Enriched Liquid Xenon Time Projection Chambers (TPCs) of increasing sensitivity 1. Liquid enriched xenon (>80% 136 Xe) 2. EXO-200 (Phases 1/2) 
 (200 kg; opened kmole era; ν mass sensitivity ~100 meV) 3. nEXO, R&D underway, towards a project 
 (5 tonnes; ν mass sensitivity ~10 meV, cover inverted mass ordering) 4. nEXO “Phase 2” with Ba-daughter ID (~ meV) Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 11

  12. Enriched LXe TPCs • Monolythic (efficient background mapping) Why xenon? • In-line purification of xenon • Simple-minded enrichment 228 Th source, SS Liquid xenon TPC’s • Active self-shielding (improves with size) • Good energy resolution 
 (ionization+scintillation, 0 ν /2 ν separation) • Particle ID (scintillation vs. ionization) β , ββ • Event topology (single-/multi-site events) Scale-up: EXO-200 (200 kg) ➔ nEXO (5,000 kg) γ Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 12

  13. The EXO-200 precursor to nEXO Low background single site Scin*lla*on# multiple site Ioniza*on# e"# e"# events (SS) events (MS) e"# e"# data e"# e"# e"# e"# e"# e"# 2 νββ e"# e"# e"# 228 Th calibration source γ γ Phase I+II: 234.1 kg yr 136 Xe exposure ⋅ Limit T 1/20 νββ > 3.5 x 10 25 yr (90% C.L.) 〈 m ββ 〉 < (93 – 286) meV Sensitivity 5.0x10 25 yr PRL 123(2019)161802 Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 13

  14. the EXO-200 TPC half TPC Teflon reflector tiles Cathode mesh (two ‘bikinis’) ~40 cm Charge collection wires in front of LAAPDS (sensitive to 175 nm) acrylic supports Field shaping rings Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 14

  15. the EXO-200 full Phase II results PRL 123(2019)161802 Phase I+II: 234.1 kg yr 136 Xe exposure 2019 release uses machine ⋅ Limit T 1/20 νββ > 3.5 x 10 25 yr (90% C.L.) learning (DNN) for improved signal-to-background 〈 m ββ 〉 < (93 – 286) meV discrimination Sensitivity 5.0x10 25 yr Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 15

  16. nEXO: a homogeneous detector 5kg 150kg Attenuation Length of a 2.4 MeV γ -ray in LXe (~ 8.5 cm) take full advantage of: 1) Compton tag and rejection 5000kg 2) External background identification and rejection The larger and monolithic the detector, the more useful this is. ➔ Ton scale is where these features become dominant. Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 16

  17. Preliminary artist view of nEXO in the SNOLAB Cryopit Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 17

  18. nEXO: a 5 tonnes LXe TPC in-xenon cold charge readout electronics pads (anode) • < 1% energy resolution (charge and SiPMs) • no central cathode • ≳ 10 ms electron lifetime SiPM ‘staves’ • ~500 Rn atoms coating the barrel (behind the cathode field cage) 1.3 m electron • no plastics, in-Xe cold electronics drift • VUV-sensitive SiPMs behind field cage • charge readout strips d i a m e t e r 
 ( 1 . 3 m ) • sensitivity (10 years): 9 x 10 27 yr • 25x EXO-200 • energy, topology, standoff & particle ID • enhanced self-shielding • x100 better T 1/2 sensitivity Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 18

  19. nEXO TPC highlights • A pad-like charge collection detector to replace a more traditional wire readout. • VUV-sensitive SiPMs • in-LXe readout electronics under development ~6 cm Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 19

  20. Charge collection ‘tiles’ (ionization detector) JINST 13, P01006 (2018) • Prototype 3mm pitch, crossed strips deposited on a 10 cm x 10 cm quartz tile produced and tested in liquid xenon. 80 fF at crossings 0.86 pF between 10 μ m adjacent strips M.Jewell et al., “Characterization of an Ionization Readout Tile for nEXO’’, 3 mm J.Inst. 13 P01006 (2018) no shielding Frisch grid Bi-207 source Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 20

  21. Detailed charge reconstruction 2019 JINST 14 P09020 charge-average distance to center BDT parameter event channel number ~20% sensitivity improved with EXO-200-derived multi-variate analysis ~30% improvement possible with DNN treatment of rise time distribution charge waveforms Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 21

  22. Charge calibration arXiv:1911.11580 Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 22

  23. Progress on VUV-sensitive SiPM’s IEEE Trans NS 65 (2018) 2823 nEXO goal NIM A 940, 371 (2019) Some 1cm 2 VUV devices now match our desired properties, with a bias of ~30V Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 23

  24. SiPM reflectivity (LXe) arXiv:1910.06438 LIXO setup at Alabama PDE 
 (VUV4 SiPM) specular reflectivity 
 (VUV4 SiPM) Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 24

  25. Optics / SiPM reflectivity arXiv:1912.01841 setup at IHEP Beijing (in gas/vacuum) Andrea Pocar — UMass Amherst CPAD 2019 — Madison, 8-10 December 2019 � 25

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