XVII Mexican Workshop on Particles and Fields 21 – 25 Nov 2019 León, Guanajuato Recent results of WIMP dark matter quest with the DEAP-3600 experiment Ariel Zuñiga Reyes* on behalf of DEAP-3600 collaboration *Astrophysics Ph.D. student (arzure89@gmail.com) Institute of Physics & Institute of Astronomy (UNAM)
• Dark Matter (DM) direct detection • DEAP detector main features • Pulse-Shape Discrimination approach in liquid argon Presentation outline • Backgrounds • WIMP region-of-interest (ROI) and sensitivity • Final remarks
Obligatory DM intro slide Planck 2015 • Rotation curves of galaxies and of galaxy clusters cannot be explained by ordinary matter only. • Gravitational lensing of background galaxies by clusters requires a significant dark matter component. • 1E0657-56, aka the Bullet Cluster, shows separation of ordinary matter (gas) from dark matter. • Cosmic microwave background radiation shows gravitational wells created by dark- matter clustering. The Bullet Cluster: Mass discrepancies in galaxy clusters Cosmic Microwave Background Flat rotation curves of galaxies Separation of gas from the bulk when use gravitational lensing of gravitating mass Stars: optical Foreground cluster CL0024+1654 produces Gas: X-rays 3 multiple images of a blue background galaxy. Total mass: gravitational lensing
Particle candidates for DM Most experiments optimized to search for WIMPs How to detect WIMP? Direct detection • nuclear recoils from elastic scattering • dependence on A, J; annual modulation, directionality. • astrophysical uncertainties (local density and v-distribution) 4
Dark Matter direct detection Dark matter particles from the Galactic halo that pass through the Earth will occasionally scatter off nuclei. The resulting recoil energy of the nucleus can be measured in dedicated low background detectors. γ -rays heat Observable: recoil energy DM particle of the nucleus e- 2 𝑤 2 1 − cos 𝜄 𝐹 𝑠 = |𝑟| 2 = 𝑛 𝑠 < 100 𝑙𝑓𝑊 2𝑛 𝑂 𝑛 𝑂 Angle not measurable so for a given DM speed 5
Expected rate of events Typical event rates are < 1 event/kg/year. A great experimental challenge! • 𝑶 𝑼 : number of target nuclei in the experiment. • 𝒏 𝝍 : DM mass. Minimum velocity Detector physics • v = | v | : DM speed in the reference frame of the experiment. • 𝒘 𝒏𝒋𝒐 : minimum DM speed that can induce a recoil of. Particle/nuclear physics 𝐹 𝑢ℎ𝑠 𝑛 𝑂 • 𝝉 𝑼 : DM – nucleus scattering cross section. 𝑤 𝑛𝑗𝑜 = • 𝑔 𝒘 + 𝒘 𝑭 𝑢 : DM velocity distribution boosted to Earth’s frame. 2 2 𝑛 𝑠 Astrophysics • 𝝇 𝟏 : average local DM density. Differential cross-section Spin-dependent (SD) cross-section at zero Spin-independent (SI) cross-section at zero momentum transfer momentum transfer 6
The paper See arXiv:1902.04048 (Published: PRD) 7
DEAP underground Facility located 2 km deep underground at SNOLAB, Canada looking for direct detection of Weakly Interacting Massive Particles (WIMP). 8
Dark Matter Experiment using Argon Pulse-shape Discrimination (DEAP) Main features: • 3.3 tonnes single phase liquid argon (LAr) target in a 5 cm thick acrylic vessel. • 3 μm thick TPB (tetraphenyl butadiene) layer shifts 128 nm LAr scintillation to visible. • 45 cm long acrylic light guides transport light to 255 Hamamatsu R5912 HQE 8" PMTs, provide thermal insulation and shielding. • Foam filler blocks between light guides provide further insulation and shielding. • The whole setup is immersed in a water tank which serves as a passive shield for outside γ -rays and neutrons. • 48 outward looking PMTs work as a muon Cherenkov veto. 9
Why liquid argon? Argon is a noble chemical element that, at atmospheric pressure and a cryogenic temperature of -186 ° C (87 K), becomes liquid. Liquid argon is suitable for very large targets due to: - readily available (1% of atmosphere) - Easy to purify and high light yield - Much lower cost compared to xenon - Transparent to its own scintillation light … but there is Ar39: β decays around 1 Bq/kg in natural argon 10
Argon Pulse-Shape Discrimination approach Argon singlet and triplet excited states have well separated lifetimes Nuclear recoil (NR) Nuclear recoils excite predominantly the singlet state signal events have more prompt light! Electron recoil (ER) fprompt fprompt 11
Pulse-Shape Discrimination in DEAP Surface α decays γ -rays β decays 12
Background: Overview 13
Background (Muons & Neutrons): Cosmogenic & Radiogenic Neutrons can be produced by nuclear reactions in We measured a muon flux of (3-4) × 10 −10 μ/cm2/s. detector components - (α,n) reaction 6 km.w.e. overburden drastically reduces muon flux; - Spontaneous fission water Cherenkov muon veto tags remaining cosmogenic backgrounds. They may then scatter on argon nuclei and produce a WIMP-like signal. PMT Borosilicate Glass (dominant neutron source) Mitigated with careful material selection, passive shielding, and fiducial cuts 14
Background ( α particles): LAr bulk Distribution of mapped α particle energies from Fprompt and PE for short- lived α -decays in LAr. Lowest Rn contamination of any noble liquid dark matter experiment! 15
Background ( β and γ -rays): Electronic recoils in LAr Probability of an electron recoil being detected above a given Fprompt value in the WIMP-search ROI. Vertical lines show the values above which 90% or 50% of nuclear recoils are expected to be found. Strongest demonstration of PSD to date, due to low electronics noise and AP removal Leakage probabilities at energy threshold: +1.3 )×10^-7 • At 90% NR accept.: ( 2.8 −0.6 +0.7 )×10^-9 • At 50% NR accept.: ( 1.2 −0.3 16
After all cuts, a background expectation < 1 event was achieved WIMP acceptance* as a function of PE At a fiducial mass of 824 kg, average WIMP efficiency is 35.4%. Expected *Given a number of events that enter the fiducial backgrounds volume, the fraction of events that survive all physics cuts is taken as the acceptance for WIMP-like recoils . 17
Applying cuts to data, no WIMP candidate events are seen 231 live days after run selection and deadtime corrections 824 kg fiducial mass 0 events in ROI 18
WIMP Sensitivity This analysis excludes spin-independent WIMP-nucleon cross-sections above 3.9× 10 −45 cm² @ 100 GeV WIMP mass (90% C.L.). This is a leading result for argon detectors and complementary to the results obtained from Xe-based experiments. 19
Final remarks • DEAP-3600 updated results to excludes spin-independent WIMP-nucleon cross-sections above 3.9× 10 −45 cm² @ 100 GeV WIMP mass (90% C.L.). • Most powerful demonstration of pulse-shape discrimination (PSD) between electronic recoils (ERs) and nuclear recoils (NRs). • Lowest reported Rn222 (Rn220) backgrounds in LAr: 0.15 μ Bq/kg (4 nBq/kg). Future plans • Collecting blinded data since January 2018 - 20% left non-blind - Plan to keep collecting data at least until end of 2020 • Currently lose sensitivity to harsh cuts needed to mitigate α -decays on neck flow guide surfaces - Developing multivariate analysis techniques to more efficiently reject them (DEAP learning?) • Developing new calibration sources (83Kr, 37Ar) to improve detector response model 20
After DEAP Joining the Global Argon Dark Matter Collaboration DarkSide-20k. Competitive with LZ, start operation at LNGS in 2021. Target exposure: 100 tonne·year. Future multi-hundred tonne (300) LAr detector, Argo. It will reach down the neutrino floor. Target exposure: 1 ktonne·year. Complimentary to xenon, the only other target allowing such large exposure. 21
THANKS FOR YOUR ATTENTION! 22
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