K. Zuber, Techn. Univ. Dresden Erlangen, 4. June Status and perspectives of the COBRA double beta decay experiment

Transcription

1 K. Zuber, Techn. Univ. Dresden Erlangen, 4. June 2009 Status and perspectives of the COBRA double beta decay experiment

2 Contents General Introduction Experimental considerations COBRA Summary and Outlook

3 Double beta decay (A,Z) (A,Z+2) +2 e - + 2ν e (A,Z) (A,Z+2) + 2 e - - 2νββ 0νββ Unique process to measure the mass of the neutrino Unque process to measure character of neutrino Requires half-life measurements well beyond yrs!!!! The smaller the neutrino mass the longer the half-life

4 Example - Ge76 There are only 35 candidates

5 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

6 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

7 3 Flavour mixing (PMNS) Weak eigenstates not identical to mass eigenstates, analogue to CKM mixing in quark sector cosθ 12 sinθ 12 0 U = sinθ 12 cosθ Majorana: cosθ 13 0 sinθ 13 e iδ cosθ 23 sinθ 23 0 e iα 1 0 sinθ 13 e iδ 0 cosθ 13 0 sinθ 23 cosθ e iα 2 U= U PMNS diag(1,e iα 1,e iα 2 )

8 Oscillation evidences Neutrino oscillations: Periodic conversion of different flavours into each other Δm 2 =m 2 2 depends on i m j No absolute mass measurement Atmospheric sin 2 2θ 23 = 1.00, Δm 232 = ev 2 Solar + reactors sin 2 2θ 12 = 0.81, Δm 122 = ev 2 If all three are correct... we need more (sterile ones)

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

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

11 Neutrino mass schemes and 0νββ Best fit values M. Hirsch Neutrino 2006

12 Nuclear matrix elements The dark side of double beta decay

13 Nuclear matrix elements 2νββ: Only intermediate 1 + states contribute Several experimental activities ongoing to improve situation (see K. Zuber nucl-ex/ )

14 Latest shell model calculations 116 Cd comes of best... E. Caurier et al, arxiv: , PRL 100, (2008)

15 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

16 The search for 0νββ or

17 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 1000 moles of isotope, implying 100 kg Now you only can lose: nat. abundance, efficiency, background,...

18 Candidates 0νββ decay rate scales with Q 5 Isotope 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

19 Future projects, ideas K. Zuber, Acta Polonica B 37, 1905 (2006) CUORICINO (stopped 08) running as NEMO-3 small scale ones will expand, very likely not a complete list...

20 Heidelberg -Moscow The detectors are decaying!! 5 isotopical enriched Ge-detectors Peak at 2039 kev

21 Heidelberg -Moscow Part of collaboration: T 1/2 = 2.23 ± 0.4 x yr m = 0.32 ± 0.03 ev H.V. Klapdor-Kleingrothaus et al., Phys. Lett. B 586, 198 (2004), Mod.Phys.Lett.A21: ,2006

22 Current aims Check whether observed peak claimed in 76 Ge is true (GERDA) If yes, observe it with at least one other isotope to confirm that it is double beta decay If not, next milestone will be 50 mev suggested by oscillation results If still no observation, down to range 1-10 mev Remember: m ν ΔEB 4 Mt

23 COBRA Use large amount of CdZnTe Semiconductor Detectors Large array of CdZnTe detectors K. Zuber, Phys. Lett. B 519,1 (2001)

24 Gran Sasso (Italy) It s all about background reduction GERDA COBRA CUORE

25 COBRA collaboration TU Dresden TU Dortmund Material Research Centre Freiburg Washington University at St. Louis Czech Technical University Prague University of Bratislava University of Jyvaskyla University of La Plata JINR Dubna Laboratori Nazionali del Gran Sasso More welcome Erlangen???? Supporting institutes: University of Hamburg (Germany), Jagellonian University (Poland), Los Alamos Nat. Lab. (USA)

26 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 )

27 Background COBRA S. Pirro, Milano Beyond MeV background a priori much lower

28 Isotopes 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

29 Needed Sensitivity T 1/2 aε M t /ΔE B claim of evidence yrs yrs 50 mev yrs yrs

30 It s all about background Background from: natural radioactivity, cosmogenic produced radioisotopes, cosmic rays, neutrons Two options Energy measurement only Energy measurement and tracking 1024 pixel, pixel size: 625 μm

31 COBRA Set-up at LNGS

32 64 array

33 Background model Assumption: Major background is paint + radon

34 Cd fold non-unique beta decay (1/2 + 9/2 + ) First half-life result published as C. Goessling et al., PRC 72:064328,2005 First time theoretical model based on nuclear structure calculation (thanks to J. Suhonen) We have more than 10 independent measurements now...

35 Cd-113

36 Cd-113 First time histogram of yrs half-life measurements Half-life: Q-value: Spectral shape fits better with unique 3-fold forbidden than non-unique transition J. V. Dawson et al., Nucl. Phys. A 818,264 (2009)

37 New Results Six half-life limits above years, one world best, six limits within factor 3 of world best, coincidence analysis and analysis on colourless data ongoing based on 18 kg days of data, J.V. Dawson et al., arxiv: , acc. by Phys. Rev. C

38 Next things done Remove passivation paint Install nitrogen flushing

39 Measured spectrum Data taking with red crystals finished, now running 4 (8) clear painted dets. Background at 2.8 MeV around 5 counts/kev/kg/yr!!

40 To show the progress...(i) 113 Cd spectrum

41 To show the progress... (II) Approaching 2nu double beta of 116 Cd (T 1/2 =2.9 x10 19 yrs) more than 2 orders of magn. away real one H. Kiel, Ph.D. thesis 2005 now (2009)

42 Region of interest Region of 116 Cd peak

43 Removing the paint... If stable will be done for all remaining detectors

44 Needed Sensitivity T 1/2 M t /ΔE B 50 mev

45 KING COBRA - Below 50 mev Current idea: 40x40x40 CdZnTe detectors = 420 kg, enriched in 116Cd Down to 10 mev you might be... A real time low-energy solar neutrino and Supernova experiment? e Threshold energy: 464 kev ν e 116In e 7Be contribution g.s. alone: 227 SNU 116Cd τ = 14s 116Sn K. Zuber, Phys. Lett. B 571,148 (2003) K Balasi, T.Kosmas, P. Divari,KZ, subm.

46 The solid state TPC Semiconductor Tracker Energy resolution Tracking Massive background reduction Positive signal identification Pixelated CdZnTe detectors

47 Pixelisation Idea: Massive background reduction by particle identification α= 1 pixel, β and ββ= several connected pixel, γ= some disconnected p. 0νββ 3 MeV β α ~15μm 1-1.5mm1.5mm Beta with endpoint 3.3MeV 7.7MeV α life-time = 164.3μs

48 Pixel size Reasonable range ( 130 Te) For 200 μm 130 Te and 116 Cd would hit about 7-10 pixel

49 Pixel detectors Two options: Timepix, CZT ASICs 1024 pixel, pixel size: 625 μm Pixel detector with 200 μm pixels (10000 pixel) produced!

50 Option1: ASICs for CZT 64 pixel detectors Detectors: 2x2x0.5 cm 3 One 32x32 pixels sent for bonding to 1024 channel ASIC Second one 55x55 pixels sent fo bonding to 2048 channel ASIC Installation at LNGS autumn 2009

51 Option2: Timepix 55, 110, 220 μm option, channels (256x256) Simulation: 116Cd neutrinoless decay

52 Option 2: Timepix Timepix Si, 300μm thick (data shown) TimePix CdTe detector 1mm thick, 256x256 Pixel, 55μm pitch, 1.4x1.4 cm 2 1mm Myon candidate

53 snapshot Bi alpha-beta coincidence candidate Unfortunately I can t show you a double beta event...

54 Event pool It reacts on environment, ie. not completely dominated by internal background

55 Particles... Alphas Betas Muons

56 A nice muon...

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

58 Future activities Upgrading LNGS setup ( 64 detectors with clear paint, active vetosignal enhancer in form of scintillator, cooling, pulse shape Installing pixel detector underground (St. Louis) Run CZT in Liquid scintillator (in collab with Hamburg) Get CdTe Timepix running above ground/underground Tune pixel detectors towards low background (material screening) Work toward a Technical Design Report, to be finished in 2012

59 Summary Double beta decay is the gold plated channel to probe the fundamental character of neutrinos Near term experimental goals are driven by the Klapdor claim of a peak in Ge-76 COBRA is a rather new approach making use of CZT room temperature semiconductors First background sources have been identified and removed, working towards larger array (64), lower background The solid state TPC (semiconductor tracker) can be the major and unique step beyond existing double beta experiments

60

61 Neutrino mass schemes and 0νββ Adding errors

62 Effect of Θ 13 in normal hierarchy

63 0νββ-Inverted mass scheme

64 0νββ - Inverted hierarchy

65 Normal + inverted scheme

66 Schechter-Valle theorem

67 Pixellated detectors

68 Oscillations and 0νββ General: Rough estimate:

69 0νββ -Normal hierarchy

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

71 Latest development on CZT preliminary 36 gram detector, 6 times larger than current COBRA detectors

72 Neutrino mass vs. right handed currents H int j L J + L + κj L J + R + ηj R J + L + λj R J + R EC/ß + <λ> λ,η <<1 Possible evidence <m ν >(ev) M. Hirsch et al., Z. Phys. A 347,151 (1994)

73 Nuclear matrix elements The dark side of double beta decay

74 Uncertainties in nuclear matrix elements, example 116 Cd Check with a different isotope <m ν >=0.4eV V. Rodin et al., nucl-th/ , Nucl. Phys. A 2006

75 Current ideas Detectors (CdZnTe) Energy Pixel Shielding Passive Passive and active LSci

76 Nuclear matrix elements Most important: Short range correlations Atomic traps: Ft-values, Q-values F. Simkovic et al., arxiv: Theory Experiment Nuclear deformation D. Frekers, ECT Trento 2008