Light Dark Matter Search with Liquid Argon Masayuki Wada INFN Cagliari, Italy June 5 2019 Light Dark Matter Workshop at Fermilab � 1
� 2 FEATURES OF NOBLE LIQUID DETECTORS Dense and easy to purify (good scalability, advantage over solid targets) ▸ High scintillation & ionization (low energy threshold, not low enough to search < 1 GeV/c 2 DM) ▸ Transparent to own scintillation ▸ For TPC High electron mobility and low diffusion ▸ Amplification for ionization signal ▸ ▸ Discrimination electron/nuclear recoils (ER/NR) via ionization/scintillation ratio Liquid Argon Liquid Xenon ‣ lower temperature (Rn purification is ‣ Denser & Radio pure easier) ‣ Lower energy threshold ‣ Stronger ER discrimination ‣ Higher sensitivity at low mass WIMP ‣ Intrinsic ER BG from 39 Ar ‣ Need wavelength shifter
� 2 FEATURES OF NOBLE LIQUID DETECTORS Dense and easy to purify (good scalability, advantage over solid targets) ▸ High scintillation & ionization (low energy threshold, not low enough to search < 1 GeV/c 2 DM) ▸ Transparent to own scintillation ▸ For TPC High electron mobility and low diffusion ▸ Amplification for ionization signal ▸ ▸ Discrimination electron/nuclear recoils (ER/NR) via ionization/scintillation ratio Liquid Argon Liquid Xenon ‣ lower temperature (Rn purification is ‣ Denser & Radio pure easier) ‣ Lower energy threshold ‣ Stronger ER discrimination ‣ Higher sensitivity at low mass WIMP ‣ Intrinsic ER BG from 39 Ar ‣ Need wavelength shifter
SENSITIVE TO LOW DARK MATTER � 3 COMPARISON WITH XENON100 Phys. Rev. D 94, 092001 (2016) 2 10 day] All DS-50 XENON100 × r kg i e h t One S2 h t i w t r e Single Scatters v n o × C d l e i y Events / [keV n o 10 i t a z i n o i XENON100 1 FIG. 4. Energy distribution of the events remaining in the data set after all data selection cuts. As an example, the expected spectrum for a WIMP of 6 GeV =c 2 and a spin-independent WIMP-nucleon scattering cross section of 1 . 5 × 10 − 41 cm 2 is also shown. The corresponding nuclear recoil energy scale is indicated on the top axis. The charge yield model assumed 1 − 10 here has a cutoff at 0.7 keV, which truncates the WIMP spectrum. 0 0.5 1 1.5 2 2.5 3 3.5 ne NR energy E [keV] recoil XENON100 DarkSide-50 WIMP spectra in Xe and Ar [1/keV/kg/day] 0.5 BG [evt/keVnr/kg/d] 0.2 @ 1.1 keVnr 10 in [0.7, 1.7] keVnr 2 -41 2 M =3 GeV/c , σ =10 cm W 0.07 Xe BG [evt/keVnr/kg/d] 0.5 @ 6 keVnr 1 in [3.4, 9.1] keVnr R dR/dE Ar − 1 10 Analysis threshold 0.7 keVnr 0.6 keVnr Xe Ar 2 − 10 ▸ DS-50 has lower BG at the lowest Ne bins. − 3 10 ▸ Ar sees more events with given WIMP mass 4 − 10 and cross section. 0 0.5 1 1.5 2 2.5 3 E [keV] R
DARKSIDE-50 XY Reconstruction Introduction � 4 THE TIME-PROJECTION CHAMBER (TPC) Ar Ar n Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar e- Ar Ar Ar Ar e- Ar e- Ar Ar Electron Recoil (ER) Ar e- e- Ar x-y position acquired charge Ar Ar Ar 23 from S2 light fraction S1 S2 Ar Ar Ar ∫ (S1) ≪ ∫ (S2) Ar WIMP-like signal! Ar time E mult Ar Nuclear Recoil (NR) S2 e- Ar e- acquired charge S2 e- z ∝ t drift S1 e- Ar S1 E drift ∫ (S1) ≤ ∫ (S2) liquid argon (LAr) time S2/S1 ratio and Pulse Shape Discrimination (PSD) WIMPs will generate nuclear recoils (NRs) DM
DARKSIDE-50 XY Reconstruction Introduction � 4 THE TIME-PROJECTION CHAMBER (TPC) Ar Ar n Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar e- Ar Ar Ar Ar e- Ar e- Ar Ar Electron Recoil (ER) Ar e- e- Ar x-y position acquired charge Ar Ar Ar 23 from S2 light fraction S1 S2 Ar Ar Ar ∫ (S1) ≪ ∫ (S2) Ar Cannot See! WIMP-like signal! Ar time E mult Ar Nuclear Recoil (NR) S2 e- Ar e- acquired charge S2 e- z ∝ t drift S1 e- Ar S1 E drift ∫ (S1) ≤ ∫ (S2) liquid argon Cannot See! (LAr) time S2/S1 ratio and Pulse Shape Discrimination (PSD) WIMPs will generate nuclear recoils (NRs) DM
LOW N E EVENTS IN DARKSIDE-50 � 5 BACKGROUND Delayed electrons E [keV ] nr 1 2 3 4 5 6 7 8 9 10 11 12 131415 2 10 1 2 3 E [keV ] ee 3 10 Data -40 DM spectra =10 cm 2 σ day] χ 10 G4DS MC All 2 M =2.5 GeV/c χ Cryostat γ -rays 2 × 2 10 M =5.0 GeV/c kg χ PMTs -rays γ 2 1 39 85 M =10.0 GeV/c Ar + Kr χ × - 10 e Events / [N 1 − 10 1 2 − 10 10 3 − 10 0 5 10 15 20 25 30 35 40 45 50 N - e ▸ The events in Ne<4 are delayed electrons related to impurities. ▸ The origin of the excess at low Ne events (4<Ne<10) is unknown and under investigation.
DARKSIDE LOW MASS � 6 CRITERIA FOR FUTURE LAr TPC Membrane Cryostat AAr ▸ Low activity of 39 Ar ▸ Low impurity SiPM array Light guide Acrylic ▸ good electron lifetime Gas pocket ▸ low rate of the single electron events Field cage Fiducial volume UAr ▸ Ultra-pure photo-sensor ▸ Pure (or no) cryostat
39 Ar SUPPRESSION � 7 FURTHER DEPLETION OF Ar Urania (Underground Argon): ‣ Expansion of the argon extraction plant in Cortez, CO, to reach capacity of 100 kg/day of Underground Argon Aria (UAr Purification): ‣ Very tall column in the Seruci mine in Sardinia, Italy, for high-volume r chemical and isotopic purification of ~350 m Underground Argon. A factor 10 reduction of 39 Ar per pass is expected.
LOW MASS WIMP SEARCH � 8 SENSITIVITY 39 − 10 ] 40 − 10 2 [cm 41 − 10 SI σ 42 − 10 Dark Matter-Nucleon 1 tonne year projection threshold: 2 Ne 43 − 10 39 Ar: 1µBq/kg 44 − 10 45 − 10 DS-LM 1 t yr proj. × DS-50 2018 46 − 10 DS-50 BQF DS-50 No QF − 47 10 COGENT 2013 LUX 2017 XENON1T 2017 PICO-60 2017 CDMSLite 2017 CRESST-III 2017 48 − 10 PandaX-II 2016 XENON100 2016 CDMS 2013 CRESST 2012 DAMA/LIBRA 2008 Neutrino Floor 49 − 10 1 − 5 10 1 2 3 4 5 6 7 8 910 × M [GeV/c ] 2 χ ▸ Exposure: 1 tonne year ▸ Acrylic: 5 mm thickness with the activities achieved by JUNO collaboration. ▸ 39 Ar: 1µBq/kg (currently ~1mBq/kg in ▸ No cryostat DS-50) with 39 Ar depletion in Aria plant ▸ Analysis threshold: 2 Ne (~0.4 keVnr) ▸ SiPM: 50 times lower contribution than currently achieved in DS-20k (cleaner and ▸ No systematic uncertainties are included reduced electronics)
� 9 ASSUMPTIONS ▸ No BGs except the internal 39 Ar BG, external gamma BGs from the detector components, and coherent neutrino BGs (the neutrino electron scattering is an order smaller and ignored). ▸ Low Ne events will be suppressed via deep fiduciallization, pulse shape, and reduced activity in the active volume.
DARKSIDE-50 � 10 SUB-GEV DARK MATTER SEARCH ▸ Ultra-light DM (m 𝜓 ≪ 1 GeV) scatter off electrons ▸ DM signals are also ER. 36 − 10 37 − 10 ▸ The same measured spectrum as the ] 38 − 10 WIMP search can be used. 2 [cm 39 − 10 F = 1 DM DarkSide-50 e DarkSide-LM Proj. ▸ Two extreme cases of Dark Matter σ 40 − 10 39 1 µ Bq/kg Ar 1 µ Bq/PDM Dark Matter-Electron XENON100 form-factor are considered XENON10 41 − 10 − 42 10 − 31 10 ▸ F DM =1 heavy mediator 32 − 10 33 − 10 ▸ F DM ∝ 1/q 2 light mediator 34 − 10 2 F 1/q − 35 ∝ 10 DM DarkSide-50 ▸ The dashed lines are with assumptions DarkSide-LM Proj. − 36 10 39 1 Bq/kg Ar 1 Bq/PDM µ µ XENON100 of 1 uBq/kg for 39 Ar, 1 uBq/PDM, Cu 37 − 10 XENON10 38 − 10 cryostat, 80,000 kg day, and 2e- 39 − 10 2 3 10 10 10 threshold 2 m [MeV/c ] χ
TODO � 11 NR IONIZATION YIELDS AmBe neutron source AmC neutron source 900 100 241 241 13 AmBe Data 90 Am C Data 800 G4DS Fit G4DS Fit 80 700 Single S2 Single S2 S1 + S2 S1 + S2 70 600 241 13 Am C γ - - e e Events / N Events / N 60 500 50 400 40 300 30 200 20 100 10 0 0 10 20 30 40 50 60 70 80 90 100 10 20 30 40 50 60 70 80 90 100 N number of electrons N number of electrons - - e e E [keV ] Ar nr 2 10 1 10 10 ▸ MC + Ionization model [1] fit to 9 8 Argon NR data from AmBe and AmC. ] 7 nr /keV 6 5 ▸ Need calibration points at low - [e 4 y 3 Ar Data Q recoil energies ARIS 2 SCENE AmBe - AmC - ARIS - SCENE 1 Joshi et al. 2014 Joshi et al. 2014 Cross Calibrated 0 − 2 − 1 10 10 1 ε Reduced Energy [1] F . Bezrukov, F . Kahlhoefer, and M. Lindner, Astropart. Phys. 35, 119 (2011).
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