The Quest for Dark Matter with the LUX-ZEPLIN Experiment

Alvine Kamaha

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University of California, Los Angeles

March 14-18, 2022

The Dark Matter Case

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• Started back in ~1930’s

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• Numerous evidence to-date from astrophysical and cosmological observations

Cosmic Microwave Background

Galaxy rotational curve

X-ray:: NASA/CXC/CfA/M. Markevitch et al.; optical: NASA/STScI, Magellan/U. Arizona/D. Clowe et al.; lensing map: NASA/STScI ESO WFI, Magellan/U. Arizona/D. Clowe et al.)

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Bullet cluster

© phys.org

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Dark Matter Particle Candidates

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A wide field of possible candidates all beyond the Standard Model.

• Preferred ones are WIMPs but not yet found

“While the observational evidence for dark matter is exceptionally convincing, our current level of ignorance of the basic properties of dark matter is remarkable”

(pg.142, Briefing Book for European Strategy for Particle Physics Update 2020)

*outdated

(WIMP) Dark Matter Searches in our galaxy

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1) Collider production

2) Indirect Detection

3) Direct Detection

Recent Progress in Search for Dark Sector Signatures

Homestate

LUX, LZ

Dark Matter Search: past, present & future

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Dominated by dual-phase noble liquid detectors at high mass ⇒ LUX-ZEPLIN

LUX-ZEPLIN (LZ) Collaboration

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Rutherford Appleton Laboratory, UK, January 2020

35 Institutions: 250 scientists, engineers, and technical staff

• Black Hills State University
• Brandeis University
• Brookhaven National Laboratory
• Brown University
• Center for Underground Physics
• Edinburgh University
• Fermi National Accelerator Lab.
• Imperial College London
• Lawrence Berkeley National Lab.
• Lawrence Livermore National Lab.
• LIP Coimbra
• Northwestern University
• Pennsylvania State University
• Royal Holloway University of London
• SLAC National Accelerator Lab.
• South Dakota School of Mines & Tech
• South Dakota Science & Technology Authority
• STFC Rutherford Appleton Lab.
• Texas A&M University
• University of Albany, SUNY
• University of Alabama
• University of Bristol
• University College London
• University of California Berkeley
• University of California Davis
• University of California Los Angeles
• University of California Santa Barbara
• University of Liverpool
• University of Maryland
• University of Massachusetts, Amherst
• University of Michigan
• University of Oxford
• University of Rochester
• University of Sheffield
• University of Wisconsin, Madison

US UK Portugal Korea

Thanks to our sponsors and participating institutions!

U.S. Department of Energy Office of Science

https://lz.lbl.gov/

@lzdarkmatter

The LZ Experiment

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• Dual phase noble LXe TPC primarily searching for WIMPs via low-energy NRs

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• 494 3” PMTs viewing 7 tonnes of LXe/GXe
• S1: Primary scintillation
• S2: Proportional scintillation (light emitted by electrons extracted into gas phase)
• Excellent 3D position reconstruction → TPC volume fiducialisation
• Z from S1-S2 timing
• X-Y from light patterns in PMT array(s)

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• Size of S1, S2 allows for NR/ER discrimination (S2/S1)_ER >> (S2/S1)_NR

→ 99.5% ER background (β,𝛾) discriminations against signal (WIMPs)

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[LZ Technical Design Report, NIM 2019, 163047]

The LZ Experiment

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SNOLAB

• Located at the Sanford Underground Research Facility (SURF) in South Dakota (USA)

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• 1 mile of rock overburden to reduce muon flux contributing to experiment background

• Muon flux reduced by O(10^7)
• Radiogenic backgrounds become the dominant ones

LZ detector: nested detectors system

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Gd-loaded liquid scintillator

veto

LZ core: dual-phase Xe TPC

Instrumented with PMTs to reject neutron & 𝛄 backgrounds inside TPC

High purity water

LXe Skin region

OD

LZ core

LZ detector design [NIM 2019, 163047]

LZ veto: outer detector and skin region

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Purpose:

• Characterization, tagging and rejection of background (n,𝛄) events to enhance discovery potential
• Coupled to LXe self-shielding enable to increase fiducial volume & increase sensitivity.

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Without veto (3.2t fid.vol) With veto (5.6t fid.vol)

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Distributions of single-scatter nuclear recoils in 40Gev WIMP ROI (6-30 keV)

Expected bckg NR: 10.31 cts/1000 days 1.03 cts/1000 days

LZ Projected WIMP Sensitivity

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LZ has been designed to reach unprecedented sensitivity in (heavy) Dark Matter. It is one of best experiments with strong chance of discovery in the near future (DOE flagship experiment)

big dip!

Projected sensitivity unprecedented (1.4 x10 cm @40GeV!)

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[LZ Projected WIMP sensitivity for 1000 live days, 5.6 tonnes FV

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LZ Projected WIMP Sensitivity

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[LZ Projected WIMP sensitivity for 1000 live days, 5.6 tonnes FV

However, this unprecedented sensitivity could only be achieved if backgrounds under control, through strong background mitigation & discrimation at all stages of the experiment ⇒…

big dip!

Projected sensitivity unprecedented (1.4 x10 cm @40GeV!)

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Background Origins and Mitigations

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Xenon Contaminants

• ^222/220Rn,
• ^natKr,
• ^natAr

Physics

ER

NR

72%

23%

4%

49%

38%

7%

6%

Total (expected): 1131 ER 1.03 NR

After cuts: 5.66 ER 0.52 NR

[LZ Projected WIMP sensitivity for 1000 live days, 5.6 tonnes FV, PRD.101.052002]

Background control during construction

• First elaborated radio-contaminants control program in the Dark Matter field!
• Surface contamination below requirements
• Dust: <500 ng/cm → <1g of dust on all LXe wetted surfaces
• Rn/ Pb plate-out <0.5 mBq/m on TPC walls
• Eur. Phys. J. C 80, 1044 (2020) Corresponding authors: A. Kamaha, U. Utku

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• ~1200 screening and Rn emanation assays of detector materials during 5 years
• Detector assembly in ISO-6 Rn reduced cleanroom under strict background control protocols (~100 protocols) to mitigate against dust & Rn surface contamination

Leads to (𝞪,n)

Wall bckg

Naked 𝜷, ER bckg

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Long lived

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LZ Calibration systems

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photo-neutrons

Commercial rod sources (CSD)

Internal sources

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Four deployment systems / sources categories:

To calibrate the TPC, the skin & the OD volumes

• TPC :
• NR & ER bands
• Position reconstruction, TPC map corrections
• Trigger efficiency
• Energy calibration
• Skin & OD
• energy & threshold
• Intra-detectors timing

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(Introduced into Xe circulation

and carried into TPC)

DD generator

(Conduit thru side of water tank)

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1

4

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Different calibration systems to deploy various sources in size, shape & energy for a thorough detector calibration for further bckg mitigation

Highlights of LZ detector construction

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LZ TPC Construction Highlights

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Top & Bottom PMT array assembly & cabling

LZ Construction Highlights

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TPC detector construction in ISO-6 cleanroom

LZ Construction Highlights

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TPC detector integration into inner cryostat vessel for underground transportation

LZ Construction Highlights

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Major milestone!

TPC transportation to the underground laboratory

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LZ Construction Highlights

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Underground integration within water tank completed.

Commissioning ongoing, first results this year …

Summary

• Dark Matter exists and is being actively searched by different experiments around the world
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• LZ, the US flagship experiment, is one of the best experiments designed to probe a new WIMP Dark Matter parameter space
• Projected sensitivity unprecedented (1.4x10 cm @ 40GeV!) for 1000 live days

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• A lot of effort invested into backgrounds control and thorough detector calibration to reach the targeted sensitivity
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• Currently being operated, first physics results expected this year.

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Backup slides

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Benefit of LXe used in LZ

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Large signal rate

Event rate

(1 cts/keVee/kg/day)

• High density (2.9 g/cm³)

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• High atomic mass (A=131g/mol)

⇒ Large signal rate

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• 50% odd isotopes in natural Xenon ⇒ spin-dependent sensitivity as well

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• Self-shielding

⇒ Ultra-low background inner region (using 3D position recons)

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• No long-lived radioactive Xe isotopes

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• Easily scalable to large size

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• Large light output and fast response

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• Long electron drift lengths (~1 m)

Self shielding

direct detection discovery vs non-discovery

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Standard DM plots in cross section vs WIMP mass

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• Contour indicate a discovery

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• Parabola indicates a non-discovery.
• In which case a limit on the WIMP-nucleon

scattering cross

Section is plotted for the various masses that were probed

© https://www.arxiv-vanity.com/papers/1509.08767/

Dust fall-out onto surfaces

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• Witness coupons & tape lifts used used to measure fall-out
• 2 models developed to estimate fall-out
• modified SNO model & ASML model
• Focus on modified SNO model (originally developed by Hallman & Stokstad, 1991)

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• Both agree enabling accurate tracking & mitigation of dust deposition on TPC surfaces
• UV light used throughout for confirmation

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witness coupons image under microscope showing dust particulates of different sizes

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Witness coupon

Ambient Rn plate-out onto surfaces

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Jacobi model

• Plate-out estimated using Jacobi model validated/calibrated for this purpose

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• Plate-out rate mostly depends

on clean room parameters & exposure time of surfaces to ambient air

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