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Understanding neutrino background implications in LXe-TPC dark matter searches using 127 Xe electron captures Dylan J. Temples, Northwestern University Topics in Astroparticle and Underground Physics, Toyama, Japan September 9, 2019 LXe-TPC Dark

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Understanding neutrino background implications in LXe-TPC dark matter searches using 127Xe electron captures

Dylan J. Temples, Northwestern University

Topics in Astroparticle and Underground Physics, Toyama, Japan September 9, 2019

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LXe-TPC Dark Matter Searches

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  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan Top PMT Array Bottom PMT Array Anode Gate Cathode

e- S1 S2

E = W( ne + nγ ) = W( S1/g1 + S2/g2 )

E

extraction

E drift

Advantages: ❖ Energy deposition reconstruction ❖ Sub-mm 3-D position reconstruction ❖ Self-shielding / fiducialization ❖ Continually purify target ❖ Discrimination between NR (signal) and ER (background) ➢ Charge-to-light ratio Limitations (for DM searches) ❖ Insensitive to DM masses below ~5 GeV ❖ Approaching CEνNS sensitivity (indistinguishable from dark matter signal) ❖ Imperfect background discrimination

Observables: S1 (prompt scintillation), S2 (electroluminescence)

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ER/NR Discrimination in LXe-TPCs

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September 9, 2019 TAUP, Toyama, Japan

Dark matter searches use charge-to-light ratio to discriminate between NR (signal) and ER (background)

arXiv:1703.09144

❖ NRs produce overall smaller signals ➢ Efficient energy loss to heat (High dE/dx) ❖ NRs produce proportionally less charge for the same amount of scintillation light Rejection efficiency “quantified” by fraction of ER events below NR median -- nominally 99.5%

Xe

Recoiling particle (e,N)

Heat

Ionization Excitation

Xe Xe Xe

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Calibrating the Discrimination

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  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan

3He

6 keV 6 keV

Tritium beta decay

❖ Typical low-energy ER calibration source

Valence 𝛿/𝜉-e scatter

❖ Similar energy deposition to beta-decay

1 keV 5 keV

Inner-shell 𝜉-e scatter

❖ Atomic deexcitation: Auger e- & x-ray ❖ Larger dE/dx than similar energy beta decays

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Xenon L-shell Electron-recoil Discrimination Analyzer

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  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan

Based on design of MiX detector [arXiv:1507.0131] Photomultipliers:

Top: Hamamatsu R8520 (4x) Bottom: Hamamatsu R11410 (1x)

TPC volume: 40 mL (~117 g LXe) Operating conditions:

Drift field: 275 V/cm Extraction field: 5650 V/cm (liq.)

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Simulating Inner-shell Recoils with 127Xe Electron Captures

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  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan

Activation of 127Xe source at FNAL

Xenon samples in neutron source Activity and isotopic content assayed by HPGe *Paper in preparation*

❖ Excited nuclear state → 𝛿s ❖ Inner-shell vacancy → x-rays and Auger e-s Isolate effects of secondary particles, select events where 𝛿s escape without interacting in the detector. Effect of interest strongest for L-shell capture ❖ Always results in emission of Auger electron(s) ❖ Detector response dominated by most energetic secondary particle

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Detector Performance

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  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan

2mm pitch hexagonal grids

g1 = 0.255 phe/gamma g2top = 18.005 phe/electron

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Discrimination Parameter Space: EC vs. 𝛾

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  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan

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Discrimination Parameter Space: EC vs. 𝛾

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  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan

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Rejection Efficiency: EC vs. 𝛾

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  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan

0.5% → 2.4% 0.5% → 3.6% Nominal ER rejection efficiency (LZ): 99.5% ❖ Leakage rate: 0.5% Reject tritium (valence) and EC (inner-shell) at same discrimination parameter ❖ Leakage rate for inner-shell ERs is 5x larger: 2.4%

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Implications on Neutrino Backgrounds

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  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan

20% of physics background from solar neutrinos 13 𝜉-L scatters Nominal rejection inefficiency: 0.5% ❖ 1.3 “leaked” neutrino ERs Inner-shell rejection inefficiency: ~2.5% ❖ 0.3 “leaked” 𝜉-L ERs ❖ 25% increase in neutrino burden on unrejected ER background Looking forward: DARWIN [arXiv:1606.07001]

❖ Will see 115 𝜉-L scatters per year ❖ Nailing down the rejection efficiency for these events is critical

LUX-ZEPLIN Background Table [arXiv:1703.09144]

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The XELDA Team

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September 9, 2019 TAUP, Toyama, Japan

Northwestern University

  • C. E. Dahl
  • J. McLaughlin
  • J. Phelan

Fermilab / U.C. Santa Barbara

  • W. H.

Lippincott

  • A. Monte
  • J. Bargemann
  • D. Baxter
  • A. Cottle
  • U. Chicago

Oxford

Summer students:

  • M. Trask C. KlienStern
  • M. Fassnacht
  • D. Temples (Northwestern University)
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Conclusions

  • XELDA designed to calibrate effects of atomic deexcitation on ER/NR discrimination power in

LXe-TPCs

  • Neutrino-induced inner shell vacancies produce a different detector response than tritium 𝛾-decays
  • Preliminary results indicate this effect increases the neutrino background burden by ~25%
  • PLR may interpret these events as signal if not included as a source of background
  • This will be an important source of background to understand in larger xenon-based dark matter

searches (DARWIN)

  • Ongoing analysis work to refine measurement and beat down systematics, paper in the works

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  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan Thank you to our funding sources: DE-SC0012161 DOE SCGSR Program

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Backup Slides

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Charge yields: EC vs. 𝛾

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September 9, 2019 TAUP, Toyama, Japan

L-shell charge yield: 35.1 +/- 1.77 electron/keV

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Alternative Rejection Efficiency Calculation

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0.5% → 6.3% Overlap of shifted along line of constant energy2D Gaussian with NR band median Rejection efficiency: 93.7 %

  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan

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127Xe Electron Capture Decays

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  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan Example atomic de-excitation process resulting from L-shell electron capture on

127Xe, demonstrating full Auger cascade.

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Inner-shell Vacancies & Auger Cascades

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❖ Ejected electron ~ β

  • f same energy

❖ Electronic transition accompanied with x-ray or Auger electron emission ❖ Xe L-shell binding energy ~ 5 keV ❖ In LZ dark matter search region ❖ Larger dE/dx in LXe

  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan

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Particle Transport in Liquid Xenon

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September 9, 2019 TAUP, Toyama, Japan

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Impact on DM searches with DARWIN

❖ Nominal ER rejection: 99.98% [30% NR acceptance] ❖ WIMP search region: 2-10 keVee ❖ Solar neutrino ER background rate: 5.2×10−3 events/ton/year (after rejection) ➢ 26 events/ton/year (before rejection) ❖ Active volume: 40 ton LXe (5-yr exposure) ➢ 1040 total solar neutrino ERs per year in WIMP search ROI ➢ 115 of these will be 𝜉-L ERs ❖ At 99.98%(99.5%) valence ER rejection efficiency, our data suggests an L-shell ER efficiency of 99.8%(97.0%) ➢ 0.18 (4.63) leaked neutrino valence ERs ➢ 0.23 (3.45) leaked 𝜉-L ERs

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September 9, 2019 TAUP, Toyama, Japan

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127Xe L-Shell Contamination of Tritium Sample

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September 9, 2019 TAUP, Toyama, Japan

Observed L:K shell ratio: 0.164 Expected: 0.157

(within 5% due to selection efficiency)

Based on K-shell rate in tritium data set, estimate L-shell contamination of tritium in region of interest Contamination fraction: 3.2% L-shell K-shell

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Effect of fluctuation of gains

How much would detector gains have to change in order to move the tritium band to where the L-shell was

  • bserved?

❖ Good handle on g1

➢ K-Shell appears in same S1 region in both data sets

❖ Fluctuation of g2: liquid level changes

➢ To move tritium to L-shell value, need a g2 below 15 ➢ No runs have calculated g2 values that low

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  • D. Temples, Northwestern U.

September 9, 2019 TAUP, Toyama, Japan