Z.Djurcic, D.Leonard, A.Piepke Physics Dept, University of Alabama, Tuscaloosa AL P.Vogel Physics Dept Caltech, Pasadena CA A. Bellerive, M. Dixit, C. Hargrove, D. Sinclair Carleton University, Ottawa, Canada W.Fairbank Jr., S.Jeng, K.Hall Colorado State University, Fort Collins CO M.Moe Physics Dept UC Irvine, Irvine CA D.Akimov, A.Burenkov, M.Danilov, A.Dolgolenko, A.Kovalenko, D.Kovalenko, G.Smirnov, V.Stekhanov ITEP Moscow, Russia J. Farine, D. Hallman, C. Virtue Laurentian University, Canada M.Hauger, F.Juget, L.Ounalli, D.Schenker, J-L.Vuilleumier, J-M.Vuilleumier, P.Weber Physics Dept University of Neuchatel, Neuchatel Switzerland M.Breidenbach, R.Conley, C.Hall, A.Odian, C.Prescott, P.Rowson, J.Sevilla, K.Skarpaas, K.Wamba, SLAC, Menlo Park CA E.Conti, R.DeVoe, G.Gratta, M.Green, T.Koffas, R.Leon, F.LePort, R.Neilson, S.Waldman, J.Wodin Physics Dept Stanford University, Stanford CA Enriched Xenon Observatory for double beta decay

Last decade: the age of ν physics Discovery of ν flavor change: Solar neutrinos (MSW effect) Reactor neutrinos (vacuum oscillation) Atmospheric neutrinos (vacuum oscillation) Loose ends: LSND results So assuming that MiniBoone sees no oscillations, we know that: ν masses are non-zero, There are 2.981±0.008 v (Z lineshape), 3 ν flavors were active in Big bang Nucleosynthesis

Drastically different mass scenarios are still allowed by the data ~2.8 eV From tritium endpoint (Maintz and Troitsk) ~0.3 eV From 0νββ if ν is Majorana ~1 eV From WMAP Time of flight from SN1987A (PDG 2002) 23 eV No real understanding why M so small Dirac vs Majorana neutrinos? Need a lepton number violating process…

 decay can occur in two modes a)Via the emission of 2 ’s: A typical 2 nd order nuclear process b) A neutrinoless mode: Requires both M  0 and 

 decay a standard but small 2 nd order correction to regular  decay BUT in some cases is the only energetically allowed 2  has been observed experimentally 0  has never been observed experimentally 0  sensitive to all neutrino masses

For 0  decay due to light Majorana ν masses: where, the effective Majorana neutrino mass, and nuclear matrix elements that can be calculated, a known phase-space factor, the half-life time to be measured

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) Detection of 0νββ Decay The two e - energy sum is the primary tool from S.R. Elliott and P. Vogel, Ann.Rev.Nucl.Part.Sci. 52 (2002) 115.

For further substantial progress we need tons of an appropriate isotope exposed for a long time BUT there are problems In a bkgnd free environment mass sensitivity scales as If bkgnd scales like Nt mass sensitivity scales as Qualitatively new means are needed to suppress bkgnds and fully utilize the large fiducial mass

Xe is ideal for such a measurement It is one of the easiest isotopes to enrich; Like argon, it represents a good ionization detecting medium; It exhibits substantial scintillation that can be used to complement the ionization detection; Can be re-purified during the experiment; No long lived Xe isotopes to activate; Its  decay results in 136 Ba that can be identified in its atomic form via techniques of high resolution optical spectroscopy.

2 P 1/2 4 D 3/2 2 S 1/2 493nm 650nm metastable 47s Optical detection of a 136 Ba + atom (M. Moe PRC44(1991)931) Resonant laser detection is: Highly sensitive, yielding >10 7 photons per atom; Highly selective; Extensively used in the atomic physics community. Provides additional constraint Huge bkgnd reduction pump probe

Detector R&D Program Single ion Ba + tagging at different Xe pressures; LXe energy resolution; LXe purification for long e - lifetime and radioimpurities; Ba ion lifetime and grabbing from LXe; 136 Xe Isotopic enrichment; Procurement and characterization of low radioactivity materials; Construction/operation of a 200kg enriched 136 Xe prototype detector;

EXO spectroscopy lab Ion Trap 493nm laser 650nm laser

CCD Image of Ba + ions in the trap Trap edge

Zero ion background All above in UHV; Perform the same experiment in noble gas atmosphere Millikan type experiment with the ion trap

1Lt Test Chamber PMT Ionization Readout Preamplifier Energy Resolution Measurement Setup Reconstruct energy as linear combination of ionization and scintillation signals There are indications that correlations between the two variables help improve energy resolution; J.Seguinot et al. NIM A354 (1995) 280

Clear anti-correlation on event-to-event basis is seen… A linear combination of ionization and scintillation WILL optimize resolution

First EXO published result in Phys.Rev.B Resolution is optimized by a ~ ‘mixing angle’ Ionization only Ionization combined with scintillation E.Conti et al Phys. Rev. B 68 (2003) or

Xe purification studies-Continuous Xe Recirculation

This is already the largest non-fissile isotope enrichment program ever entertained! First 200 kg pilot production started in the Summer of 2001 and was successfully completed in May 2003 Xe leak monitoring station

200kg 136 Xe Prototype is an important step Need to test the detector technology, particularly the LXe option; Essential to understand backgrounds from radioactivity; Necessary to measure the 2  “background” mode; Test the production logistics and quality of 136 Xe; 2000kg of natural Xe are already available by our collaborators at ITEP; Already a respectable (20x)  decay experiment (no Ba-ion tagging at this stage);

Detector ~ 60 liters enriched liquid 136 Xe, –In low background teflon vessel –Surrounded and shielded by ~50 cm radially low background thermal transfer fluid –Contained in a low background Cu double walled vacuum insulated cryostat –Shielded by ~ 5 cm very low background Pb –Further shielded by ~20 cm low background Pb –Located ~800 m below ground in NaCl deposit – WIPP in Carlsbad, New Mexico. Detector is a liquid TPC with photo-detectors to provide start time and improve energy resolution of the β’s.

Detector 2D Detector schematic APD plane below crossed wire array

Cryostat Cross Section Condenser Xenon Heater should be on this area Xenon Chamber Support Outer Copper Vessel Inner Copper Vessel FC-87 1” thick Thermal Insulation (MLI- vacuum), not shown to scale Outer Door Inner Door Xenon Chamber Refrigerant feedthroughs Heat Transfer Fluid In/Out

Detector Full View

Simplified xenon handling system diagram

An experimental facility for EXO WIPP : Waste Isolation Pilot Plant Carlsbad NM

Enriched Xe in hand. Clean rooms in commercial production. WIPP agreement, including Environmental Impact, complete. Swiss collaborators building cryostat. Xe purification and refrigeration issues through R&D, purchasing of components. Detector vessel, readout, and electronics being engineered. EXO could be in WIPP by Summer 2005, if technically limited. Status

Assumptions: 1)200kg of Xe enriched to 80% in 136 2)σ(E)/E = 1.6% obtained in EXO R&D, Conti et al Phys Rev B 68 (2003) )Low but finite radioactive background: 20 events/year in the ±2σ interval centered around the 2.481MeV endpoint 20 events/year in the ±2σ interval centered around the 2.481MeV endpoint 4)Negligible background from 2νββ (T 1/2 >1·10 22 yr R.Bernabei et al. measurement) EXO 200kg prototype mass sensitivity CaseMass (ton) Eff. (%) Run Time (yr) σ E 2.5MeV (%) Radioactive Background (events) T 1/2 0ν (yr, 90%CL) Majorana mass (eV) QRPA (NSM) Prototype * 406.4* (0.53)

Assumptions: 1)80% enrichment in 136 2)Intrinsic low background + Ba tagging eliminate all radioactive background 3)Energy res only used to separate the 0ν from 2ν modes: Select 0ν events in a ±2σ interval centered around the 2.481MeV endpoint Select 0ν events in a ±2σ interval centered around the 2.481MeV endpoint 4)Use for 2νββ T 1/2 >1·10 22 yr (Bernabei et al. measurement) *  (E)/E = 1.6% obtained in EXO R&D, Conti et al Phys Rev B 68 (2003) †  (E)/E = 1.0% considered as an aggressive but realistic guess with large light collection area collection area ‡ QRPA: A.Staudt et al. Europhys. Lett.13 (1990) 31; Phys. Lett. B268 (1991) 312 # NSM: E.Caurier et al. Phys Rev Lett 77 (1996) 1954 EXO neutrino effective mass sensitivity CaseMass (ton) Eff. (%) Run Time (yr) σ E 2.5MeV (%) 2νββ Background (events) T 1/2 0ν (yr, 90%CL) Majorana mass (meV) QRPA ‡ (NSM) # Conserva tive * 0.5 (use 1)2* (95) Aggressi ve †1† 0.7 (use 1)4.1* (21)