The EXO-200 Double Beta Decay Experiment and Plans for the Future

David SinclairValday 2014

The EXOCollaboratio

n

University of Alabama, Tuscaloosa AL, USA - D. Auty, T. Didberidze, M. Hughes, A. Piepke, R. TsangUniversity 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. SinclairColorado State University, Fort Collins CO, USA - C. Benitez-Medina, C. Chambers, A. Craycraft, W. Fairbank, Jr., T. WaltonDrexel University, Philadelphia PA, USA - M.J. Dolinski, M.J. Jewell, Y.H. Lin, E. SmithDuke University, Durham NC, USA – P.S. BarbeauIHEP 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. KaufmanUniversity of California, Irvine, Irvine CA, USA - M. MoeITEP Moscow, Russia - D. Akimov, I. Alexandrov, V. Belov, A. Burenkov, M. Danilov, A. Dolgolenko, A. Karelin, A. Kovalenko, A. Kuchenkov, V. Stekhanov, O. ZeldovichLaurentian University, Sudbury ON, Canada - B. Cleveland, J. Farine, B. Mong, U. WichoskiUniversity of Maryland, College Park MD, USA - C. Davis, A. Dobi, C. Hall, S. Slutsky, Y-R. YenUniversity of Massachusetts, Amherst MA, USA - T. Daniels, S. Johnston, K. Kumar, A. Pocar, D. Shy, J.D. WrightUniversity of Seoul, South Korea - D.S. LeonardSLAC 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. WittgenStanford 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. WeberTechnical University of Munich, Garching, Germany - W. Feldmeier, P. Fierlinger, M. MarinoTRIUMF, Vancouver BC, Canada – J. Dilling, R. Krucken, F. Retière, V. Strickland

Outline of talk

• Some thoughts on double beta physics• Description of the EXO-200 Detector• Detection of 2 nbb decay in 136Xe• Limits on 0nbb decay in 136Xe• Plans for the future

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

• 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

AtmosphericMinosT2K

ReactorT2KMinosSolar

LBNE

SolarKAMLAND

0bb n

Pontecorvo Maki Nakagawa Sakata Matrix

(Cosmology favours about 4 but evidence is weakening)

What do we know about mixing angles

• With good accuracy• F12 = 33.8o from solar, kamland

• F23 = 45o from SuperK, Minos…

• F13 = 9o from reactors• d CP phase not known• 1, 2 a a Majorana phases not known

Slide from Yvonne WongTaup 2011

OferLahav

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

• Neutrino masses are very small because of mR in denominator. mR is at the gut scale

• If mL is not zero it can dominate and give degenerate neutrino masses

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

• 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

• 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.)

• G is known, scales with E5

• M is a nuclear matrix element. Calculations are converging (factor of 2)

• m2bb contains neutrino mixing information

Nucl. Phys. B659 359

Dark areasShow 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

Candidate Isotopes

Isotope Energy (keV) Abundance %76Ge 2039. 7.8136Xe 2462 8.9130Te 2530 34.582Se 2996 9.2100Mo 3035 9.6150Nd 3367 5.648Ca 4274 0.19

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

• 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?

• 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

The EXO-200 Detector

Measuring Electron lifetime

EXO-200 has achievedVery long lifetimes

Supports plans for largerDetector

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:

http://link.aps.org/doi/10.1103/PhysRevC.89.015502• DOI: 10.1103/PhysRevC.89.015502

Current state of sourceReproductionThere are no freeParameters except overallnormalization

Moving on to Neutrinoless Decay

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

nEXO at SNOLAB

Water

Cryostat

Detector

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

nEXO Exclusion Limit (90% confidence 4 years)

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?

• 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?

• 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

2P1/2

4D3/2

2S1/2

493nm

650nm

metastable 80s

Requires Ba+ ion

Double beta decay produces Ba++

Extraction of Ions from gas

• Test process using atmospheric pressure electrospray source and a quadrupole mass spec

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

• 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