Improved Measurements of the -Decay Response of Liquid Xenon with - - PowerPoint PPT Presentation

Improved Measurements of the β-Decay Response of Liquid Xenon with the LUX Detector

Jon Balajthy

UC Davis, Department of Physics September 12, 2019

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LUX Detector

• 4850’ level (4300 m w.e.) of the Sanford Underground

Research Facility in Lead, SD.

• Water tank for neutron shield/ muon veto
• 2-Phase (liquid/gas) xenon TPC

○ Sensitive to light (S1) and charge (S2) ○ 3-D position reconstruction

• Total/ fiducial mass of 370 kg/ 118 kg
• Combined science runs:

○ Excluded scattering cross sections below 1.1×10−46 cm2 for a 50 GeV WIMP ○ Exposure of 3.35×104 kg days ○ ~1100 fiducial counts

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Combined Energy Model

Charge and light work functions are averaged into a single W = 13.7 ±0.2 eV/quanta. Some fraction, r, of Ni will be converted to N* α is empirically constant for ER

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Nr will be randomly distributed with mean value, Ni ·PR, and standard deviation, σR. ER energy can equivalently be written:

• r, in terms of measurables:

Which can be rewritten:

ER Yields

Energy and field dependence of the recombination process is characterized by the yields: Important note- Ly and Qy are assumed to be complementary:

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ER Yields in Post-WS 14C and 3H Data

Begin with S1 and S2 measurements in a collection of energy and field bins for both 14C and

3H.

• Fields: 43, 53, 65, 80, 98, 120, 147, 180, 220, 269, 329, 403, 491 V/cm

○ The drift field in Run04 was highly non-uniform (arXiv:1709.00095)

• Energies: 1-145 keVee

○ The 14C spectrum overlapped with 131mXe above this range

Procedure: 1. Model detector resolution in post-WS 2. Develop model of yields and recombination fluctuations (σR) 3. Numerically de-smear the reconstructed energy, S1, and S2 spectra 4. Measure Nγ/Ne in each energy/field bin (can then be used to calculate LY, QY, and PR)

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Post-WS Doke Plot

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• Doke plot measured

from Kr-83 + Xe-131 at a collection of drift times

• Only two lines, but

smearing due to E-field gives very good measurement

• f g1, g2

Post-WS Doke Plot

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Errors dominated by systematic variation Two-point Doke plots in each drift bin gives a measurement of how g1 and g2 change as a function of position Introduces subdominant- dominant errors on final Ly, Qy measurements

S2 Tails in Post-WS LUX

Thought to be caused by emission

• f electron trains

Modeled by adding area to the LibNEST S2 output ○ Area drawn from an exponential distribution with mean = b*S2 ○ Added only to a certain fraction, R, of MC events ○ b and R found by matching 37Ar and 131mXe MC to data

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S2 Tails in Post-WS LUX

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The addition of the S2 to libNEST significantly improves the agreement with both 3H and

14C beta spectra

σR should be proportional to Ni

1/2 for a binomial process.

σR has been instead observed to be roughly proportional to ~Ni In LUX post-WS carbon-14 data, we add a Gaussian adjustment to the Ni dependence Where:

Recombination Fluctuations

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Comparison to LUX WS2013

The adjusted σR model better reproduces the substructure of the LUX WS2013 measurements When Ne ≈ Nγ, the new model reproduces the WS2013 result

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arXiv:1512.03133

De-Smearing a Beta Spectrum

• Continuous spectra will be smeared by

finite detector resolution.

• Similar effect is found in 2-D Ne, Nγ space
• Can be accounted for analytically by

integration

• In Post-WS, this is unwieldy due to S2 tails.

Instead we desmear numerically ○ Calculate ratio of true to measured Erec in MC & apply correction to data ○ Calculate ratio of true to measured Ne, Nγ in MC, and apply correction to data

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The average Etrue in the 80 keV bin is 79.53 ±0.009 keV

Results- Snapshot

Results are reported in terms of photon-electron ratio Very low statistical error (shown in black) Systematic error is taken as quadrature sum of:

• g1/g2 uncertainty
• Absolute magnitude of

smearing correction

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Post-WS Qy Compared to WS2013

We construct an empirical model to help compare LUX Post-WS to previous measurements. All the datasets considered agree with the model to about 1-σ. The QY results are consistent with WS2013 measurements at 180 and 100 V/cm, except...

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arXiv:1512.03133 arXiv:1709.00800 arXiv:1610.02076 arXiv:1705.08958

Post-WS Qy Compared to WS2013

… Run03 tritium has distinct kink near endpoint This is due to mistake in WS2013 de-smearing. Recombination fluctuations are double counted- they are corrected for in both reconstructed energy and individual quanta.

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Wrong

Beta vs EC

Also… 127Xe EC L-shell is consistent within systematic error bars, but just barely As per Dylan Temples’ talk on Monday- this discrepancy appears to be physical

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Beta vs Photo-absorption

131mXe and 83mKr lines also show discrepancy with beta yields

• Photoabsorption events give higher light yield
• 83mKr is composite, and has even more intricacies

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Compared to World Data

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Compared to World Data

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Good agreement with 3H data from Xenon100

Compared to World Data

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Systematically below the Compton scattering data from neriX, Shanghai, and E. Dahl

Could point to physical difference in Compton/ beta yields?

Compared to World Data

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Systematically below

37Ar EC data from

PIXeY.

Summary

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• Developed model for S2 tails in LUX Post-WS data
• Added Gaussian adjustment to σR model
• Numerically de-smeared S1 and S2 spectra
• Measured LY, QY, and PR as functions of energy and electric field

○ Energy range from 1-145 keV ○ Field range from 43-491 V/cm

• Compared results to existing data

○ Very consistent with LUX WS2013 3H data ○ Consistent with WS2013 127Xe K- and M-shell ○ Hints of discrepancy with 127Xe L-shell ○ Agrees with 3H data from Xenon100 ○ Disagrees with all available Compton data

Extra Slides

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Energy Deposition in LXe

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An energetic particle interacting with a xenon nucleus will create:

• Excited xenon atoms (Xe*)
• Electron-ion pairs (e-, Xe+)
• Heat (not detected)

The excitons and ions form dimers with ground state atoms (Xe2*, Xe2

+).

After the initial partitioning, there is a recombination period (~50 ns) where:

e-+Xe2

+→ Xe**+Xe → Xe*+Xe + heat

The net result of this is that an ion is converted to an exciton.

ER Yields

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For ER, PR, LY, and QY can all be written in terms of the Nγ/Ne ratio only:

Post-Run04 Injection Campaign

Following the primary WIMP-search run (WS2014-16), a series of NR and ER calibrations were performed.

• D-D
• 83Kr
• 131mXe
• 3H
• 14C
• 222Rn
• 37Ar

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Beta Model from Post-Run04 14C and 3H Data

Fully empirical model of QY and σR

• QY is fit to the sum of two

asymmetric sigmoids

• The m1 and m7 parameters are

allowed to vary with field

• σR is the Gaussian adjusted

linear model presented in slide 6 Compton data from neriX is in tension

• n the 1-2 sigma level

27 Non-LUX datasets used to constrain model: Low energy: Boulton et al., 2017 High energy: Doke et al., 2002 0-field: Baudis et al., 2013 Aprile et al., 2012 High-field: Akimov et al., 2014

Results- 14C

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Results- 3H

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