Charles Prescott - SLAC Neutrino Day - April 18, 2003 The Search for Neutrinoless Double Beta Decay...
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Transcript of 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
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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.
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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
Charles Prescott - SLAC Neutrino Day - April 18, 2003
<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
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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”
Charles Prescott - SLAC Neutrino Day - April 18, 2003
EXO
http://doe-hep.hep.net/HEPFacSub/HEPAP_FacilitiesMar03.pdf
Charles Prescott - SLAC Neutrino Day - April 18, 2003
Majorana
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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.)
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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
Charles Prescott - SLAC Neutrino Day - April 18, 2003
http://majorana.pnl.gov/
Charles Prescott - SLAC Neutrino Day - April 18, 2003
http://majorana.pnl.gov/
Charles Prescott - SLAC Neutrino Day - April 18, 2003
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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).
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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
Charles Prescott - SLAC Neutrino Day - April 18, 2003
CCD image of an single ion in the trap
trapedge
Identification of 136Ba with red and blue lasers
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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.
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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
Charles Prescott - SLAC Neutrino Day - April 18, 2003
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.