1. Determining the neutrino mass:
The search for the neutrinoless double beta decay
Tobias Bode

2. Dirac vs Majorana neutrino Neutrino mass mixing
Outline
Introduction
Theory
Dirac vs Majorana neutrino
Neutrino mass mixing
Nuclei undergoing double beta decay
Experiments
Heidelberg-Moscow experiment
GerDA
CUORE
Enriched Xenon Observatory
Conclusion and outlook
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

3. Why look for neutrinoless double beta decay (0νββ)?
Introduction
Why look for neutrinoless double beta decay (0νββ)?
Dirac or Majorana neutrino?
Physics beyond the Standard Model?
ΔL≠0 ?
Determine the neutrino mass
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

4. Feynman graph of hypothetical neutrinoless double beta decay (0νββ)
Introduction
Feynman graph of hypothetical neutrinoless double beta decay (0νββ)
2nd order processes of weak interaction
4th order in GWS-model
0νββ forbidden in SM(ΔL≠0)
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

5. 2νββ decay is four particle decay process
ββ-decay plot
2νββ decay is four particle decay process
Continuous electron spectrum
0νββ is two particle decay process
Sharp peak at Q-value in energy sum spectrum
energy sum spectrum
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

6. Dirac vs Majorana neutrino
Dirac neutrino
Charge conjugation changes the neutrino to an antineutrino
Majorana neutrino
neutrino is charge self-conjugated → its own antiparticle
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

7. No known majorana fermions in nature
Majorana neutrino
No known majorana fermions in nature
If it exists →Physics beyond the SM
ΔL≠0
No more neutrino/antineutrino but right-handed & left-handed neutrino
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

8. Requirements for 0νββ Neutrino is Majorana particle Neutrino has mass
Theory
Requirements for 0νββ
Neutrino is Majorana particle
Neutrino has mass
Handedness changes due to massive ν
Also possible by :
Right handed weak interaction
RH weak current couples to RH antineutrino
Other exchange particles (neutralino etc.)
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

9. Decay through a continuum of virtual intermediate states odd/odd
ββ decay theory
ββ decay possible if the next even/even nucleus energetically lower than the mother nucleus
Decay through a continuum of virtual intermediate states odd/odd
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

10. Nuclei which undergo double beta decay
Isotope
Q-Value [MeV]
Isotopic abundance
Observed halflife [y]
48-Ca
4.271 %
4.0*10^19
76-Ge
2.039
7.8%
1.4*10^21
82-Se
2.995
9.2%
0.9*10^20
96-Zr
3.350
2.8%
2.1*10^19
100-Mo
3.034
9.6%
8.0*10^18
116-Cd
2.802
7.5%
3.3*10^19
128-Te
0.868
31.7%
2.5*10^24
130-Te
2.533
34.5%
0.9*10^21
136-Xe
2.479
8.9%
Not obs.
150-Nd
3.367
5.6%
7.0*10^18
ββ decay observable only if β decay energetically forbidden
For all other isotopes :
β decay rate much higher
ββ decay is suppressed
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

11. Calculate the effective majorana mass
effektive majorana
mass
Assumes no right-handed weak currents
The halflife is experimentally determined
The phasespace factor is larger for 0νββ than for 2νββ due to the virtual character of the neutrino
the nuclear matrix elements are very difficult to compute
Uncertainties factor 3
The calculated effective majorana mass depends heavily on choice of those matrix elements
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

12. Why effective neutrino mass?
Effective mass term
Majorana mass
Emission of antineutrino:
Absorption of neutrino:
elements of neutrino mass mixing matrix
Total amplitude of 0νββ decay p p n n
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

13. Effective majorana mass
In contrast to β decay majorana phases α also relevant in neutrino mass mixing
coherent sum over mass eigenstates
→ destructive interference possible
Single mass eigenstates could be larger than
if CP conserved α=±1
Only range of neutrino mass can be determined by 0νββ
asff
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

14. Experiments
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

15. Passive targets Active targets Source ≠ detector
Experiments
Passive targets
Source ≠ detector
ββ emitter in thin foils between detectors
Easy to change isotopes
NEMO-3
Active targets
Source = detector(no self absorption)
Bolometer (CUORE)
Semiconductor detector(Heidelberg-Moskau, GERDA)
TPC (EXO)
Ge-Diode
TPC
Bolometer
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

16. Semiconductor detector experiments
Active target
High energy resolution
Material: 76-Ge
High nat. abundance (7.8%)
Low Q-value of 2.04 MeV
Hard to discriminate from natural radioactive background
passive shielding & active veto counters needed
Solid shielding source of radioactivity
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

17. Heidelberg-Moscow experiment (HDMS)
Operated at Gran Sasso Underground Lab from
Target & detector: 10.9kg enriched 76-Ge in 5 diodes
Lead and copper shielding
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

18. Data analysis Heidelberg-Moscow 2004
71.7 kg years of data
Author claims signal at Q=2039 keV
28.75 ± 6.86 events detected (4.2σ)
Problem: background simulation, discriminate γ and β counts
Heidelberg-Moscow solution: Pulse shape analysis
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

19. Pulse shape analysis of HDMS data
90% of ββ events are localized in a small volume in the detector (single site event)
Normal γ events are multiple site events (MSE)
Calculation of SSE Library
Comparison with all events
Rejection of identified MSE
11±1.8 events
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

20. How to increase the sensitivity of 0νββ experiments I
a=isotope abundance
M=target mass
B=background
ΔE=energy resolution
t=measurement time
→ enrichment more effective than target mass increase
Easy to enrich isotopes best suited for future experiments
If B=0 →
If B≠0 →
(Poisson fluctuations)
Background reduction!
Low level shielding
Radon free environment
Selected materials
Underground labs
Detector segmentation
µ-veto, neutron veto
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

21. How to increase the sensitivity of 0νββ experiments II
Increase of target mass
HDMS=11kg
→ future Ge experiment 35kg
→ future Xe experiment 1t
Modular design(scaleable)
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

22. Current and future 0νββ experiments
name
target nuclei
mass[kg]
method
laboratory
status
COURICINO
130-Te
40.7
bolometer
Gran Sasso
finished
NEMO-3
100-Mo/82-Se
6.9
tracking calorimeter
Fréjus
taking data
GerDA
76-Ge
15/35/500
semiconductor
by 2009/10
EXO
136-Xe
200/1000
TPC/Iontagging
WIPP
by 2009
CUORE
750
2011
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

23. GerDA (GERmanium Detector Array)
located at Gran Sasso
Similar to HDMS
Bare 76-Ge diodes, immersed in cryogenic fluid(LN/LAr)
No radiation from solid shielding
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

24. Phase I: 15kg HDMS Ge-diodes background≈0.01 cts(keV kg y)
GerDA build-up
Phase I: 15kg HDMS Ge-diodes
background≈0.01 cts(keV kg y)
Sensitivity: = eV
Phase II: 35kg segmented new Ge-diodes
background≈0.001 cts(keV kg y)
Sensitivity: = eV
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

25. GerDA background reduction
Main goal is to further reduce external γ-background
LAr-Anticoincidence
ββ-decay localized event
If scintillation light detected in LAr at same time
Event rejected because it was a γ-event
Segmentation
ββ-decay localized event
If ionization detected in more than 1-2 segments
Event is rejected
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

26. Using 62 -crystals in bolometer setup Debye-Law : T→ 0 :
CUORICINO
Using crystals in bolometer setup
Debye-Law :
T→ 0 :
E: deposited energy
Placed in dilution refrigerator at ≈10mK
At E=2.53MeV (Q-value) ΔT=0.18mK
3 years, backgroundrate: 0.19 cts/(kg keV y)
5x5x5 cm³
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

27. Cryogenic Underground Observatory for Rare Events
Next phase of CUORICINO, 750 kg
19 towers with 53 crystals each=988 crystals
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

28. Enriched Xenon Observatory (EXO)
Liquid/gaseous Xe Time Projection Chamber
Xenon easy to purify and enrich (Russian centrifuges)
Q-value higher than nat. radioactivity
Possible to “laser-tag” Barium ions
coincidence of ion and ββ event
→ reduction of background
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

29. Energy resolution not good in LXe TPC
EXO-200
First phase: 200 kg LXe
Energy resolution not good in LXe TPC
→ Combination of scintillation & ionization
Dense material →
Small volume →
Good spatial resolution
Electrons drifting to ground
Electron trajectory reconstructed by anode segmentation & drift time
Scintillation light used as timing signal for electron drift time measurement
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

30. 1t (10t) of enriched LXe/Gxe in full-scale EXO
EXO Barium tagging
1t (10t) of enriched LXe/Gxe in full-scale EXO
Laser fluorescence will be used to identify ions to reduce background
2νββ and 0νββ not destinguishable! Good enough energy resolution needed
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

31. Projected sensitivity of EXO
EXO also looks for 2νββ decay
Test for matrix elements
Knowledge of important for background estimates of 0νββ decay
Mass[t]
136-Xe[%]
Effiency[%]
Time[y]
Background[cts/(kg keV y]
0.2 80 70 2 40
6.4 x 10^25
186 1 5
2 x 10^27 33 10
4.1 x 10^28
7.3
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

32. Importance of different approaches to 0νββ decay
Conclusion
Importance of different approaches to 0νββ decay
To increase sensitivity a quadratic increase in target mass is needed
International collaborations and funding is needed
If mass range is determined, it will give new impulses and limitations for theories&experiments in particle- & astrophysics
Seminar talk , T. Bode, , Determining the netrino mass: Search for the neutrinless double beta decay

33. Thank you