The GAPS experiment a search for cosmic-ray antinuclei from dark - PowerPoint PPT Presentation

The GAPS experiment – a search for cosmic-ray antinuclei from dark matter M. Kozai (ISAS/JAXA) on behalf of the GAPS collaboration 1

The GAPS Experiment  International collaboration between US, Japanese, and Italian institutes 2

 General Anti-Particle Spectrometer ➢ High-sensitivity survey for GAPS Payload low-energy antinuclei in cosmic-rays  Balloon experiment ➢ Instrument size : ~4 m x 4 m x 4 m ➢ Payload mass : 3.5 t  NASA long duration balloon (LDB) flight from Antarctica ➢ Suitable for observation of low-rigidity particles ➢ ~35 days/flight ➢ Nominal 3 flights are proposed ➢ The first flight is scheduled in late 2021 NASA 3

Scientific motivation Antiproton spectra observed and predicted (GAPS) ➢ Antiproton ✓ Below ~0.3 GeV/n is unexplored or has low statistics ✓ Helpful constraints on cosmic-ray propagation/source ➢ Antideuteron (our primary target) Background-free signal from dark matter in galactic halo if dark matter is WIMPs (weakly interactive massive particles) GAPS ➢ Antihelium < 0.25 GeV ✓ “Candidate” events by AMS -02 ✓ Complementary search by GAPS T. Aramaki+, 2014, Astropart. Phys. 59, 12-17 4

Antideuteron ( ഥ 𝐄 ) as a dark matter signal Predicted ഥ ➢ Background ഥ 𝐄 fluxes 𝐄 (known physics) ✓ Cosmic-ray collision with interstellar medium ✓ Kinematics & cosmic-ray soft spectrum → Suppressed below several GeV ➢ Signal ഥ 𝐄 (beyond-the-standard-model physics) ✓ WIMP annihilation/decay at rest ✓ Soft spectrum peaking <1 GeV for O (10) GeV WIMPs → 10 3 - 10 4 signal/background ratio ✓ GAPS will provide the highest sensitivity ✓ GAPS is also sensitive to O (100) GeV ~ O (1) TeV WIMPs T. Aramaki+, 2016 Astropart. Phys. 74, 6-13 in certain models 5

GAPS detection concept Novel detection concept utilizing exotic atom Time-of-Flight system (plastic scintillator paddles) Silicon detector array 1. Time-of-Flight system triggers event and measures β , incident angle and d E /d x 2. Silicon detector array tracks particle trajectory and measures d E /d x 3. Cosmic-ray antinucleus is slowed and captured by silicon detector array, 4. and forms an excited exotic atom with silicon atom 5. Deexcitation of exotic atom i. Characteristic X-rays are emitted by ladder deexcitation ii. Pions and protons are emitted by nucleus annihilation → Identifying cosmic-ray antinucleus 6

Lithium drifted silicon (Si(Li)) detector cf. M. Kozai+, 2019, NIM A accepted, arXiv:1906.05577 F. Rogers+, 2019, JINST accepted, arXiv:1906.00054 / K. Perez+, 2018, NIM A 905, 12-21  Silicon detector is optimal for exotic-atom technique ➢ Low atomic number (14) of Si → Appropriate energy distribution of characteristic X-rays ➢ Sufficient energy resolution to distinguish characteristic X-ray energies  Thick sensitive layer can be obtained with relatively low cost → Suitable for GAPS silicon detector array which functions as a particle degrader Goal design ➢ Energy resolution <4 keV FWHM for 60 keV X-rays at ~-40 ℃ ➢ Thickness 2.5 mm (sensitive layer >90%), Diameter 10 cm, 8 readout strips ➢ Mass production of >1000 detectors 7

➢ Uniform Li drift into large-area (~10cm) wafer ✓ Si ingot optimized for Si(Li) detector ✓ Li evaporation and drift methods copper-stained cross section Li-drifted region ➢ Leakage current suppression Energy spectrum with X-ray sources at -41C ✓ ~0.5 nA/cm 2 at -35 ℃ achieved ➢ Below 4 keV FWHM for 60 keV at ~-40 ℃ X-ray achieved ➢ Collaboration with Shimadzu corp. and SUMCO corp. ➢ Detailed design and fabrication process fixed ➢ Mass production of >1000 detectors started from Jan., 2019 ➢ ~70 Detectors/month, Yield ~90% 8

Silicon detector Integration 4 detectors/module Polyethylene foam 36 modules Tracker : 10 layers Front-End Electronics • Custom ASIC • Wide dynamic range for both X-rays and particles (10 keV – 100 MeV) cf. V. Scotti+, 2019 ICRC ➢ Test modules and test boards have been fabricated ➢ Integration/performance tests are under way 9

Silicon detector cooling  Our challenge: Cooling silicon detector using a passive cooling system ➢ Unprecedent challenge in balloon experiment ➢ High albedo in Antarctic flight makes it more difficult  New heat pipe system for GAPS ➢ Transferring heat input from preamp to radiator ➢ 2-phase coolant fluid → Large heat transfer and uniform temperature distribution ➢ Drivers of coolant : Gravity and pressure difference → Basically passive and low-power system ➢ Developments ✓ Study of self-excited flow ✓ Some technical elements 2-phase coolant, Reservoir, Check-valve, Auxiliary pump and heater ➢ Tests with the engineering model (EM) in thermal chamber 10

 Balloon experiment (piggy back) to demonstrate the utmost cooling (<-55 ℃ ) of radiator Ft. Sumner, NM GAPS Radiator tests 2018 & 2019 Scaled radiator Thermal analysis of the entire payload 11

Time-of-Flight system ➢ ~200 scintillator paddles (T6.5 mm x W16 cm x L1.2~1.8 m) ➢ Both-sides-readout → position information in each paddle ➢ Requirements ✓ Timing resolution <500 ps ✓ Charge reconstruction <0.25e ➢ 6 SiPMs on each side of paddle ✓ Custom preamplifier with built-in SiPMs ✓ Custom DAQ board with DRS4 ASIC ➢ Performance test with prototype system is under way Achieved 340 ps timing resolution and dynamic range ➢ Construction of scintillator paddles already started 12 S. Quinn+, 2019 ICRC

Simulation study ഥ 𝐄 sensitivity estimation : 2 x 10 -6 (m 2 s sr GeV/n) -1 for 105 days flight (T. Aramaki et al., 2016 Astropart. Phys. 74, 6-13) G4 model ➢ More precise implementation of simulation code ➢ Improvement of analysis algorithm ➢ Geant 4 model ✓ Implementing full payload designs ✓ Validation of physical processes Trajectory reconstruction ➢ Particle detection/identification scheme R. Munini+, 2019 ICRC ✓ Trajectory reconstruction algorithm ✓ Likelihood analysis among beta, dE/dx, stopping range, etc. ✓ Neural network techniques 13

Summary  Low-energy cosmic-ray antideuteron provides a clean channel for dark matter search.  GAPS experiment ➢ Survey for unexplored low-energy region of antinuclei spectra with high sensitivity ➢ Search for antideuterons as a smoking gun signature of WIMP DM.  Key techniques for the novel detection concept utilizing the exotic atom had been developed and validated.  Full scale system testing will be demonstrated in early 2020. ➢ Silicon detectors integrated into detector modules ➢ Time-of-Flight paddles with SiPMs ➢ DAQ/PS system  The first balloon flight from Antarctica is scheduled in late 2021. 14

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Antideuteron ( ഥ 𝐄 ) as a DM signal Cosmic-ray origin ഥ 𝐄 (background, known physics) 𝐪 ഥ Collision Cosmic-ray 𝐪 ഥ 𝐨 ഥ (p, He, ...) 𝐨 ഥ ഥ 𝐄 Coalescence Interstellar medium (H, He, ...) p, n, γ, ν, ... ➢ Cosmic-ray energy threshold for the particle productions ➢ Steeply falling cosmic-ray energy spectrum Suppressed ഥ D flux below several GeV Dark matter origin ഥ 𝐄 (signal, exotic physics) 𝐪 ഥ DM annihilation/decay 𝐪 ഥ 𝐨 ഥ ഥ 𝐨 ഥ 𝐄 Coalescence p, n, γ, ν, ... 16

GAPS sensitivity as a function of DM mass • Accessible cross section of WIMP annihilation into u-ubar • Sensitivity reaches thermal relic cross section for DM mass below 100 GeV in the MAX propagation model (favored by AMS-02 antiproton; Brauninger, 2009) dbar is also detectable for heavy DM (0.5-20 TeV) In MAX propagation and certain DM models such as: • Enhanced cross section motivated by AMS-02 positron and antiproton data • Pure-Wino DM 17

Exotic atom 1. Target material degrades incoming antinucleus. 2. Antinucleus is captured by atomic orbit of the target nucleus material. 3. Generation of the exited exotic atom non-electron particle (antiproton, antideuteron…) Decay of the exited exotic atom Auger transition Energy level Ladder deexcitations → characteristic X-rays Annihilation with nucleus → multiple π/p emissions Characteristic X-rays and π/p emissions → Detection and identification of incoming antinucleus 18

Predicted ഥ 𝐄 spectrum ➢ Background ഥ 𝐄 (Ibarra&Wild, 2013) ➢ Signal ഥ 𝐄 from several 10 GeV WIMP models (Donato+ 2008, Dal&Raklev 2014, Baer+ 2005) ✓ SUSY model • Annihilation of 30 GeV neutralino • Decay of 50 GeV gravitino ✓ Extra-dimensional GUT • Annihilation of 40 GeV Kaluza-Klein neutrino (LZP) ✓ Shaded regions : Galactic propagation uncertainties ✓ Boost factor for DM annihilation Effect of density clump Set minimum = 1 → Conservative estimation 19

➢ Uniform Li drift into large-area (~10cm) wafer ✓ Si ingot optimized for Si(Li) detector ✓ Li evaporation and drift apparatuses/methods copper-stained cross section Li-drifted region Energy spectrum with X-ray sources at -41C ➢ Leakage current suppression ✓ Guard-ring structure ✓ Surface treatment ✓ New contact structure ~0.5 nA/cm 2 at -35 ℃ achieved ➢ Collaboration with Shimadzu corp. and SUMCO corp. <4 keV FWHM for 60 keV X-ray achieved ➢ Detailed design and fabrication process fixed ➢ Mass production started from Jan., 2019 ➢ ~70 Detectors/month, Yield ~90% 20