K. Zuber, University of Sussex TU Dresden, 15. Oct Double beta decay

Transcription

1 K. Zuber, University of Sussex TU Dresden, 15. Oct Double beta decay

2 Contents General Introduction Neutrino oscillations and DBD Experimental considerations GERDA COBRA SNO+ Outlook and summary

3 Beta and double beta decay Beta decay (A,Z) (A,Z+1) + e - + ν - e n p + e - + ν e (A,Z) (A,Z+2) +2 e - + 2ν e (A,Z) (A,Z+2) + 2 e - - Double beta decay - β-decay 2νββ 0νββ changing Z by two units while leaving A constant

4 Requirements Weizsäcker formula for A=const near minimum well approximated by m(z,a) = const + 2b S (A /2 Z) 2 A 2 + b C Z 2 A 1/3 + m ez + δ m E-E O-O β β A Even β β β + β + β + β + Pairing energy δ leads to splitting: δ = 0 for even-odd, odd-even δ = - 12 MeV/A 1/2 for even-even δ = + 12 MeV/A 1/2 for odd-odd Z o Z There are 35 β - β - isotopes in nature Single beta decay must be forbidden

5 Example - Ge76

6 History 1934: E. Fermi theory of weak interaction 1935: M. Goeppert-Mayer discussed 2νββ 1937: E. Majorana two component neutrino 1937,39: G. Racah, W.H. Furry discussed 0νββ 1949: First half-life limits (Fireman, Fremlin,...) 1967: First geochemical evidence for 2νββ 1987: First laboratory evidence for 2νββ 2002: First laboratory evidence for 0νββ???

7 Spectral shapes 0νββ: Peak at Q-value of nuclear transition Measured quantity: Half-life Dependencies (BG limited) T 1/2 a ε (M t/δe B) 1/2 Sum energy spectrum of both electrons link to neutrino mass 1 / T 1/2 = PS * ME 2 * (m ν / m e ) 2

8 0νββ Any L=2 process can contribute to 0νββ R p violating SUSY V+A interactions Leptoquarks Double charged Higgs bosons Compositeness Heavy Majorana neutrino exchange Light Majorana neutrino exchange... 1 / T 1/2 = PS * NME 2 *ε 2

9 The standard lore Light Majorana neutrino exchange Measured quantity Quantity of interest Effective Majorana neutrino mass 1 / T 1/2 = PS * NME 2 * (<m ν > / m e ) 2 Phase space integral calculable Nuclear transition matrix element

10 Oscillation evidences Δm 2 =m 2 2 depends on 2 m 1 No absolute mass measurement LSND (not confirmed by MiniBooNE) sin 2 2θ = , Δm 2 = ev 2 Atmospheric sin 2 2θ = 1.00, Δm 2 = ev 2 Solar + reactors sin 2 2θ = 0.81, Δm 2 = ev 2 If all three are correct... we need more (sterile ones)

11 Neutrino mass schemes almost degenerate neutrinos m 1 m 2 m 3 hierarchical neutrino mass schemes normal inverted

12 3 Flavour oscillations (PMNS) ν ν ν ν ν ν = ν ν ν τ μ ν τ τ τ μ μ μ τ μ e 2 i e3 e2 e1 e 2E m U U U U U U U U U solar If sin θ 13 0 CP-violation atmospheric U= U PMNS diag(1,e iα 1,e iα 2 ) Majorana: U = cosθ 12 sinθ 12 0 sinθ 12 cosθ cosθ 13 0 sinθ 13 e iδ sinθ 13 e iδ 0 cosθ cosθ 23 sinθ 23 0 sinθ 23 cosθ e iα e iα 2

13 Physical quantities Experimental observable: Half-life Double beta decay: Effective Majorana neutrino mass m ν = U 2 ei m i = m 1 U 2 e1 + m 2 U 2 e2 e iα 1 + m 3U 2 e3 e iα 2 CP-invariance: m ν = U 2 2 ei m i = m 1 U e1 ± m 2 U e 2 2 ± m 3 U e 3 2 Beta decay m ν = Σ U ek 2 m k Measurements are complementary

14 Isotope Phase space 0νββ decay rate scales with Q 5 2νββ decay rate scales with Q 11 Q-value (kev) Nat. abund. (%) (PS 0v) 1 (yrs x ev 2 ) (PS 2v) 1 (yrs) Ca E E16 Ge E E18 Se E E17 Zr E E16 Mo E E17 Pd E E18 Cd E E17 Sn E E17 Te E E17 Xe E E17 Nd E E15

15 Nuclear matrix elements The dark side of double beta decay

16 Uncertainties in nuclear matrix elements, example 116 Cd <m ν >=0.4eV Nuclear Model SK-01(?) SK-01(?) RFSV-05(1.25) RFSV-05(1.0) RFSV-05(1.25) RFSV-05(1.0) SPVF-99(1.25) CS-03(1.25) ASaws098(1.25) ASaws098(1.0) Asws-98(1.25) ASws-98(1.0) SK-01(?) SK-01(?) Results discussed by RFSV SPVF-99(1.25) SPVF-99(1.0) PSVF-96(1.25) Without higher order terms of nuclear current T 1/2 for Cd (years) 27 V. Rodin et al., nucl-th/ , Nucl.. Phys. A 766,107 (2006)

17 β + β + -modes In general: Double charged higgs bosons, R-parity violating SUSY couplings, leptoquarks... n n (A,Z) (A,Z-2) + 2 e + (+2ν e ) β+β+ e - + (A,Z) (A,Z-2) + e + (+2ν e ) β+/ec 2 e - + (A,Z) (A,Z-2) (+2ν e ) EC/EC p e p e Q-4m e c 2 Q-2m e c 2 Q Important to reveal mechanism if 0νββ is discovered Enhanced sensitivity to right handed weak currents (V+A)

18 Neutrino mass vs. right handed currents 0ν ( T 1/2 ) 1 m = C ν mm m e 2 + C ηη η 2 + C λλ λ 2 + C mλ λ EC/ß + m ν m e + C mη η <λ> m ν m e + C ηλ η λ λ,η <<1 Possible evidence <m ν >(ev) M. Hirsch et al., Z. Phys. A 347,151 (1994)

19 The search for 0νββ or

20 Back of the envelope Τ 1/2 = ln2 a N A M t / N ββ (τ>>t) ( Background free) For half-life measurements of yrs 1 event/yr you need source atoms This is about 10 moles of isotope, implying 1 kg Now you only can loose: nat. abundance, efficiency, background,...

21 Signal information (A,Z) (A,Z+2) e - Signal: One new isotope (ionised), two electrons (fixed total energy) Single electron energies Angle between electrons Sum energy of both electrons Daughter ion (A,Z+2) Gamma rays (eg. four 511 kev photons in β + β + )

22 The dominant problem - Background How to measure half-lives beyond years??? The first thing you need is a mountain, mine,... The usual suspects (U, Th nat. decay chains) Alphas, Betas, Gammas Cosmogenics thermal neutrons High energy neutrons from muon interactions 2νββ

23 Experimental techniques Source = detector Source detector Semiconductors Heidelberg-Moscow, IGEX, COBRA, GERDA, MAJORANA Cryogenic bolometers CUORICINO, CUORE Scintillators SNO+, CANDLES, MOON, GSO, XMASS Time projection chambers (TPC) NEMO-3, SuperNEMO, DCBA, EXO

24 GERDA COBRA

25 Heidelberg -Moscow Five Ge diodes (overall mass 10.9 kg) isotopically enriched ( 86%) in 76 Ge Lead box and nitrogen flushing of the detectors Digital Pulse Shape Analysis Peak at 2039 kev

26 S p e c t r u m 0ν peak region

27 Latest HD-Moscow results Statistical significance: kg x yr Including pulse shape analysis: 35.5 kg x yr (installed Nov. 95, only 4 detectors) SSE T 1/2 > 1.9 x yr (90% CL) m < 0.35 ev

28 Heidelberg -Moscow more statistics Recalibration Subgroup of collaboration T 1/2 = x yr m = ev H.V. Klapdor-Kleingrothaus et al, Phys. Lett. B 586, 198 (2004)

29 Gerda - Motivation Improvement of neutrino mass sensitivity Check of possible evidence Hd-Moscow Improve Background by ultra-pure shielding HPGe detectors in Liquid Argon GERDA 10-3 (kg y kev) -1 [IGEX (kg y kev) -1 ] [Hd-M 0.17 (kg y kev) -1 ] GERDA

30 GERDA sensitivity 76 Ge P I 15 kg y at 10-2 (kev kg y) -1 T 1/2 0ν > y ( HdM: y) y GERDA Phase II 3y 35kg GERDA Phase I 1y Effective 15kg Neutrino Mass Determination or Limit P II P III 100 kg y at 10-3 (kev kg y) -1 T 1/2 0ν > y 1 ton 76 Ge exp. (GERDA/Majorana) depending on Phase I/II outcome Background goal 10-4 (kev kg y) -1

31 Gerda Detector Overview LNGS 3600 mwe Water shield Cherenkov muon veto LAr shield in steel cryostat ~20 Ge crystals on 7 Cu strings ~35kg of 76 Ge (a>85%) Q ββ =2039 kev

32 Ge Detectors Phase II front-end electronics 7 strings with 5 detectors 18-fold segmented detector 3 layers, 6 angular

33 18-fold segmented n-type detector 3-fold segment in height I. Abt et al. NIMA 577 (2007) fold segmented in azimuthal angle

34 COBRA Use large amount of CdZnTe Semiconductor Detectors Array of 1cm 3 CdZnTe detectors K. Zuber, Phys. Lett. B 519,1 (2001)

35 Advantages Source = detector Semiconductor (Good energy resolution, clean) Room temperature Modular design (Coincidences) Two isotopes at once Industrial development of CdTe detectors 116 Cd above MeV Tracking ( Solid state TPC )

36 Isotopes COBRA: CdZnTe semiconductors nat. ab. (%) Q (kev) Decay mode Zn ß-ß- Cd ß-ß- Cd ß-ß- Te ß-ß- Te ß-ß- Zn ß+/EC Cd ß+ß+ Cd EC/EC Te ß+/EC

37 116 Cd comes of best... V.K.B. Kota, priv. comm.

38 COBRA collaboration University of Sussex University of Warwick University of Liverpool Rutherford Appleton Laboratory University of Birmingham University of York University of Dortmund Material Research Centre Freiburg Laboratori Nazionali del Gran Sasso Washington University at St. Louis University of Bratislava University of Jyvaskyla University of La Plata Technical University Prague More welcome University of Surrey (UK), University of Hamburg (Germany), Jagellonian University (Poland), Louisianna State University (USA), Technical University Dresden (Germany), Daresbury Lab. (UK)

39 First COBRA Double beta results world best Copper Wax CZT Pertinax Based on 4 detectors, total of 4.34 kg x days T. Bloxham et al., arxiv: , acc.by Phys. Rev. C

40 The first layer Installed at LNGS in april 2006, world wide largest array of this type of detector

41 The first layer - some spectra Cd-113 beta decay with half-life of about yrs

42 The first layer - Coincidences Coincidences Preliminary Coincidences around Det 7 Example: Powerful tool!!! 3-coincidence Just starting to analyse/understand the power of that..

43 Example: 130 Te 1 st excited state 130 Te 130 Xe + 2e + γ(536kev ) 1. 1 Detector with 2530 kev 2. 1 Detector with 1994 kev 3. 2 Detectors (One with 1994 kev, one with 536 kev)) 4. 2 Detectors (One with 1994 kev, two sum up to 536 kev) Have performed a search for mode 3 in 4.2 kg x days and found no event!

44 New Results PRELIMINARY PRELIMINARY About 8 kg days

45 New Results PRELIMINARY PRELIMINARY Some new world-best limits

46 New passivation (4 detectors) Major background so far: Red paint on detector surface Sample measurements at LNGS suggest improvement of about 3 orders of magnitude Monte Carlo expectation Paint contribution at 2.8 MeV: about 0.2 counts/kev/kg/yr

47 New passivation Around 10 counts/kev/kg/yr Very preliminary: At least a factor 10 better, lot of construction work around COBRA at LNGS, no coincidences, no nitrogen flushing...

48 Sensitivity T 1/2 M t /ΔE B 50 mev

49 The solid state TPC Energy resolution Tracking Massive background reduction Positive signal information Pixelated CdZnTe detectors

50 Pixelisation - I Massive BG reduction by particle ID, 200μm pixels (example simulations): α= 1 pixel, β and ββ= several connected pixel, γ= some disconnected p. Y pixel Total E=2805 0νββ Total E = MeV β α ~15μm mm1.5mm X pixel eg. Could achieve nearly 100% identification of 214 Bi events ( 214 Bi 214 Po 210 Pb). Beta with endpoint 3.3MeV 7.7MeV α life-time = 164.3μs

51 Rejection power of pixels First look on rejection power Likelihood using: -Number of pixels - spatial separation - energy loss (de/dx) T. Bloxham, M. Freer, Nucl. Inst. Meth. A (2007) Suggests a background reduction of 1000!

52 Pixelisation - II Running 256 pixel det with ASIC, 1.6mm pixel size crystal ASIC readout 122 kev Additional 16 pixel detector with conventional readout running Single pixel 57 Co spectrum 136 kev

53 Pixel detectors - next step 64 pixel detectors 2x2x0.5 cm 3 Pixel electrodes will be replaced by 200 μm pixels, mask in hand By end 2007 we ll have two high resolution pixel detectors

54 Pixellated detectors Solid state TPC 2D - Pixelisation on both electrodes

55 Nobody said it was going to be easy, and nobody was right George W. Bush

56 SNO The smoking gun 1000 t heavy water (D 2 0) CC ν e + d p + p + - e NC ν + d p + n + x ν x e ES - e x ν x ν CC ES = ν e ν e ( ν μ + ν τ ) CC NC = ν e ν e + ν μ + ν τ

57

58 Test <m ν > = ev 0ν: 1000 events per year with 1% natural Nd-loaded liquid scintillator in SNO++ simulation: one year of data maximum likelihood statistical test of the shape to extract 0ν and 2ν components ~240 units of Δχ 2 significance after only 1 year!

59 SNO+ broad-brush schedule 2007: removal of SNO heavy water, SNO+ design 2008: SNO+ installation 2009: Fill, run with pure scintillator 2010: Add Nd

60 Summary Neutrinoless double beta decay crucial for neutrino physics Gold plated channel for Majorana character and neutrino mass Sensitivity of 50 mev neutrino mass requires hundreds of kg of isotopes (enrichment) A lot of experimental proposals/ideas GERDA, COBRA are the seminconductor approaches (energy resolution) SNO+ could be a quick, very sensitive experiment Revived interest in neutrinoless double EC Progress is fast...