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The EXO-200 Double Beta Decay Experiment and Plans for the Future. David Sinclair Valday 2014. The EXO Collaboration. University of Alabama, Tuscaloosa AL, USA - D. Auty, T. Didberidze, M. Hughes, A. Piepke, R. Tsang

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University of Alabama, Tuscaloosa AL, USA - D. Auty, T. Didberidze, M. Hughes, A. Piepke, R. Tsang

University of Bern, Switzerland - S. Delaquis, G. Giroux, R. Gornea, T. Tolba, J-L. Vuilleumier

California Institute of Technology, Pasadena CA, USA - P. Vogel

Carleton University, Ottawa ON, Canada - V. Basque, M. Dunford, K. Graham, C. Hargrove, R. Killick, T. Koffas, F. Leonard, C. Licciardi, M.P. Rozo, D. Sinclair

Colorado State University, Fort Collins CO, USA - C. Benitez-Medina, C. Chambers, A. Craycraft, W. Fairbank, Jr., T. Walton

Drexel University, Philadelphia PA, USA - M.J. Dolinski, M.J. Jewell, Y.H. Lin, E. Smith

Duke University, Durham NC, USA – P.S. Barbeau

IHEP Beijing, People’s Republic of China - G. Cao, X. Jiang, L. Wen, Y. Zhao

University of Illinois, Urbana-Champaign IL, USA - D. Beck, M. Coon, J. Ling, M. Tarka, J. Walton, L. Yang

Indiana University, Bloomington IN, USA - J. Albert, S. Daugherty, T. Johnson, L.J. Kaufman

University of California, Irvine, Irvine CA, USA - M. Moe

ITEP Moscow, Russia - D. Akimov, I. Alexandrov, V. Belov, A. Burenkov, M. Danilov, A. Dolgolenko, A. Karelin, A. Kovalenko, A. Kuchenkov, V. Stekhanov, O. Zeldovich

Laurentian University, Sudbury ON, Canada - B. Cleveland, J. Farine, B. Mong, U. Wichoski

University of Maryland, College Park MD, USA - C. Davis, A. Dobi, C. Hall, S. Slutsky, Y-R. Yen

University of Massachusetts, Amherst MA, USA - T. Daniels, S. Johnston, K. Kumar, A. Pocar, D. Shy, J.D. Wright

University of Seoul, South Korea - D.S. Leonard

SLAC National Accelerator Laboratory, Menlo Park CA, USA - M. Breidenbach, R. Conley, A. Dragone, K. Fouts, R. Herbst, S. Herrin, A. Johnson, R. MacLellan, K. Nishimura, A. Odian, C.Y. Prescott, P.C. Rowson, J.J. Russell, K. Skarpaas, M. Swift, A. Waite, M. Wittgen

Stanford University, Stanford CA, USA - J. Bonatt, T. Brunner, J. Chaves, J. Davis, R. DeVoe, D. Fudenberg, G. Gratta, S.Kravitz, D. Moore, I. Ostrovskiy, A. Rivas, A. Schubert, D. Tosi, K. Twelker, M. Weber

Technical University of Munich, Garching, Germany - W. Feldmeier, P. Fierlinger, M. Marino

TRIUMF, Vancouver BC, Canada – J. Dilling, R. Krucken, F. Retière, V. Strickland

outline of talk
Outline of talk
  • Some thoughts on double beta physics
  • Description of the EXO-200 Detector
  • Detection of 2nbb decay in 136Xe
  • Limits on 0nbb decay in 136Xe
  • Plans for the future
2 neutrino double beta decay
2 Neutrino Double Beta Decay
  • Nemo has done a great job of measuring most of the 2 neutrino double beta decay rates
  • 136Xe is an exception because NEMO cannot use a gas source
  • Earlier work suggested limits on the 136Xe rate which would make it exceptionally slow
physics of double beta decay
Physics of double beta decay
  • Understanding Neutrinoless DBD is closely coupled to understanding neutrino masses and mixing
  • We therefore make a diversion to look at what we know
assuming 3 families
Assuming 3 families

(Cosmology favours about 4 but evidence is weakening)

Pontecorvo Maki Nakagawa Sakata Matrix











bb 0n

what do we know about mixing angles
What do we know about mixing angles
  • With good accuracy
  • F12 = 33.8ofrom solar, kamland
  • F23 = 45o from SuperK, Minos…
  • F13= 9o from reactors
  • d CP phase not known
  • a1, a2 Majorana phases not known



neutrino mass in the standard model
Neutrino mass in the Standard Model
  • In the standard model neutrino masses are 0
  • Because we only observe left handed neutrinos we cannot form a Dirac mass term this way
  • Possible to form a Majorana mass term
seesaw model
Seesaw Model
  • Neutrino masses are very small because of mR in denominator. mRis at the gut scale
  • If mL is not zero it can dominate and give degenerate neutrino masses
neutrinos and leptogenesis
Neutrinos and Leptogenesis
  • The only neutrinos which can impact the baryon asymmetry are the very heavy right handed neutrinos
  • We would like to understand CP violation in this sector
  • This is far beyond the reach of experimental physics
  • May be related to CP violation in light sector
    • See e.g. Pascoli, Petcov and Riotto, CERN-PH-TH/2006-213
  • This can come from either Dirac CP term d or from the Majorana phases a or both
what would we like to learn about neutrinos
What would we like to learn about neutrinos
  • Determine the mass hierarchy critical
  • Determine d
  • Are neutrinos Majorana
  • Determine the a parameters
  • Show violation of total lepton number
neutrino less double beta decay
Neutrino-less double beta decay
  • Observation of neutrino-less double beta decay would
    • Demonstrate that neutrinos are Majorana particles
    • Demonstrate DL=2 total lepton number violating process
    • Set mass scale for the neutrino
  • Rate is given by
double beta cont
Double Beta (cont.)
  • G is known, scales with E5
  • M is a nuclear matrix element. Calculations are converging (factor of 2)
  • m2bbcontains neutrino mixing information

Nucl. Phys. B659 359

Dark areas

Show variation due to phases only

Light colours include experimental errors

Assumed q13 =0


Klapdor-Kleingrothaus Results for Ge

double beta decay

57 kg years of 76Ge data

Apply single site criterion

exo 200
EXO 200
  • Tracking Liquid TPC
  • 200 kg enriched 136Xe
  • Ionization + scintilation
  • No gain in ionization channel – demanding on electronics
  • Lead shield + HFE (heat transfer fluid)
why xenon
Why Xenon
  • Favourable Q value
  • Easy to make very pure
  • Easiest (least expensive!) isotope to produce
  • Possibility of background control through tagging of daughter
what form to use
What form to use?
  • Gas (eg NEXT, Gotthard)
    • Excellent energy resolution
    • Good tracking
    • Detector is large so shielding is more challenging
  • Liquid Scintillator
    • Refer to Kozlov’s talk
  • Liquid Xenon
    • Compact, reasonable resolution, event reconstruction

EXO-200 has achieved

Very long lifetimes

Supports plans for larger


new analysis out this week
New Analysis out this week
  • After a lot of work to fully understand the detector response a more precise value has been obtained.
  • T1/2 = 2.172 +-0.017 (stat) +-0.060 (syst)x1021 y
  • Most precisely measured 2 neutrino double beta decay rate to date
  • Possible because of the homogeneous detector design
  • URL:
  • DOI: 10.1103/PhysRevC.89.015502

Current state of source


There are no free

Parameters except overall


exo future
EXO Future
  • Next step will be nEXO
  • 5 T liquid xenon enriched in 136Xe
  • Location likely to be SNOLAB
  • 5T is chosen as the mass required to cover the inverted hierarchy
  • Replace lead with large water shield





some changes from exo 200
Some changes from EXO-200
  • Need internal electronics to cut noise
  • Have to deal with heat
  • Go to single ended TPC design to give maximum self-shielded fiducial mass
the big challenge
The Big Challenge
  • The biggest challenge for the project will be securing 5 T of enriched 136Xe
  • Russia is the only country that has the capability of producing such an enormous amount of isotopically separated material
  • We need to look at this project as a global endeavor
tpc or scintillator
TPC or Scintillator?
  • Scintillator can proceed with minor changes to existing detector
  • Good self shielding from clean scintillator
  • Great detector for exclusion limit
  • TPC has better energy resolution (we aim for 1%)
  • TPC gives more handles to discriminate against backgrounds
  • Probably better ability to make discovery
can we reach the normal hierarchy
Can we reach the normal hierarchy?
  • Need to control even better the backgrounds
  • We may be able to tag events with the production of 136Ba
  • Process involves extraction of the Ba ion from xenon, trapping it, and identification by laser spectroscopy
barium tagging
Barium tagging

Requires Ba+ ion

Double beta decay produces Ba++





metastable 80s


extraction of ions from gas
Extraction of Ions from gas
  • Test process using atmospheric pressure electrospray source and a quadrupole mass spec
conversion from ba to ba
Conversion from Ba++ to Ba+
  • Pass ions through low pressure TEA
  • TEA has low IP and can give up an electron to Ba++ but not to Ba+
  • Use triple quadrupole system. First quad selects Ba++, second contains the TEA, third analyses the products
  • Conversion efficiency looks very high and no evidence for molecular formation
timescale for next phase
Timescale for Next Phase
  • EXO is taking 0 neutrino search data now
  • Will probably reach background limit in couple of years
  • DOE has indicated it wants to make a decision on next generation detector in ~ 2 years
  • We need to have a developed proposal on this timescale