Directional Dark Matter Search with Nuclear Emulsion

1. Directional Dark Matter Search with Nuclear Emulsion Giuliana Galati, PhD. INFN Naples EPS-HEP Conference 2019 July 10-17, Ghent, Belgium

2. Introduction Nuclear Emulsion for WIMP Search with directional measurement 70 physicists 14 institutes ITALY INFN e Univ. Bari, LNGS, INFN e Univ. Napoli, INFN e Univ. Roma GSSI Institute JAPAN Chiba, Nagoya SOUTH KOREA Gyeongsang RUSSIA LPI RAS Moscow, JINR Dubna SINP MSU Moscow, INR Moscow Yandex School of Data Analysis TURKEY METU Ankara Website: news-dm.lngs.infn.it Letter of intent: https://arxiv.org/pdf/1604.04199.pdf

3. Impinging direction of DM particle is (preferentially) opposite to the velocity of the Sun in the Galaxy, i. e. from Cygnus Constellation Unambiguous proof of the galactic origin of Dark Matter Unique possibility to overcome the “neutrino floor”, where coherent neutrino scattering creates an irreducible background • Introduction POWER OF DIRECTIONALITY

4. Introduction Current approach: low pressure gas detectors Recoil track length O(mm) Small achievable detector mass due to the low gas density DIRECTIONAL APPROACH • Use solid target: • Large detector mass • Smaller recoil track length O(100 nm) • Very high resolution tracking detector Solution: Nuclear Emulsion based detector acting both as target and tracking device

5. Introduction Fixedpointing: target mounted on equatorial telescope constantly pointing to the Cygnus Constellation Backgroundreduction: neutron shied surrounding the target Location: Gran Sasso Underground Laboratory NEWSdm PRINCIPLE • Aim: detect the direction of nuclear recoils produced in WIMP interactions • Target: nanometric nuclear emulsions acting both as target and tracking detector

6. Introduction Nuclear emulsions: AgBr crystals in organic gelatine Passage of charged particle produce latent image Chemical treatment make Ag grains visible New kind of emulsion for DM search: Smaller crystal size NIT: NANO EMULSION IMAGING TRACKERS 100 keV Carbon A long history, from the discovery of the Pion (1947) to the discovery of νμ→ ντ oscillation in appearance mode (OPERA, 2015) 44 nm 25 nm 44 nm 500 nm

7. Introduction AgBr-I: sensitive elements Organic gelatine: retaining structure PVA to stabilise the crystal growth NIT EMULSIONS N C O Br Track length (micron) Ag Recoil energy (MeV) heavy nuclei Each nucleus gives different contribution to the overall sensitivity light nuclei Lighter nuclei ⇒ longer range at same recoil energy ⇒ Sensitivity to low WIMP mass 7

8. READOUT TECHNOLOGY READOUT TECHNOLOGY

9. Fast and completely automated optical microscopes Challenge: detect tracks with lengths comparable/shorter than optical resolution Baseline strategy: two-steps approach • READOUT TECHNOLOGY TRACK IDENTIFICATION CANDIDATE IDENTIFICATION Pros: Fast scanning profiting of the improvements driven by the OPERA experiment, dedicated measurement stations in each lab Limit: Resolution with standard technologies ~200 nm • CANDIDATE VALIDATION • Resonant light scattering • Pros: Super resolution ~6 nm

10. Scanning with optical microscope and shape recognition analysis Signal: clusters with elliptical shape: major axis along track direction Background: spherical clusters Automatic selection of candidate signals by optical microscopy Resolution 200 nm (one order of magnitude better than the OPERA scanning system), scanning speed 20 cm2/h READOUT TECHNOLOGY Direction detected! Step 1: Candidate Identification θ Test using 400 keV Kr ions Nucl.Instrum.Meth. A680 (2012) 12-17  ANGULAR RESOLUTION: σ2 = σ2intrinsic+σ2scattering σ = 360 mrad

11. Occurring when the light is scattering off a nanometric metallic (silver) grains dispersed in a dielectric medium (Applied Phys Letters 80 (2002) 1826) Sensitive to the shape of nanometric grains: when silver grains are not spherical, the resonant response depends on the polarization of the incident light. Each grain is emphasized at different polarization values polarization direction polarization direction READOUT TECHNOLOGY Step 2: Resonant Light Scattering Scattering intensity Scattering intensity polarization direction of incident light polarization direction of incident light

12. Taking multiple measurements over the whole polarization range produces a displacement of the barycenter of the cluster Measure the displacement of cluster barycentre as a function of polarization angle (dx, dy) READOUT TECHNOLOGY SUPER RESOLUTION ANALYSIS Signal-like events (100keV C ion) Background grain Max barshift Barycenter of the cluster Polarizationangle No barycenter shift

13. Barycenter displacement > 20 nm (Displaced) • Barycenter displacement ≤ 20 nm (Non-displaced) READOUT TECHNOLOGY Results for 100keV C-ions x-axis Horizontal ions, signal-like events 90 nm Measurement of track slope and length beyond optical resolution

14. Unprecedented accuracy of 6 nm achieved on both coordinates READOUT TECHNOLOGY Position accuracy Results for 10keV C-ions Background-like events Breakthrough!!! 14

15. BACKGROUND STUDIES BACKGROUND STUDIES

16. BACKGROUND STUDIES EXTERNAL SOURCES • Environmental photons • Environmental neutrons • Cosmogenic neutrons BACKGROUND SOURCES Negligible with appropriate shield ⇒ Full Geant4 simulation in progress • NIT emulsions largely insensitive to electrons • Additional level arms under investigation: • Dedicated chemical treatments • Low temperatures • Polarized light scattering • Use of synthetic polymers • INTRINSIC SOURCES • Radioactivity from 14C • Intrinsic neutrons Detectable background yield from the intrinsic radioactive contamination of NIT: ~0.06 n/kg year NEWSdm Collaboration Astroparticle Physics 80 (2016) 16

17. BACKGROUND STUDIES TELESCOPE AND SHIELD PRELIMINARY DESIGN

18. TEST TEST IN PROGRESS

19. Aim: measure the detectable background from environmental and intrinsic sources and validate estimates from simulations Confirmation of a negligible background will pave the way for the construction of a pilot experiment with an exposure on the 10 kg year scale Pilot experiment will act as a demonstrator to further extend the mass range TEST FIRST TECHNICAL TEST: 10g • Experimental setup: • shield from environmental background • cooling system to ensure required temperature to NIT emulsions • Polyethylene slabs 40 cm-thick to absorb environmental and cosmogenic neutrons • Lead bricks 10 cm-thick to absorb environmental photons

20. Installed in Underground Gran Sasso INFN Laboratories in June 2019 TEST FIRST TECHNICAL TEST: 10g

21. TEST FIRST TECHNICAL TEST: 10g Cooling System NIT POLYETHYLENE/LEAD SHIELD Cryostat

22. NEWSdm: novel approach for directional Dark Matter searches • Fine-grained nuclear emulsion used as target and tracking system • Breakthrough in readout technologies to go beyond optical resolution • 10 kg scale (pilot) experiment as a demonstrator of the technology • Aim: large mass scale detector to go beyond “neutrino floor” • Status: • Letter of Intent submitted to LNGSC in 2015 • 10g technical test in progress • Conceptual Design Study to be submitted by September 2019 CONCLUSIONS

23. BACKUP SLIDES

24. Given the carbon content in the emulsion and the 14C activity, β-rays amount to ~108 per kg⋅year NIT emulsions largely insensitive to electrons Additional level arms under investigation: BACKGROUND STUDIES RADIOACTIVITY FROM 14C NIM A 845 (2017) 373 • Dedicated chemical treatments • Reduce sensitivity to electrons at low temperatures • Exploit the response of electrons to the polarized light scattering • Use of a color camera to distinguish nuclear recoils from electrons • Replace the gelatine with synthetic polymers

25. Target mass = 10 kg Exposure time = 1 year Minimum detectable track length from 200 nm to 50 nm Zero background hypothesis BACKGROUND STUDIES 10 kg PILOT EXPERIMENT • Directionality discrimination of the signal not included