Double beta decay and EXO - SLAC

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Status of the EXO double beta decay project Andreas Piepke for the EXO Collaboration University of Alabama

Transcript of Double beta decay and EXO - SLAC

Status of the EXO double beta decay project

Andreas Piepke for the EXO Collaboration University of Alabama

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Double beta research is relevant for:• particle physics• cosmology

No further elaboration possible in a 15 min talk!

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Double beta decay:

Simultaneous decay of two neutrons into two protons viavirtual intermediate nucleus.

Neutrinoless mode:

2n → 2p + 2e- Mediated by exchange of light neutrinos or super symmetric particles. Requires ν to be Majoranaparticles. For neutrino mass mode decay rateproportional to squared average ν mass.

Two neutrino mode:

2n → 2p + 2e- + 2νe Very rare second order decay, allowed in standard model. Observed for 10 nuclides but not 136Xe. e- have continuous energy distribution.

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Decay modes distinguished by measurement of electron sum energy.

Left: expected spectrafor 200 kg 136Xe inone year.

Right: leakage ofββ2ν-events intoββ0ν−analysis interval.5.8th power!

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EXO goals:

• Probe degenerate and inversely hierarchical mass scenariosfor Majorana neutrinos.

• Use high resolution tracking detection of decay electrons andfinal state (Ba) tagging for unambiguous ββ−decay signature.

• If decay is observed, compare to yield determined by otherexperiments to exclude unaccounted background andtest consistency of nuclear matrix elements.

136Xe → 136Ba++ + 2e- (+ 2νe)

detect the 2 electrons (ionization + scintillation in xenon detector)

Xe TPC

e-e

-e-

e-e

-

other Ba+ identification strategies are also being investigated within the EXO collaboration

EXO detection strategy

positively identify daughter via optical spectroscopy of Ba+

[M. Moe, Phys. Rev. C 44 (1991) R931]

CCD observe single ion

ion “tip”

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EXO’s two parallel research thrusts:

EXO-200 uses 200 kg of isotopically enriched 136Xe.Demonstrate sufficient energy resolution, background,test Heidelberg claim for neutrinoless decay and measuretwo neutrino decay mode. Use simultaneous detection ofcharge and scintillation light. Light read-out via APDs. No Ba tagging.Large next generation double beta decay experiment inadvanced state of assembly at WIPP (NM).Development of single Ba ion detection using laser pumping,Ba retrieval. A lab demonstrator is being set up to quantifyall efficiencies.If required by physics build a large 1 to 10 ton experimentcombining both aspects.

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EXO-200:An intermediate detector without Ba tagging using 200 kg liquid xenon,

isotopically enriched to 80%136Xe

(−108° C, 3.02 g/cm3)

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25 cm Pb5 cm Cu cryostat 50 cm cryogenic

fluid HFE-7000

Thin walled CuTPCEXO-200

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Lab preparation

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EXO-200 schedule and location• Jun, 2006 Cryostat and lower Pb installed (Stanford)• Sep, 2006 First empty cool-down (Stanford)• May, 2007 Replace super insulation (Stanford) • June, 2007 Move clean rooms to WIPP• 2008 Finish underground manufacturing of TPC

components. E-beam weld pressure vessel.• 2009 Assemble and install TPC and veto at WIPP.

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October 2007: clean-rooms and gowning areainstalled at WIPP.

Staging container forcomponent pre-cleaninginstalled at WIPP.

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Background control

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Natural, cosmogenic and anthropogenic radioactivity content of all construction materials quantified using various techniques.EXO testing program: more than 450 material measurements.

Low background γ-counting (Th/U): 1 ppb (UA) 10 ppt (Bern)Techniques:

Mass spec (Th/U): 10 ppt GDMS, 1 ppt ICPMS (INMS Canada)NAA utilizing MIT reactor (Th/U): 0.02 to 0.3 ppt (UA)α counting for 210Pb analysis of shielding lead (via210Po): 5 Bq/kg (UA)Rn counting, PIN diode with electrostatic collection: 10 atoms (Laurentian, Canada)

Keep track of results through an elog data base.

D. Leonard et al., Nucl. Inst. Meth. A 591 (2008) 3. 15

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Use Monte Carloto quantify backgroundimpact of every det.component.

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The TPC

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5/20/2009 IPMU

Ionization alone:σ(E)/E = 3.8% @ 570 keV

or 1.8% @ Qββ

Ionization & Scintillation:σ(E)/E = 3.0% @ 570 keV

or 1.4% @ Qββ(a factor of 2 better than the

Gotthard TPC)E.Conti et al. Phys. Rev. B (68) 054201

EXO-200 will collect 3-4 timesas much scintillation…

further improvement possible

Compilationof Xe resolution

results

EXO ioniz + scint

EXO ioniz only

Energy resolution improvement in LXe

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Charge Detection

• Double-ended TPC chamber with ~20 cm drift regions. In Xe about 50 e/keV → at 2480 keV results in 124,000 e-.

• Mid-plane cathode biased up to -75 kV• 38 Inductive “Y” wires per side at -4 kV, 100% charge

transparent.• 38 “X” wires at virtual ground to collect the charge.

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Light detection:• 516 16 mm (active area) APD’s.• QE measured to be 120% at 175 nm by NIST.• Geometrical photo-coverage ~17%. Compared to PMTs withabout 30% QE corresponds to 70% coverage.

• Read-out: gangs of seven APD’s• Yield enhanced by reflective Teflon reflectors in TPC.

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September 2008

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Because of limited overburden at WIPP (1585 mw.e.) weare setting up an active scintillation muon veto system.

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Assumptions: 1) 200kg of Xe enriched to 80% in 1362) σ(E)/E = 1.4% obtained in EXO R&D, Conti et al Phys Rev B 68 (2003) 0542013) Low but finite radioactive background:

20 events/year in the 2σ interval centered around the 2.46MeV endpoint4) Negligible background from 2νββ (T1/2>1·1022yr R.Bernabei et al. measurement)

EXO-200 Majorana mass sensitivity

Case Mass(ton)

Eff.(%)

Run Time(yr)

σE/E @ 2.5MeV

(%)

RadioactiveBackground

(events)

T1/20ν

(yr, 90%CL)

Majorana mass(meV)

QRPA1 NSM2

EXO-200 0.2 70 2 1.6* 40 6.4 1025 133 186

1) Rodin, et. al., Nucl. Phys. A 793 (2007) 213-2152) Caurier, et. al., arXiv:0709.2137v2

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Ba single ion detection

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Out of time?Skip to end

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ββ decay observables

Daughter nucleus

Energy deposition from two e-

136Xe 136Ba++

e- e-

νe νe

a.u.

(T1+T2)/Qββ

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Under development:• Ba ion retrieval system based on charged probe (grabber)• Release technology e.g. by coating probe with thin Xe ice• Ba+ single ion detection via laser spectroscopy in iontrap

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Cryogenic dipstick•Capture ion on SXecoating•LHe cooling (~20K) to

maintain stable SXecoatingin 10-8 torr vacuum•Microcapacitor used to

measure and stabilizeSXe with accuracy ofa few monolayersin LXe andvacuum

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Prototype grabber andlinear ion trap at Stanford.

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Reference Cavities

Stable laser tagging system

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Linear trapGas purifier systemGrabber insertion port

Laser injectionoptics

RF/DC feed

Microscope& readout

Lasers

Referencetrap

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Ba+ Linear Ion Trap

Vcos(Ωt) + U

U

Ba oven

DC

pot

entia

l [V

]

0 Volts

-5 Volts

BaBuffer gas

CCD

e- gun

Spectroscopy lasers

Scope

• High ion loading efficiency observed.

• Ions loaded at one end will travel to the other.

• Ions can be manipulated by changing DC

potential configuration.

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Ion signal as a function of time as ions are loaded and unloaded from the linear trap. The quantized structure demonstrates our ability to detect single atoms in a buffer gas with high S/N.

0 ions

1 ion

2 ions

3 ions

4 ions

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Assumptions: 1) 80% enrichment in 1362) Intrinsic low background + Ba tagging eliminate all radioactive background3) Energy res only used to separate the 0ν from 2ν modes:

Select 0ν events in a ±2σ interval centered around the 2.46 MeV endpoint4) Use for 2νββ T1/2 > 1·1022 yr (Bernabei et al. measurement)

* s(E)/E = 1.4% obtained in EXO R&D, Conti et al Phys Rev B 68 (2003) 054201† s(E)/E = 1.0% considered as an aggressive but realistic guess with large light

collection area1) Rodin, et. al., Nucl. Phys. A 793 (2007) 213-2152) Caurier, et. al., arXiv:0709.2137v2

EXO neutrino effective mass sensitivity

Aggressive

Conservative

Case

7.3

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5.3

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Majorana mass(meV)

QRPA1 NSM2

0.7 (use 1)

0.5 (use 1)

2νββBackground(events)

4.1*10281†107010

2*10271.6*5701

T1/20ν

(yr, 90%CL)

σE/E @ 2.5MeV

(%)

Run Time(yr)

Eff.(%)

Mass(ton)

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ConclusionThe investigation of neutrinoless double beta decay allowsto test the properties of neutrinos under CP conjugation,paired with nuclear structure calculation it probes the neutrino mass scale.Oscillation results now set a scale for these searches andthe next generation of experiments is being set up. Anunambiguous decay signature will be important for a positiveclaim.EXO is being set up to provide such signature in form offinal state detection.The next step is EXO-200 without Ba tagging. The Baretrieval and detection technology developed in parallel.First EXO-200 data is expected this year.

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The EXO team

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What if the Heidelberg signal is due to ββ0ν−decay ? Central value T1/2 (Ge) = 2.23+0.44

-0.31 ·1025, ( 3σ) (MPL A 1547 (06)

20νXe

0νXe

0νXe1/2,

2ββ20ν

Ge0νGe

0νGe1/2, MGT

1mMGT

1

⋅⋅=⟩⟨=

⋅⋅

In 200 kg EXO, after 2 yrs of life time:

Worst case (RQRPA, upper limit, α=0.53): 46 events on top of 40 events bkgd 5.0 σ

Best case (NSM, lower limit, α=0.20): 170 events on top of 40 bkgd 11.7 σ

A Ge and Xe experiment have the neutrino mass in common:

0νGe1/2,

0νGe1/2,20ν

Xe0νXe

20νGe

0νGe0ν

Xe1/2, TαTMG

MGT ⋅=⋅

⋅=

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Xe offers a qualitatively new tool against background:136Xe 136Ba++ e- e- final state can be identified using optical spectroscopy (M.Moe PRC44 (1991) 931)

Ba+ system best studied(Neuhauser, Hohenstatt,Toshek, Dehmelt 1980)Very specific signature

“shelving”Single ions can be detectedfrom a photon rate of 107/s

•Important additionalconstraint

•Drastic backgroundreduction

2P1/2

4D3/2

2S1/2

493nm

650nm

metastable 47s

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