GERDA: The GERmanium Detector Array for the search for neutrinoless decays of 76 Ge. Allen Caldwell Max-Planck-Institut für Physik

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1 GERDA: The GERmanium Detector Array for the search for neutrinoless decays of 76 Ge Allen Caldwell Max-Planck-Institut für Physik

2 What we know Mass Scale NORMAL INVERTED m 12 2 known m 13 2 known Mixing Matrix U ij can be characterized by three mixing angles, 12, 23, 13, one Dirac CP phase,, and two Majorana phases 2, 3 12, 23 measured, upper limit on 13

3 Current (lack of) Understanding What we do not know about neutrinos: 1. absolute mass scale (offset) 2. mass hierarchy (1,2,3 or 3,1,2) 3. nature of neutrino (Majorana, Dirac particle) 4. value of third mixing angle ( 13 ) 5. CP phases (, 2, 3 ) Double beta decay experiment can address 3, and, if neutrinos are Majorana particles, then also a combination of 1,2,5

4 Majorana vs Dirac How can we test if neutrinos are Dirac or Majorana particles? Experimental Problem: P( L R ) m E 2 m ev, E MeV or more Only known technique is neutrinoless double beta decay: e e W - W - Nuclear Physics A,Z A,Z+2

5 Double Beta Decay (A,Z) (A,Z+1)+e+ energetically forbidden (A,Z) (A,Z+2)+2e+2 is allowed. Then, for Majorana particle (A,Z) (A,Z+2)+2e possible Note: process would violate lepton number conservation! Very rare decay lifetimes >10 20 years!

6 Decay Rate & Spectrum Normalized energy spectrum 2 0 If resolution poor If resolution good 0 -DBD rate Phase Q5 space Nuclear matrix element Effective Majorana mass 1/ = G(Q,Z) M nucl 2 <m ee > 2

7 Effective Neutrino Mass Im m ee = m (1) ee +e i 2 m (2) ee +e i 3 m (3) ee m (1) ee = U e1 2 m 1 m (2) ee = U e2 2 m m 21 m (3) ee = U e3 2 m m 31 m (3) ee e i 3 m ee m (2) ee e i 2 m (1) ee Re Complicated relationship between effective mass in neutrinoless double beta decay and neutrino masses, mixing angles and phases Cancellation possible: m ee could be vanishingly small o o (

8 Effective Neutrino Mass m ee in ev Inverted hierarchy Degenerate F.Feruglio, A. Strumia, F. Vissani, NPB % CL Normal hierarchy Negligible errors from oscillations; width due to CP phases Lightest neutrino (m 1,m 3 ) in ev

9 0 DBD Claims Note: 0 predicted to have shorter lifetime from phase space arguments Positive result corresponds to m ee 30 kev (my estimate)

10 Heidelberg-Moscow Experiment H.V. Klapdor-Kleingrothaus, I.V. Krivosheina, A. Dietz, O. Chkvorets Phys.Lett.B586: ,2004 Experiment with Ge detectors enriched in 76 Ge Exposure 71.7 kg-yr Experiment carried out in Gran Sasso lab Background: 0.11/(keV kg yr) Known Bi lines Claim: 4.2 signal T 1/2 = yr m ee =440 mev (best fit) KK Matrix Element m ee 700 mev Rodin et al. Matrix Element

11 GERDA GERDA (GERmanium Detector Array) is a collaboration of 12 institutes, ca. 80 physicists, from Germany, Italy, Russia, Poland, Belgium. The experiment has been approved by the LNGS (Gran Sasso) 1400 m ~ m.w.e GERDA

12 GERDA Locations Underground detector laboratory (LArGe-facility)

13 GERDA

14 Proposed & Ongoing Experiments Some of the possible isotopes Decay Q(keV) Nat. Abundance Experiments 48 Ca 48 Ti % CANDLES 76 Ge 76 Se % GERDA,Majorana 82 Se 82 Kr % NEMO 96 Zr 96 Mo % 100 Mo 100 Ru % NEMO,MOON 116 Cd 116 Sn % 128 Te 128 Xe % 130 Te 130 Xe % COBRA,CUORE 136 Xe 136 Ba % EXO,XMASS 150 Nd 150 Sm %

15 GERDA We like Germanium because: excellent energy resolution (3 2 MeV) considerable experience built up over the years - best background levels, best limits to date! still improvements possible 2 0 There are also some downsides: Q=2039 kev in region of backgrounds Q=2039 kev not among the higher Q values (recall 1/Q 5 ) enrichment possible, but expensive! limited number of crystal growers, detector makers

16 Detector Setup Maximum charge Organized in strings Start with existing detectors

17 Sensitivity Bayesian analysis: discovery defined as P(background only spectrum)< KK et al. Phase I: 15 kg-yr, existing enr Ge crystals Phase II: 100 kg-yr, new segmented enr Ge crystals

18 Central value from KK et al. Sensitivity

19 Background Suppression Source Action s external to crystals from 208 Tl( 232 Th), 214 Bi( 226 Ra), 60 Co, Front-end electronics μ Induced prompt signals μ Induced delayed signals (e.g. n+ 76 Ge 77 Ge 77 As) Internal to crystal (cosmogenic) Shield: high-purity liquid argon shield. Minimize material close to detector. Cold ASIC Underground location (LNGS mwe); Water Cerenkov veto Low-Z material shield (Ar) Minimize time above ground after enrichment ( 68 Ge), crystal pulling ( 60 Co) In addition: segmented detectors and pulse shape analysis (Phase II)

20 Backgrounds The types of things we worry about: e.g., cosmogenic activation of 68 Ge (about 6/(day kg) in enriched Ge) 68 Ge 68 Ga via EC (10.6 KeV X-ray) =271 days 68 Ga 68 Zn via + (90%, 1.9 MeV) + (0.511 MeV) + (0.511 MeV) =68 minutes MeV

21 Existing Detectors Heidelberg-Moscow detectors for Phase I of GERDA. In addition, three detectors from IGEX experiment. Total mass approx 18 kg. Detectors need to be refurbished to fit into GERDA scheme. This process is well underway.

22 New Detectors Phase II detectors 18-fold segmented detectors (true-coaxial, 3x6, n-type) Kapton cable bonded contacts Total of 30g mounting material / detector

23 New Detectors

24 Material Screening Sample screening coordinated between: MPIK, GEEL, Baksan, LNGS All materials which can produce background are measured GeMPI at LNGS:

25 Active Background Suppression Background sources: Cosmogenically produced 68 Ge and 60 Co U/Th contamination, 210 Pb on surface External gammas Signatures: Signal has two electrons in final state range ~mm Background sources mostly with E >2 MeV Compton scattering dominant interaction, range ~few cm Signal: Background ( 60 Co):

26 Background Suppression Counts/keV/kg/day Energy (kev) With prototype 18-fold segmented detector

27 Summary Claim m ee in ev Inverted hierarchy GERDA I,II GERDA III Degenerate Normal hierarchy Different M.E. Lightest neutrino (m 1,m 3 ) in ev 1. We will confirm or rule out the Klapdor-Kleingrothaus et al. claim 2. If not verified and background reduction to the level 10-3 /(kg yr kev) demonstrated, go for Phase III (ca. 1 ton, 20 mev level)