Prospects for kev-dm searches with the GERDA experiment

Transcription

1 Physik-Institut Prospects for kev-dm searches with the GERDA experiment Roman Hiller for the GERDA collaboration 29/03/2017 Page 1

2 GERDA concept Cleanroom Lock systen Enriched 76 Ge detectors Cryostat /w liquid argon Innermost active shielding via LAr scintillation Wavelength shifting + PMT, SiPM Passive shielding WCD as µ veto Ge detector array Steel cryostat 64 m 3 LAr Copper shield Water tank 590 m 3 University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 2

3 GERDA experiment LNGS, Italy, 3500 m.w.e. 7 enriched + 3 natural coaxial type det. since Dec. 2015: Phase II added 30 enriched BEGe type det. 36 kg total LAr veto Eur. Phys. J. C 73 (2013) 2330 University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 3

4 0νββ - the main mission Search for lepton number violation and study ν properties O() candidate isotopes with forbidden β-decay but allowed 2νββ 2νββ in 76 Ge T 1/2 = ± yr Q (MeV) 0νββ signature = peak at endpoint of 2νββ spectrum Ge 2039 kev As Se n n Events 2νββ 2νββ Eur. Phys. J. C 75 (2015) 416 p e - ν ν e - p n n 0νββ 0νββ p e - e - p Energy Qβ β University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 4

5 kev Dark Matter Sub-GeV DM via "super weak" coupling Super-WIMP Well studied (hardly detectable) fermions (sterile ν, gravitino) Bosons in some cases detectable via axio-electric effect Candidates: Pseudo-scalar Super-WIMPs (Axion-like particles) Vector Super-WIMPs (Hidden photon) SuperWIMPs e - Z +,e - Z +,e - Pospelov et. al. Phys. Rev. D 78, University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 5

6 Rates σ proportional to photoelectric effect σ pe Using estimates of the local DM density, the expected rates are: Pseudo-scalar ( ) ( R A g 2 ma σpe ) Ae kev b 5 kg 1 yr 1 R A 9 7 α α Vector ( ) ( kev σpe m V b ) kg 1 yr 1 Event rate (kg 1 yr 1 ) gaee = 2.5e-12 gaee = 5e-12 gaee = 1e-11 gaee = 2e-11 gaee = 4e mass (kev) Event rate (kg 1 yr 1 ) α α = 1e-24 α α = 3e-24 α α = 9e-24 α α = 2.7e-23 α α = 8.1e mass (kev) University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 6

7 Signature Analogous to photoeffect: complete absorption in atom with emission of monoenergetic electron Peak of single-site events at the mass of the Super-WIMP 4 3 data Super-WIMPs, gaee = 4 11 BG model + Super-WIMPs cts/1 kev E (kev) University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 7

8 Existing limits limits in -0 kev region XMASS, XENON0, EDELWEISS, MAJORANA Several indirect constrains 11 EDELWEISS γ BG Ae g 12 MAJORANA XENON0 XMASS log(α /α) Abundance MAJORANA HB Stars XMASS 28 XENON0 RG Stars 13 (kev) m A m V (kev) arxiv: University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 8

9 What about GERDA? Sensitivity M t σ E BI...but GERDA has higher energy threshold 175 kev (DAQ) Cross section Ge and Xe similar...but decreasing with energy However, no direct constrains above 145 kev M t kg yr BI kg yr kev FWHM kev GERDA ( kev) (0) (/w LAr) XENON kev GERDA background index 5 4 Ge Xe [bn]/a BI (1/(kg yr kev)) mass [kev] energy (kev) University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 9

10 Energy reconstruction weekly calibrations with 228 Th sources DAQ threshold > 500 kev Special calibration campaign /w 226 Ra source also at < 500 kev Enr BEGe (226Ra) Enr Coax (226Ra) Model from Th FWHM [kev] energy [kev] University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page

11 Background MC Background model Main contributions < 1 MeV: 2νββ (irreducible internal SSE) 39 Ar (n-surface; LAr suppression factor 3-5, PSD?) cts/ kev data Ar39 Bi212 Bi214 K40 K42 alpha nubb E (kev) University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 11

12 Background reduction - Pulse shape discrimination Discrimination of SSE/MSE, surface events Charge drift time pulse shape Current trace amplitude/energy = amplitude/area HPGe MULTI SITE EVENT (MSE) HPGe SINGLE SITE EVENT (SSE) signal (a.u.) Charge Current SSE A E signal (a.u.) Charge Current MSE A E time (µs) time (µs) University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 12

13 Background rejection A/E too sensitive to noise? Check by adding measured noise to signal traces might be alright Alternative: compare pulse to template of SSE, FEP energy (kev) FEP University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 13

14 Conclusion Bosonic Super-WIMPs are a candidate for kev mass DM They can be detected via a peak signature GERDA performs well down to 175 kev (exposure, energy resolution, background)... and consequently might be sensitive to Super-WIMPs at the lower end of its spectrum University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 14

15 Recent GERDA results Phase II 28.5 kg yr exposure background index BEGe cts and coax 2.2 kgyrkev < 1 background event in ROI at design exposure 0 kg yr T 0ν 1/2 > yr(90%c.l.) counts / 25 kev 4 enriched coaxial kg yr prior liquid argon (LAr) veto 3 2 after LAr veto after LAr veto and PSD 50 kev blinding overflow bin 1 counts / 25 kev 4 enriched BEGe kg yr Ar 40 K 42 K Q ββ Po energy [kev] GERDA energy [kev] Accepted by Nature, arxiv: University of Zurich, Physik-Institut 29/03/2017 kev Dark Matter search with GERDA Page 15