Charles Prescott - SLAC Neutrino Day - April 18, 2003 The Search for Neutrinoless Double Beta Decay...

Charles Prescott - SLAC Neutrino Day - April 18, 2003

The Search forNeutrinoless

Double Beta Decay

- Physics Motivations

- Limits from ν oscillations

- Backgrounds

- Experiments

- Future Prospects


from S.R. Elliott and P. Vogel, Ann. Rev. Nucl. Part. Sci. 52 (2002)

“Moore’s Law” for Double Beta Decay

Range suggested byatmospheric oscillation

Range suggested by

atmospheric oscillation data


A brief digression into nuclear physics

Consider a nucleus of Z protons and N neutrons with A = Z+NTwo 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: 124Te52)

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-oddeven-even

even-odd


48Ca→48Ti 4.271 0.18776Ge→76Se 2.040 7.882Se→82Kr 2.995 9.296Zr→96Mo 3.350 2.8100Mo→100Ru 3.034 9.6110Pd→110Cd 2.013 11.8116Cd→116Sn 2.802 7.5124Sn→124Te 2.228 5.64130Te→130Xe 2.533 34.5136Xe→136Ba 2.479 8.9150Nd→150Sm 3.367 5.6

Candidate Q Abund.Candidate Q Abund. (MeV) (%)(MeV) (%)

Candidate Nuclei for Double Beta Decay

Issues include : Q value (10th or 11th power dependence), abundance, ease of purification (chemical and isotopic), radioactivity (incl. cosmogenesis), & experimental ease of use.


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 ~ <m> 2.

• 0 decay does not conserve lepton number


Effective Majorana mass

Thus cannot be directly related to informationfrom oscillations...but only indirectly related within specific models... giving a range of values

i

measured half-life

phase space factornuclear 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


<m> 90% C.L. ranges from all data :

~(1 - 4 meV)normal hierarchy ~(15 - 60 meV)inverted hierarchy

{S.R. Elliott and P.Vogel, Ann.Rev.Nucl.Part.Sci. 52 (2002) 115-151. }

NormalHierarchy

InvertedHierarchy

Neutrino Oscillation Results

large mixing, N=3


Other possibilities

Quasi-degenerate model: ____________

msol

matm

m0

sterile neutrinos ?

non-Standard Model couplingslepton number violating RH currentsSUSY . . “Standard Model”

SUSY examples

Conclusions:(1) meff 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


DOE Office of Science Facilities Roadmap for the next 20 years

December 2002Ray 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”


EXO

http://doe-hep.hep.net/HEPFacSub/HEPAP_FacilitiesMar03.pdf


Majorana


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 10-2)

0 peak (5% FWHM)(normalized to 10-6)

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.)


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 (136Xe gas TPC - 3.3 kg at 5 atms)R. Luescher et al., PL B 434, 407 (1998)

T1/2 >4.4x1023 yr (90% cl)

m < 2-2.5 eV(model dependent)

neutrons


Heidelberg-Moscow

11 Kg 76Ge – 5 p-type HPGe detectors - for ~4 years (in Gran Sasso)

2

0

hep-ph/0103062 6 March 2001

T1/2 > 1.9 1025 y (90% cl)m < 0.27-0.35 eV


http://majorana.pnl.gov/


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 Ba136 identification.

Compact and scalable (3 m3 for 10 tons).


Background reduction by coincidence measurement

136Xe 136Ba++ + e– + e–

Identify event-by-event

Barium detection from 136Xe 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 Xenontest cell with probe tip


CCD image of an single ion in the trap

trapedge

Identification of 136Ba with red and blue lasers


Isotopic enrichment for a gaseous Xe is most economically achieved by ultracentrifugation

136Xe, being the heaviestXe isotope, is particularly

easy to separate.

The separation step thatrejects the light fraction is

also very effective in removing85Kr (T1/2=10.7 yr) that is

constantly produced by fissionin nuclear reactors.

Large facilities existIn Russia – 500,000centrifuges per plant.

EXO now has 100 kgof enriched Xenon, with another 100 kg

due in March.


WIPP : Waste Isolation Pilot PlantCarlsbad NM

Excavated in underground salt – lower U/Th activity.

~2,000 m.w.e. depth

An experimental facility for EXO


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


Question : Comparison with other potential experiments ?

Ge76

Te130

Xe136

Mo100

1.3 1028 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-forlifetimes do not differ by much

from exp. to exp.

The Ge76 exps. offer good E resol.

but neverthelessmust improve bkgrd

by factor >1000.

Charles Prescott - SLAC Neutrino Day - April 18, 2003 The Search for Neutrinoless Double Beta Decay...

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