Outline What is ββ and why study it? Assumes you heard previous talks. General Experimental Issues The New Proposals

Transcription

1 Neutrinoless Double-Beta Decay Experiments Steve Elliott - Los Alamos Nat. Lab. Neutrinos and Implications for Physics Beyond the Standard Model SUNY Stony Brook, Oct , 2002 Outline What is ββ and why study it? Assumes you heard previous talks. General Experimental Issues The New Proposals

2 ββ(2ν) vs. ββ(0ν) ν e e n p n p ν e ν e n p n p ββ(2ν): Allowed and observed 2 nd order weak process. ββ(0ν): requires massive Majorana neutrinos even in presence of alternative mechanisms.

3 ββ Decay Rates Γ ν ν ν 2 = G M 2 Γ 0 = G 0 M 0 m ν ν ν ν G are calculable phase space factors. G 0ν ~ Q 5 M are nuclear physics matrix elements. Hard to calculate. Present estimate of uncertainty is not well founded. m ν is where the interesting physics lies.

4 Neutrino Masses Direct mass and ββ experiments set absolute mass scale. <m β > < 3 ev The results of oscillation experiments set the relative mass scale. Sets m ν scale > 50 mev ββ also addresses Dirac/Majorana nature of ν.

5 3 Complementary Experimental Techniques ββ Absolute Mass Scale Relative Mass Scale Mixing Matrix Elements β Oscil. CP nature of ν Need all three types of experiments to fully understand the nature of the ν.

6 What about mixing, m ν & ββ(0ν)? No mixing: mββ = mν e = m ββ ei i ε i i= 1 m = U m virtual ν exchange ε = ±1, CP cons. Compare to β decay result: real ν emission 2 2 i m = β Uei m 3 i= 1

7 An exciting time for ββ! For at least one neutrino: m > δm 2 50meV i atmos For the next experiments: mββ 50meV < m ββ > in the range of mev is very interesting.

8 Classes of Background ββ(2ν) tail Need good energy resolution. Natural U, Th in source and shielding Pure materials, identify ββ daughter, pulse shape, timing, position. Cosmic ray activation Store and prepare materials underground.

9 An Ideal Experiment Maximize Rate/Minimize Background b E m ββ Mt live 1 4 Large Mass (~ 1 ton) Good source radiopurity Demonstrated technology Natural isotope Small volume, source = detector Good energy resolution Ease of operation Large Q value, fast ββ(0ν) Slow ββ(2ν) rate Identify daughter Event reconstruction Nuclear theory

10 ββ(2ν) as a Background. Sum Energy Cut Only next generation experimental goal S B 2ν me τ1/ 2 = 7 Q 0ν δ τ 1/ 2 6 dn/d(k e /Q) x K e /Q <m ββ > Sensitivity(meV) Mo 136 Xe 76 Ge 130 Te K e /Q Resolution (%) 4 5

11 Natural Activity The Problem: τ(u, Th) ~ 10 9 years Target: τ(ββ(0ν) ~ years Detector Shielding Cryostat, or other experimental support Front End Electronics etc.

12 Cosmic Ray Induced Activity Material dependent. Need for depth to avoid activation. Need for storage to allow activation to decay.

13 A Great Number of Proposed Experiments COBRA DCBA NEMO CAMEO CANDLES CUORE EXO GEM GENIUS GSO Majorana MOON Xe XMASS Te-130 Nd-150 Mo-100, Various Cd-114 Ca-48 Te-130 Xe-136 Ge-76 Ge-76 Gd-160 Ge-76 Mo-100 Xe-136 Xe kg CdTe semiconductors 20 kg Nd layers between tracking chambers 10 kg of ββ isotopes (7 kg of Mo) 1 t CdWO 4 crystals Several tons CaF 2 crystals in liquid scint. 750 kg TeO 2 bolometers 1 ton Xe TPC (gas or liquid) 1 ton Ge diodes in liquid nitrogen 1 ton Ge diodes in liquid nitrogen 2 t Gd 2 SiO 5 :Ce crystal scint. in liquid scint. 500 kg Ge diodes Mo sheets between plastic scint., or liq. scint t of Xe in liq. Scint. 10 t of liquid Xe

14 Present Experimental Limits m ee 1 M G 0ν τ 1/ 2 Ge (IGEX) NP of RAS 63, 1299 (2000) Ge (Heid-Mosc) Dark Matter 2000 Mo (ELEGANTS) NP A611, 85 (1996) Te-130 (Cuoricino) PL B486, 13 (2000) Te-128 (Geochem) PR C47, 806 (1993) Xe (Gotthard) PL B 434, 407 (1998) Half Life <m ee > 160 x y ~330 mev 190 x y ~300 mev 0.52 x y ~6600 mev 1.44 x y ~1700 mev 6.9 x y ~1100 mev 4.4 x y ~2500 mev

15 20 The Controversy Counts 10 Locations of claimed peaks Counts Energy (kev) Energy (kev) 2060 Mod. Phys. Lett. A16, 2409 (2001) If one had to summarize the controversy in a short statement: Consider two extreme background models: 1. Entirely flat in kev region. 2. Many peaks in larger region, only ββ peak in small region. These 2 extremes give very different significances for peak at 2039 kev. KDHK chose Model 2 but did not consider a systematic uncertainty associated with that choice. 2080

16 Found Peaks: These experiments are hard PR D45, 2548 (1992) Need more than one experiment A 2527-keV Ge-det. peak that was an electronic artifact. A ~2528-keV Te-det. peak that was a 2σ Statisticalflucuation. NP B35 (Proc. Supp.), 366 (1994).

17 Potential Large-Mass Future Experiments Experiments in Europe/Japan CUORE, GENIUS, XMASS NUSL Candidates EXO, MAJORANA, MOON Smaller Mass Experiments Cobra, DCBA, NEMO, CAMEO, CANDLES, GSO

18 Molybdenum Observatory Of Neutrinos - MOON U. of Washington U. of North Carolina U. of Wisconsin Research Center for Nuclear Physics, Osaka Plus others as collaboration is forming. Spokesperson Hiro Ejiri RCNP

19 MOON Overview 3.3 tons 100 Mo, 34 tons Mo Doesn t require enriched material (but would want it). Scintillator/source sandwich Position and single E β data play big role in ββ(2ν) and U, Th rejection. 14% efficiency ELEGANTS is precursor.

20 MOON Scintillator A liquid scintillator option is also being considered.

21 Cryogenic Underground Observatory for Rare Events - CUORE Berkeley Firenze Gran Sasso Insubria (COMO) Leiden Milano Neuchatel U. of South Carolina Zaragoza Spokesperson Ettore Fiorini Milano

22 CUORE Overview 0.21 ton, 34% natural abund. 130 Te TeO 2 bolometers, 750 g crystals Doesn t require enriched material x5x5 cm 3 crystals 17 towers of 15 modules of 4 crystals Gran Sasso Laboratory CUORICINO begins operating this fall (45 kg).

23 CUORE Detector Detector Damping Suspension Dilution Unit Thermal Shields

24 Enriched Xenon Observatory - EXO U. of Alabama Caltech IBM Almaden ITEP Moscow U. of Neuchatel INFN Padova SLAC Stanford U. U. of Torino U. of Trieste WIPP Carlsbad Spokesperson Giorgio Gratta Stanford

25 EXO Overview 10 ton, ~70% enriched 136 Xe 70% effic., ~10 atm gas TPC or LXe chamber Optical identification of Ba ion. Drift ion in gas to laser path or extract on cold probe to trap. TPC performance similar to that at Gottard. 100-kg enr Xe prototype (no Ba ID) Isotope in hand

26 Stanford Optics Lab with Ba Trap Ba Trap 136 Xe -> 136 Ba ++ e - e - -> 136 Ba + Optically observe final state. (Moe, PRC44 (1991) 931

27 EXO Single Ion Observation

28 Two Ge Detector Background Philosophies Internal Background External Background ββ β+ <Use pulse shape & segmentation to eliminate background. ββ >Minimize the materials near the crystal

29 Ge expts.: Different Approaches Suppressing constants: Majorana Approach If no signal seen: T 1 2 > If background, B~MT (i.e. within Ge mass), then: T 1 2 > MT Leads to emphasis on enrichment balanced mass and time background rejection B MT MT T1 2 = mν e = C 1 T 1 2 GENIUS Approach If we assume background is in structural materials only, B=bT then: T 1 2 > M T b Leads to emphasis on minimized structure increased natural mass decreased time

30 GErmanium NItrogen Underground Setup - GENIUS MPI, Heidelberg Kurchatov Inst., Moscow Inst. Of Radiophysical Research, Nishnij Novgorod Braunschweig und Technische Universität, Braunschweig U. of L'Aquila, Italy Int. Center for Theor. Physics, Trieste Spokesperson JINR, Dubna Hans Klapdor-Kleingrothaus MPI Northeastern U., Boston U. of Maryland, USA University of Valencia, Spain Texas A & M U. GENIUS

31 GENIUS Overview 1 ton, ~86% enriched 76 Ge Naked Ge crystals in LN Very little material near Ge. 1.4x10 6 liters LN 40 kg test facility is approved. 100% efficient Heid.-Moscow experiment is precursor GENIUS

32 GENIUS Layout Data Acquisition Clean Room Insulation Liquid Nitrogen Steel Vessel 12 m Setup for operation of three 'naked Germanium detectors in liquid nitrogen. GENIUS

33 The Majorana Project Duke U. North Carolina State U. TUNL Argonne Nat. Lab. JINR, Dubna ITEP, Moscow New Mexico State U. Pacific Northwest Nat. Lab. U. of Washington LANL U. of South Carolina Brown Univ. of Chicago RCNP, Osaka Univ. Univ. of Tenn. Co-Spokespersons Frank Avignone Harry Miley

34 Majorana Overview 0.5 ton of 86% enriched 76 Ge Segmented detectors using pulse shape discrimination to improve background rejection. Prototypes being assembled. (18 crystals array, 1 enriched segmented detector) 100% efficient Can do excited state decay. IGEX is precursor

35 Majorana Layout

36 Recent Majorana pictures

37 Summary of Proposals Proposed ton-year = M * T * ε Anticipated <m ee >, (QRPA) CUORE 0.21*5*1 = 1 60 mev EXO 6.5*10*0.7 = mev GENIUS 1*2*1 = 2 20 mev MAJORANA 0.5*10*1 = 5 25 mev MOON 3.3*3*0.14 = mev The <m ββ > limits depend on background assumptions and matrix elements which vary from proposal to proposal.

38 Conclusions Research and Development are needed for all the future proposed detectors. UG Lab space will be needed for some of that R&D. The next generation ββ experiments have a good possibility of reaching an interesting <m ββ > region.