Latest results from the DEAP-3600 dark matter search at SNOLAB On - PowerPoint PPT Presentation

Latest results from the DEAP-3600 dark matter search at SNOLAB On behalf of the DEAP-3600 collaboration DMUK Meeting, Kings College London, 11th April 2019, R. Ajaj et al. Ashlea Kemp, https://arxiv.org/pdf/1902.04048.pdf Royal Holloway, University of London. � 1

Outline • The DEAP-3600 experiment • Detector design • Liquid Argon scintillation • SNOLAB • The story so far • Updated analysis • Backgrounds • Cuts & Region of Interest (ROI) • New WIMP limit • Conclusions & Outlook � 2

The DEAP-3600 detector • Dark matter Experiment using Argon Pulse-shape discrimination, • Single-phase Liquid Argon (LAr) scintillation light detector, holding 3279 kg of target LAr, • Optimised for collection of scintillation light emitted from recoiling Ar nuclei after interaction with dark matter particle (WIMP), ➡ 128 nm VUV scintillation photons ( ɣ ) wavelength shifted to 420 nm by TPB layer for PMT detection (~75% coverage). Ar χ � 3

The DEAP-3600 detector • Dark matter Experiment using Argon Pulse-shape discrimination, • Single-phase Liquid Argon (LAr) scintillation light detector, holding 3279 kg of target LAr, • Optimised for collection of scintillation light emitted from recoiling Ar nuclei after interaction with dark matter particle (WIMP), ➡ 128 nm VUV scintillation photons ( ɣ ) wavelength shifted to 420 nm by TPB layer for PMT detection (~75% coverage). Ar χ � 4

The DEAP-3600 detector • Dark matter Experiment using Argon Pulse-shape discrimination, • Single-phase Liquid Argon (LAr) scintillation light detector, holding 3279 kg of target LAr, • Optimised for collection of scintillation light emitted from recoiling Ar nuclei after interaction with dark matter particle (WIMP), ➡ 128 nm VUV scintillation photons ( ɣ ) wavelength shifted to 420 nm by TPB layer for PMT detection (~75% coverage). χ γ γ Ar γ γ γ γ γ γ γ γ γ � 5

Liquid Argon scintillation • Ionising particles traversing LAr produce excited Ar atoms (Ar*) and Ar ions (Ar+), • Ar*, Ar+ hit other Ar atoms to form excimers, which decay by emitting VUV scintillation photons, • Ionising density depends on ionising particle - higher for nuclear recoils than electron recoils, • Excimers produced in either triplet or singlet state, • Well separated lifetimes between triplet (~1300ns) and singlet (~6ns), ➡ Pulse-shape discrimination (PSD). Prompt light Prompt + Late light = PSD http://darkmatter.ethz.ch/ Published PSD paper from DEAP-1: Astroparticle Physics 85 (2016) 1-23. � 6

SNOLAB • One of the deapest & cleanest laboratories in the world, • Located 2km underground in an active nickel mine, A. Ianni, TAUP 2017 https://phys.org/news/2018-05-world-sensitive-dark.html • Being based at SNOLAB provides excellent shielding from cosmic rays, • Muons directly interacting with nuclei in rock can produce neutrons… ➡ Neutrons mimic WIMPs! • Muon flux reduced by factor of ~ 10 7 . � 7

Outline • The DEAP-3600 experiment • Detector design • Liquid Argon scintillation • SNOLAB • The story so far • Updated analysis • Backgrounds • Cuts & Region of Interest (ROI) • New WIMP limit • Conclusions & Outlook � 8

Epoch of DEAP-3600 2006 Nov 2016 - Dec Aug 2016 Jan 2018 - Present May 2016 2017 t Initial design First injection with First physics data Collect further physics data Continue to collect with blinding scheme (20% phase begins purified argon collected* physics data (open)** open) * Data used for first dark matter search: Phys. Rev. Lett. 121, 071801 (2018), “Design and Construction of the DEAP-3600 Dark ** Data used for second dark matter search: https:// Matter Detector”: Astropart. Phys. 108 (2019) 1-23. � 9 arxiv.org/pdf/1902.04048.pdf (2019),

DEAP-3600 Collaboration 80+ researchers � 10

Outline • The DEAP-3600 experiment • Detector design • Liquid Argon scintillation • SNOLAB • The story so far • Updated analysis • Backgrounds • Cuts & Region of Interest (ROI) • New WIMP limit • Conclusions & Outlook � 11

Pulse-shape discrimination • Main advantage of using LAr as detection medium, • Scintillation photons from recoils occurring in LAr arrive at the PMTs over a range of time, ➡ More photons emitted, hence detected, later from electronic recoils ERs ( β / ɣ ) compared to nuclear recoils NRs (neutrons, WIMPs), • Define PSD variable called Fprompt , ∑ 60 ns t = − 28 ns PE( t ) Fprompt = ∑ 10 μ s t = − 28 ns PE( t ) • Reject ERs with up to 10 10 power. ER + NR pile- Data from AmBe (neutron emitter) run. � 12

Pulse-shape discrimination • Empirical function to describe Fprompt distribution for ERs: ➡ For ER-type event for which q PE reconstructed, the probability of observing given Fprompt value, f , is described by convolution of Gamma function with the Gaussian smearing term. F ER ( f , q ) = Γ ( f ; ¯ f , b ) * Gauss( f ; σ ) Mean Fprompt, ¯ f ( q ) Width of Gaussian response, σ ( q ) Shape parameter, b ( q ) • For Fprompt < 0.25, trigger efficiency < 100%, ➡ Trigger efficiency based on prompt PE, ➡ “Lose” events, data and model diverge. � 13

Pulse-shape discrimination • Latest result demonstrates best achieved PSD between ERs and NRs in LAr, • Efficient removal of significant source of background events in detector from the dark matter WIMP search. • Vertical lines show Fprompt values above which we expect 90% (green) and 50% (purple) of NRs, • At 90% NR acceptance, PSD leakage probability of 2.8 +1.3-0.6 x 10 -7 at low energy threshold for WIMP search. In energy range of WIMP search region [15.6 - 32.8 keVee], average PSD leakage probability of 4.1 +2.1-1.0 x 10 -9 at 90% NR acceptance. ‘Leakage’ probability: probability of ER type event being detected above given Fprompt value at the low energy WIMP threshold (15.6 keVee), � 14

Position reconstruction • Fiducalisation can be used to reject surface backgrounds, ➡ Apply radial cut of 630 mm to remove Cherenkov, external neutron and surface ⍺ events that reconstruct with R > 630 mm, ➡ Position resolution of 35 mm for events near radius of 630 mm in WIMP PE search region, ➡ Leakage probability at 630 mm radial cut from surface ⍺ decays into ROI is ~ 10 -5 • Two position reconstruction algorithms used: 1. PE-based (spatial distribution of PMT hits), 2. PE + timing (charge and time information of early pulses used to determine position). • Validation of algorithms performed on Ar39 β decays in data distributed uniformly across LAr volume (see back- ‘Leakage’ probability of simulated ⍺ decays in WIMP PE range vs contained LAr mass up slides). as determined by events within given reconstructed radius. � 15

Defining the WIMP ROI WIMP ROI driven by signal and background models, 2D WIMP ROI drawn in PE, Fprompt, Designed such that background expectation < 1. � 16

Backgrounds in DEAP-3600 External: LAr: Ar39 β decays, ⍺ Cosmogenic- decays from Rn222/ induced neutrons Rn220, produced inside Acrylic Vessel (AV) water tank/ rock. surface: Po210 ⍺ decays. PMTs & other detector Neck: Po210 ⍺ decays components: Radiogenic neutrons, ɣ / β produced in through LAr ‘film’ on glass… surface of acrylic flowguides, originating ➡ Cherenkov light from long-lived Pb210 produced in light guide acrylic. (Rn222). � 17

Backgrounds in DEAP-3600 LAr: Ar39 β decays, ⍺ decays from Rn222/ Rn220, Acrylic Vessel (AV) surface: Po210 ⍺ decays. � 18

Background #1: Alphas AV surface Po210 ⍺ decays with degraded energies arise from long-lived Pb210 nuclei residual on AV surface. LAr ⍺ decays from Rn222/ Rn220 chain, Produce high energy events which reside outside of the WIMP PE search region. The three decay scenarios for surface ⍺ in DEAP-3600 (decays in the LAr, the TPB layer or the AV acrylic) Image taken from the PhD thesis of P . Giampa. � 19

Background #1: Alphas AV surface Po210 ⍺ decays with degraded energies arise from long-lived Pb210 nuclei residual on AV surface. LAr ⍺ decays from Rn222/ Rn220 chain, Produce high energy events which reside outside of the WIMP PE search region. Lowest bulk Rn contamination of any noble liquid experiment Rn222 LAr: (0.153 ± 0.005) µBq/kg Predict < 0.08 events in ROI from surface ⍺ � 20

Backgrounds in DEAP-3600 External: LAr: Ar39 β decays, ⍺ Cosmogenic- decays from Rn222/ induced neutrons Rn220, produced inside Acrylic Vessel (AV) water tank/ rock. surface: Po210 ⍺ decays. PMTs & other detector components: Radiogenic neutrons, ɣ / β produced in glass… ➡ Cherenkov light produced in light guide acrylic. � 21

Background(s) #2: Neutrons & Cherenkov Cosmogenic neutrons: Veto 0.1 s before a Muon Veto PMT is fired and Veto 1 s after a Muon Veto PMT is fired to mitigate cosmogenic neutron backgrounds. Radiogenic neutrons: Estimate radiogenic neutron background expectation using data-driven neutron capture coincidence tagging analysis, ➡ After scattering in LAr, neutrons thermalise and capture, to produce ɣ ray. Cherenkov: Physics data with U232 calibration source (2.6 MeV ɣ ’s) deployed collected, ➡ Develop cut flow to remove Cherenkov. Background expectations: Cosmogenic n: < 0.11 ROI events, Radiogenic n: 0.10 +0.10-0.09 ROI events, Cherenkov: < 0.14 ROI events. Courtesy of Andrew Erlandson. � 22