Charles Prescott - SLAC Neutrino Day - April 18, 2003 The Search for Neutrinoless Double Beta Decay - Physics Motivations - Limits from ν oscillations - Backgrounds - Experiments - Future Prospects

Charles Prescott - SLAC Neutrino Day - April 18, 2003 from S.R. Elliott and P. Vogel, Ann. Rev. Nucl. Part. Sci. 52 (2002) “Moore’s Law” for Double Beta Decay Range suggested by atmospheric oscillation Range suggested by atmospheric oscillation data

Charles Prescott - SLAC Neutrino Day - April 18, 2003 A brief digression into nuclear physics Consider a nucleus of Z protons and N neutrons with A = Z+N Two spin ½ protons like to pair up…likewise for neutrons The energy levels for nuclear isobars (A = constant) are illustrated below for A=124 (ex: 124 Te 52 ) This pattern is repeated many times in the periodic chart. Proton richNeutron rich From Physics of the Nucleus, Preston (1962) -- ++ forbidden Double beta decay odd-odd even-even even-odd

Charles Prescott - SLAC Neutrino Day - April 18, Ca → 48 Ti Ge → 76 Se Se → 82 Kr Zr → 96 Mo Mo → 100 Ru Pd → 110 Cd Cd → 116 Sn Sn → 124 Te Te → 130 Xe Xe → 136 Ba Nd → 150 Sm Candidate Q Abund. (MeV) (%) (MeV) (%) Candidate Nuclei for Double Beta Decay Issues include : Q value (10 th or 11 th power dependence), abundance, ease of purification (chemical and isotopic), radioactivity (incl. cosmogenesis), & experimental ease of use.

Charles Prescott - SLAC Neutrino Day - April 18, 2003 Process a) occurs in the Standard model: Doubly weak process Process b) only proceeds : If ’s are their own antiparticles (Majorana) If the ’s are massive 4-body decay continuous spectrum for e  energy sum 2-body decay e  energy sum is at the max. (Q) Nuclear Double Beta Decay Assuming the neutrino has no right-handed couplings then 0  decay rate ~ 2. 0  decay does not conserve lepton number

Charles Prescott - SLAC Neutrino Day - April 18, 2003 Effective Majorana mass Thus cannot be directly related to information from oscillations...but only indirectly related within specific models... giving a range of values ii measured half-life phase space factor nuclear matrix elements which in principal differs from the mass mixing for oscillations by the phases Experiments measure or limit the rate for 0  events, which can be related to

Charles Prescott - SLAC Neutrino Day - April 18, % C.L. ranges from all data : ~(1 - 4 meV) normal hierarchy ~( meV) inverted hierarchy {S.R. Elliott and P.Vogel, Ann.Rev.Nucl.Part.Sci. 52 (2002) } Normal Hierarchy Inverted Hierarchy Neutrino Oscillation Results large mixing, N=3

Charles Prescott - SLAC Neutrino Day - April 18, 2003 Other possibilities Quasi-degenerate model: ____________  m sol  m atm m0m0 sterile neutrinos ? non-Standard Model couplings lepton number violating RH currents SUSY. “Standard Model” SUSY examples Conclusions: (1)m eff related to number of 0  events only under a limited set of assumptions (2) very little guidance from theory; very little guidance from oscillations data (3) 0  an “absolutely central” experimental issue 

Charles Prescott - SLAC Neutrino Day - April 18, 2003 DOE Office of Science Facilities Roadmap for the next 20 years December 2002 Ray Orbach (Office of Science) asks its advisory boards for advice on FACILITIES for the next 20 years High Energy Physics – HEPAP Nuclear Physics - NSAC Basic Energy Sciences – BESAC Fusion Energy - FESAC Advanced Scientific Computing – ASCAC Criteria: - new or upgraded facilities - forefront science - > 50 M$ Charge - to classify according to: (1) Importance of the science - “absolutely central; important; or don’t know enough yet” (2) Readiness – ready to go; significant scientific/engineering challenges remain; not yet fully defined”

Charles Prescott - SLAC Neutrino Day - April 18, 2003 EXO

Charles Prescott - SLAC Neutrino Day - April 18, 2003 Majorana

Charles Prescott - SLAC Neutrino Day - April 18, 2003 Detection of 0  Decay The two e  energy sum is the primary tool In this rare decay search, superb E resolution is essential for bkgrd. control, particularly bkgrd. due to the Standard Model 2  decay. 2  spectrum (normalized to 1) 0  peak (5% FWHM) (normalized to ) 0  peak (5% FWHM) (normalized to ) Summed electron energy in units of the kinematic endpoint (Q) Important issue : 2  rate must be determined. (A smaller 2  : 0  rate ratio is experimentally favorable.)

Charles Prescott - SLAC Neutrino Day - April 18, 2003 Examples of backgrounds Generic underground detector Rock Detector materials (stainless, welds,etc.) Glass (U) U Th Rn Cosmic rays Examples of experiments limited by backgrounds Gotthard ( 136 Xe gas TPC kg at 5 atms) R. Luescher et al., PL B 434, 407 (1998) T 1/2 >4.4x10 23 yr (90% cl) m < eV (model dependent) neutrons

Charles Prescott - SLAC Neutrino Day - April 18, 2003 Heidelberg-Moscow 11 Kg 76 Ge – 5 p-type HPGe detectors - for ~4 years (in Gran Sasso) 2  0  hep-ph/ March 2001 T 1/2 > y (90% cl) m < eV

Charles Prescott - SLAC Neutrino Day - April 18,

Charles Prescott - SLAC Neutrino Day - April 18,

Charles Prescott - SLAC Neutrino Day - April 18, 2003

EXO - Enriched Xenon Observatory The basic concept, shown here for a LXe option, is : Use ionization and scintillation light in the TPC to determine the event location, and to do precise calorimetry. Extract the Barium ion from the event location (electrostatic probe eg.) Deliver the Barium to a laser system for Ba 136 identification. Compact and scalable (3 m 3 for 10 tons).

Charles Prescott - SLAC Neutrino Day - April 18, 2003 Background reduction by coincidence measurement 136 Xe  136 Ba ++ + e – + e – Identify event-by-event Barium detection from 136 Xe decay Described in 1991 by M. Moe (PRC, 44, R931,(1991)). The method exploits the well-studied spectroscopy of Ba and the demonstrated sensitivity to a single Ba + ion in an ion trap. SLAC R&D Liquid Xenon test cell with probe tip

Charles Prescott - SLAC Neutrino Day - April 18, 2003 CCD image of an single ion in the trap trap edge Identification of 136 Ba with red and blue lasers

Charles Prescott - SLAC Neutrino Day - April 18, 2003 Isotopic enrichment for a gaseous Xe is most economically achieved by ultracentrifugation 136 Xe, being the heaviest Xe isotope, is particularly easy to separate. The separation step that rejects the light fraction is also very effective in removing 85 Kr (T 1/2 =10.7 yr) that is constantly produced by fission in nuclear reactors. Large facilities exist In Russia – 500,000 centrifuges per plant. EXO now has 100 kg of enriched Xenon, with another 100 kg due in March.

Charles Prescott - SLAC Neutrino Day - April 18, 2003 WIPP : Waste Isolation Pilot Plant Carlsbad NM Excavated in underground salt – lower U/Th activity. ~2,000 m.w.e. depth An experimental facility for EXO

Charles Prescott - SLAC Neutrino Day - April 18, 2003 EXO Performance Projections Building on previous Xe-based experiments, and using the available nuclear physics calcs. R&D efforts are underway. If successful, EXO should reach a sensitivity of 10’s of meV

Charles Prescott - SLAC Neutrino Day - April 18, 2003 Question : Comparison with other potential experiments ? Ge 7 6 Te 130 Xe 136 Mo y Especially for the ton scale proposals, background control is critical, and will be a limiting factor for all experiments – only EXO proposes a qualitatively different approach to bkgrnd control. Xe also has advantages for bulk enrichment/purity. The sought-for lifetimes do not differ by much from exp. to exp. The Ge 76 exps. offer good E resol. but nevertheless must improve bkgrd by factor >1000.