Transcript Determining the neutrino mass:
Determining the neutrino mass: The search for the neutrinoless double beta decay
Tobias Bode
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
Introduction (
Z
(
Z
e
1
e
2 1
e
2 2 2 0 • • • • 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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
ββ decay theory
const
.) ~
Z
Z
2
P
• • 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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
Nuclei which undergo double beta decay
Isotope 48-Ca 76-Ge 82-Se 96-Zr 100-Mo 116-Cd 128-Te 130-Te 136-Xe 150-Nd Q-Value [MeV]
4.271
2.039
2.995
3.350
3.034
2.802
0.868
2.533
2.479
3.367
Isotopic abundance
0.0035 % 7.8% 9.2% 2.8% 9.6% 7.5% 31.7% 34.5% 8.9% 5.6%
Observed halflife [y]
4.0*10^19 1.4*10^21 0.9*10^20 2.1*10^19 8.0*10^18 3.3*10^19 2.5*10^24 0.9*10^21 Not obs.
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
Calculate the effective majorana mass effektive majorana mass • • •
m
2
T
0 1 2
G
0 ,
M
0 2 1 halflife phasespace nuclear matrix element factor 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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
Why effective neutrino mass?
• Effective mass term
m
i
3 1
U
2
i i
i
• • Majorana mass – Emission of antineutrino: – Absorption of neutrino:
i
e i
•
U ei
elements of neutrino mass mixing matrix
e
p
e CP
i i e
i CP
†
U ei
*
U ei
*
i CP
p
e i
Total amplitude of 0νββ decay
m
2
i
2
m i
n
i CP e
n
U ei
*
e
Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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
m
if CP conserved α=±1 • Only range of neutrino mass can be determined by 0νββ • asff Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
Experiments
Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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) Ge-Diode TPC (EXO) TPC Bolometer Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
Heidelberg-Moscow experiment (HDMS) • Operated at Gran Sasso Underground Lab from 1990-2003 – Target & detector: 10.9kg enriched 76-Ge in 5 diodes – Lead and copper shielding Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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 0
T
1 2
m
25
y
eV
Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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 0
T
1 2 2.23
0.44
0.31
10 25
y m
0.32
0.03
0.03
eV
Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
How to increase the sensitivity of 0νββ experiments I • • 0
T
1 2 –
a Mt
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 →
T
0 1 2 If B≠0 → 0
T
1 2 (Poisson fluctuations) Background reduction!
– Low level shielding – Radon free environment – Selected materials – – Underground labs Detector segmentation – µ-veto, neutron veto Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
Current and future 0νββ experiments
name
COURICINO NEMO-3 GerDA EXO CUORE
target nuclei
130-Te 100-Mo/82-Se
mass[kg]
40.7
6.9
method
bolometer tracking calorimeter
laboratory
Gran Sasso Fréjus
status
finished taking data 76-Ge 136-Xe 130-Te 15/35/500 semiconductor 200/1000 TPC/Iontagging 750 bolometer Gran Sasso WIPP Gran Sasso by 2009/10 by 2009 2011 Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
GerDA build-up • • Phase I : 15kg HDMS Ge diodes – background≈0.01 cts(keV kg y) – Phase II : 35kg segmented new Ge-diodes – background≈0.001 cts(keV kg y) – eV Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
CUORICINO • • • • • • Using 62 -crystals in bolometer setup Debye-Law :
T
3 T→ 0 :
T
~
E T D
– E: deposited energy Placed in dilution refrigerator at ≈10mK
m
0
T
1 2 At E=2.53MeV (Q-value) ΔT=0.18mK
3 years, backgroundrate: 0.19 cts/(kg keV y) 24 5x5x5 cm³ Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
y
eV
Cryogenic Underground Observatory for Rare Events • • Next phase of CUORICINO, 750 kg
TeO
2 19 towers with 53 crystals each=988 crystals Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
Enriched Xenon Observatory (EXO) • • • • 136
Xe
136
Ba
2
e
(Q=2.48 MeV)
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
EXO Barium tagging • • • 1t (10t) of enriched LXe/Gxe in full-scale EXO 136 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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay
Projected sensitivity of EXO • • • EXO also looks for 2νββ decay Test for matrix elements Knowledge of important for background 1 2 estimates of 0νββ decay
Mass[t]
0.2
1 10
136-Xe[%] Effiency[%] Time[y] Background[cts/ (kg keV y]
0
T
1 2 80 80 80 70 70 70 2 5 10 40 1 1
m
[
meV
] 6.4 x 10^25 186 2 x 10^27 33 4.1 x 10^28 7.3
Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta 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, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay