The GERDA Neutrinoless double beta decay experiment

Transcription

1 The GERDA Neutrinoless double beta decay experiment Stefan Schönert, MPIK Heidelberg Workshop on Precision Measurements at Low Energy January 18 th & 19 th 2007 Paul Scherrer Institut, Villingen, Switzerland

2 Outline Introduction: 0-νββ and physics implications Effective Majorana neutrino mass <m> Predictions on <m> from oscillation experiments Sensitivity with and w/o backgrounds GERDA design Concept Sensitivities: Phase I, II, III Locations at LNGS Phase I detectors Phase II detectors Front-end electronics Infrastructures: cryogenic tank, WT, clean room,.. Screening Examples of backgrounds and reduction techniques: Detector segmentation Liquid argon scintillation read out Conclusion/Outlook

3 2ν-ββ Decay Observed in more than 10 isotopes Life times ν e 21 - years e - ν

4 Mass parabolas odd-odd even-even Ground states of even-even nuclei: 0 +

5 0ν-ββ Decay 0νββ can be generated by: exchange of light Majorana neutrinos SUSY.. Not observed yet; Life time limits > 10 ν y; Claim for evidence in Ge-76 by part of Heidelberg-Moscow Collab. e - e- Schechter & Valle: if 0νββ observed ν is Majorana particle!

6 Physics motivations 1) Dirac vs. Majorana particle: (i.e. its own anti-particle)? 0νββ Majorana nature Majorana See-Saw mechanism 2 D m ν = << R Majorana CP violation in M R higgs + lepton Leptogenesis B asymmetry m M For m 3 ~ ( m 2 atm )1/2, m D ~ m t M R ~ GeV m D 2) Absolute mass scale: Hierarchy: degenerate, inverted or normal (effective) neutrino mass

7 0ν-ββ Decay d u W - e - ν e = ν e L=2 d u W - e - Assume leading term is exchange of light Majorana neutrinos T 1/2 (0ν) - ¹ = G M² m² ee Effective neutrino mass Phase space Nuclear matrix element

8 Effective Majorana mass m ee = i U ei ² m i U ei complex: sensitive to CP phases (optimist ) cancellation possible (pessimist) NB: Beta-endpoint (Katrin) m νe = ( i U ei ² m i ² ) 1/2

9 If CP is conserved: m ββ U e1 2 m 1 Re[m ββ ] U e2 2 m 2 U e3 2 m 3

10 Im[m ββ ] CP: Destructive interference possible U e3 2 e iα 31 m3 m ββ =0 U e2 2 e iα 21 m2 U e1 2 m 1 Re[m ββ ]

11 Im[m ββ ] m ββ U e3 2 e iα 31 m3 U e2 2 e iα 21 m2 U e1 2 m 1 Re[m ββ ] Standard parametrization:

12 Experimental evidences for neutrino oscillations ν e ν µ,τ ; ν e ν x solar- and reactor-ν s: m 2 sol ev 2 sin2 2 θ m 2, θ ν µ ν τ atmospheric- and accelerator-ν s: m 2 atm (2-4) 10-3 ev 2 sin2 2 θ 23 1 ν e ν x reactor-ν s: sin2 2 θ 13 <0.2

13 Input for m ee from ν-oscillations Solar/Reactor -ν: θ 12, m² sol Atmosph.-ν: m² atm Reaktor-ν: θ 13 m 3 m 3 m 2 m² sol For NH: m 3 m 2 m 1 m² atm m² sol Mass m 2 m 1 NH m² atm m² sol m 1 m m² 2 = (m atm 1 ² + m² sol ) 1/2 m 3 = (m 1 ² + m² sol + m² atm ) 1/2 IH DG m ee = cos²θ 13 (m 1 cos²θ 12 + m 2 e 2iα sin²θ 12 ) + m 3 e 2iβ sin²θ 13 m ee = f(m 1, m² sol, m² atm, θ 12, θ 13, α,β)

14 Predictions from oscillation experiments m ee F.Feruglio, A. Strumia, F. Vissani, NPB % CL Negligible errors from oscillations; width due to CP phases m 1

15 Claim for evidence for ββ(0ν) H.V. Klapdor-Kleingrothaus, A. Dietz, I.V. Krivosheina, O. Chkvorets, NIM A 522 (2004) (subgroup of Heidelberg-Moscow Collaboration) Heidelberg-Moscow data: Nov May kg year Bgd 0.11 / (kg y kev) ± 6.87 events (bgd:~60) 4.2 sigma evidence for 0ν x10 25 y (3 sigma) Best fit 1.19 x10 25 y m ee = ev best fit 0.44 ev NB. Statistical significance depends on background model!

16 Experimental sensitivity: w/o background Experimental life time τ = N N N S T number of nuclides under control M live time number of detected decays Background free limit: 0 cnts in the analysis energy window Poisson upper limit: N P Remember: τ N N T M T const <m> (M T) 1/2 N P

17 Sensitivity: with background If no decay is observed in presence of N B background events in an energy window E: N S < ( N B ) 1/2 τ > N N T ( N B ) 1/2 detector energy resolution N B = b M T E b: background index [1/(kg year kev)] τ > N N T (b M T E ) 1/2 M T ( ) 1/2 b E <m> const. b E ( ) 1/4 M T

18 Comparison of DBD Isotopes T 0ν / 2 N sig = 2 2 = N Avg ln 2 Γ M < mee > isotope Q 5 ββ 1 Γ( ) M 2 <m ee > 2 1 mass t A Q ββ GERDA, Majorana nat. abund. rel. A rel Γ rel. M 2 N sig 76 Ge 76 Se 2039 kev 7.4% Se 82 Kr 2995 kev 9.2% Mo 100 Ru 3034 kev 9.6% Te 130 Xe 2529 kev 34% Xe 136 Ba 2479 kev 8.9% for 1000 kg y, <m ee > = 50 mev, M 2 from V.A.Rodin et al, Nucl. Phys. A766 (2006) 107. NEMO3 Super-Nemo Cuoricino/Cuore EXO

19 Introduction: 0-νββ and physics implications Effective Majorana neutrino mass <m> Predictions on <m> from oscillation experiments Sensitivity with and w/o backgrounds Claim of KK et al. (HdM Data) GERDA design Concept Sensitivities: Phase I, II, III Locations at LNGS Phase I detectors Phase II detectors Front-end electronics Infrastructures: cryogenic tank, WT, clean room,.. Screening Examples of backgrounds and reduction techniques: Muon veto Detector segmentation Liquid argon scintillation read out Conclusion/Outlook

20 Two new 76 Ge Projects: GERDA Majorana List of institutions: INFN LNGS, Assergi, Italy JINR Dubna, Russia Institute for Reference Materials, Geel, Belgium MPIK, Heidelberg, Germany Univ. Köln, Germany Jagiellonian University, Krakow, Poland Bare Univ. di enr Milano Ge array Bicocca in liquid e INFN, argon Milano, (nitrogen) Italy INR, Moscow, Russia Shield: ITEP Physics, high-purity Moscow, liquid Russia Argon (N) / H 2 O Phase Kurchatov I: ~18 Institute, kg (HdM/IGEX Moscow, Russia diodes) Phase MPI Physik, II: add München, ~20 kg new Germany enr. Detectors; total Univ. ~40 di Padova kg e INFN, Padova, Italy Univ. Tübingen, Germany LoI Array(s) of enr Ge housed in high-purity electroformed copper cryostat Shield: electroformed copper / lead Staged approach based on 60 kg arrays (60/120/180 kg) Physics goals: degenerate mass range Technology: study of bgds. and exp. techniques ~80 physicists, 13 institutions, 5 countries approved Nov 2004 at LNGS Status: under construction open exchange of knowledge & technologies (e.g. MaGe MC) consider to merge for O(1 ton) exp. ( inv. Hierarchy)

21 Phases and Physics reach of GERDA (90 % CL) * (90 % CL)* KK *: no event in ROI T 1/2 (y) (90% C.L., no bgd.) required for background free exp. with E~3.3 kev (FWHM): Phase I Phase II exposure (kg y) O(10-3 ) O(10-4 ) Phase III <50 mev <170 mev assuming M 0ν =2.5 [Rod06] and 89% enrichment counts/(kg y kev) Background requirement for GERDA: Background reduction by factor required w.r. to precursor exps. Degenerate mass scale O(10 2 kg y) Inverted mass scale O(10 3 kg y)

22 Phases and Physics reach of GERDA Phase I: Phase II: Phase III: m ee in ev F.Feruglio, A. Strumia, F. Vissani, NPB 659 Lightest neutrino (m 1 ) in ev

23 GERDA at LNGS GERDA location: hall A of LNGS

24 GERDA design Clean room lock Water tank / buffer/ muon veto Cryogenic vessel Liquid argon Ge Array

25 GERDA underground facilities at LNGS Underground detector laboratory (LArGe-facility)

26 Main Experimental Site June 06

27 Cryostat Vacuum insulated stainless steel cryostat with internal Cu liner (stainless steel factor ~100 more radioactive ( 238 U, 232 Th) than Cu) Ø outer height [mm mm] inner vessel volume 70 [m3] empty vessel 25,000 [kg] max. load inner vessel: LAr 98,000 [kg] Cu shield 20,000 [kg]

28 Infrastructure on Top of Platform Lock with tubes for cabels Clean room with lock on platform Rail system to lower position and lower individual strings

29 Phase I Detectors: Maintenance and Measurements in Undergrond detector laboratory (LArGe facility) IGEX HdM Since Nov. 2005: 17.9 kg of enriched Ge-detectors underground at LNGS; Characterization completed

30 Phase I Detectors: Prototype tests of (natural) low-mass detector assembly in liquid nitrogen 60 Co 1.6 kg mock-up p-type Enriched detectors are currently re-processed and prepared for testing

31 Phase II Detectors: Procurement of enriched Ge Enrichment of 37.5 kg Ge-76 completed in Sep.05 Transportation of Material to Europe by truck in spring for further processing Specially designed protective steel container reduces activation by cosmic rays by factor 20 Test transportation March 05

32 Phase II Detectors: True-coaxial natural detectors mock-up 18-fold segmented detector (in standard cryostat) 60 Co Core signal: 2 kev FWHM 6 fold φ segmented p-type 18 fold (6-φ; 3-z) segmented n-type One segment signal: 3.5 kev FWHM

33 Backgrounds in GERDA Source Ext. γ from 208 Tl ( 232 Th) Ext. neutrons Ext. muons (veto) Int. 68 Ge (t 1/2 = 270 d) Int. 60 Co (t 1/2 = 5.27 y) 222 Rn in LN/LAr 208 Tl, 238 U in holder Surface contam. B [10-3 cts/(kev kg y)] <<1 <0.05 < <0.2 <1 <0.6 Muon veto 180 days exposure after enrichment days underground storage 30 days exposure after crystal growing derived from measurements and MC simulations Target for phase II: Σ B 10-3 cts/(kev kg y) additional bgd. reduction techniques

34 Background reduction techniques Muon veto Anti-coincidence between detectors Segmentation of readout electrodes (Phase II) Pulse shape analysis (Phase I+II) Coincidence in decay chain (Ge-68) Scintillation light detection (LArGe)

35 Background reduction techniques Muon veto Anti-coincidence between detectors Segmentation of readout electrodes (Phase II) Pulse shape analysis (Phase I+II) Coincidence in decay chain (Ge-68) Scintillation light detection (LArGe)

36 Example: Internal 60 Co γ 1 : MeV β: E max = 318 kev γ 2 : MeV T 0 : 0 : crystal crystal growing µbq/kg µbq/kg per per day day exposure Test: Test: detector production in in days days Assume days days //(kev kg y)

37 Example of background topology Energy deposition in surrounding medium γ β Co-60 γ Multi-site energy deposition inside HP-Ge diode

38 60 Co background spectrum counts/kev 10 4 MC simulation β γ 1 γ 2 Q ββ γ 1 + γ kg energy (MeV)

39 60 Co: suppression by segmentation counts/kev 10 4 MC simulation Q ββ kg energy (MeV)

40 60 Co: suppression by segmentation N hit = 3 counts/kev 10 4 MC simulation Q ββ N seg = 1 ~10 (7 seg.) 2 kg 10 illustration: Simple 7-fold segmentation energy (MeV)

41 MaGe: 60 Co suppression by segmentation and anti-coincidence Number of crystals N crystals = 1 Number of segments probability per decay to deposit energy within Q ββ ROI per 1 kev energy bin after combined cuts: (18-fold segm.) P = /kev 6φ-3z segmentation; Prototype under construction N segments = 1 (factor ~35 reduction w/r to single unseg. detector)

42 Phase II detectors 1.6 kg 18-fold segmented true-coaxial n-type Co-60 Goal: Study of γ identification and suppression factors at Q ββ : depending on source location DE Bi-214 Th-228

43 232 Th suppression by LAr Ge-anticoinc: (20 cm diameter prototype setup) Th-232 PMT DATA Compton continuum RoI: 95 % suppr. by LAr veto Ge-diode 1 ton liquid argon detector under Liquid construction argon (20 kg) Suppression limited by size of Dewar (20 cm Ø )

44 60 Co: segmentation and LAr Ge-anticoinc Liquid Argon counts/kev 10 4 MC simulation Q ββ N seg = 1 AND LAr anticoinc ~ kg energy (MeV)

45 Off-spin of GERDA LAr R&D for DM search Pulse shape discrimination studies Data with AmBe n/γ source 60 kev visible energy Neutron recoils Liquid Argon for DM Search (WARP, ArDM, CLEAN) Discrimination of Ar-39 background? 20 kg active LAr target

46 Summary & Outlook GERDA: probe Majorana nature of neutrino with sensitivity down to inverse mass hierarchy scale phase I : background 0.01 cts / (kg kev y) scrutinize KKDC result within 1 year phase II : background cts / (kg kev y) T 1/2 > y, <m ee > < ev phase III : world wide collaboration T 1/2 > ~10 28 y, <m ee > ~10 mev 2007: Experimental installations (Cryotank, water tank, building etc.) 2008: target for detector readiness

47 GERDA collaboration