Determining the neutrino mass:

Determining the neutrino mass:

The search for the neutrinoless double beta decay

Tobias Bode

Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decay

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

Introduction

• Why look for neutrinoless double beta decay (0νββ)?

– Dirac or Majorana neutrino?

– Physics beyond the Standard Model?

– ΔL≠0 ?

– Determine the neutrino mass


Introduction


1 2 1 2

1 2

( , ) ( 2, ) 2( , ) ( 2, ) 0Z A Z A e eZ A Z A e e

• 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)

• 2νββ decay is four particle decay processContinuous electron

spectrum

• 0νββ is two particle decay process Sharp peak at Q-value in

energy sum spectrum

ββ-decay plot


energy sum spectrum

• Dirac neutrino– Charge conjugation

changes the neutrino to an antineutrino

• Majorana neutrino– neutrino is charge self-

conjugated → its own antiparticle

Dirac vs Majorana neutrino


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


• 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.)

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


ββ decay theory

2( , .) ~ Pm Z A const Z Z

Isotope Q-Value [MeV]

Isotopic abundance

Observed halflife [y]

48-Ca 4.271 0.0035 % 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

Nuclei which undergo double beta decay


• 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

Calculate the effective majorana mass


122 0 0 01 2 0 ,

halflife phasespace nuclear matrix element factor

m T G E Z M

effektive majoranamass

• Effective mass term

• Majorana mass– Emission of antineutrino:– Absorption of neutrino:

• elements of neutrino mass mixing matrix

• Total amplitude of 0νββ decay

e

Why effective neutrino mass?


32,

1

iie i i

i

m U m e

* *CP CP CP CPi e i ei i ei

i

U U † *

e i i e eiU

22 *ei i

i

m U m n n

pp

CP CPi e

e i

eeiU

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

• asffm

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)

Experiments

Seminar talk , T. Bode, 12.06.09, Determining the netrino mass: Search for the neutrinless double beta decayBolometer

Ge-Diode

TPC

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


• Operated at Gran Sasso Underground Lab from 1990-2003– Target & detector:

10.9kg enriched 76-Ge in 5 diodes

– Lead and copper shielding

Heidelberg-Moscow experiment (HDMS)


• 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

Data analysis Heidelberg-Moscow 2004


0 251 2 (0.6 4.18) 10T y

(0.2 0.6)m eV

• 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

Pulse shape analysis of HDMS data


0 0.44 251 2 0.312.23 10T y

0.03

0.030.32m eV

– 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

How to increase the sensitivity of 0νββ experiments I


01 2 MtT a

B E

0

1 2T N t 0

1 2T N t

• Increase of target mass• HDMS=11kg• → future Ge

experiment 35kg • → future Xe experiment

1t• Modular

design(scaleable)

How to increase the sensitivity of 0νββ experiments II


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 Gran Sasso by 2009/10

EXO 136-Xe 200/1000 TPC/Iontagging WIPP by 2009

CUORE 130-Te 750 bolometer Gran Sasso 2011


• located at Gran Sasso• Similar to HDMS– Bare 76-Ge diodes,

immersed in cryogenic fluid(LN/LAr)

No radiation from solid shielding

GerDA (GERmanium Detector Array)


GerDA build-up

• Phase I: 15kg HDMS Ge-diodes– background≈0.01 cts(keV

kg y)– Sensitivity: =0.3-0.9 eV

• Phase II: 35kg segmented new Ge-diodes– background≈0.001 cts(keV

kg y)– Sensitivity: =0.09-0.29

eV


m

m

• 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

GerDA background reduction

• Main goal is to further reduce external γ-background


• Using 62 -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)

CUORICINO


3

( ) ~D

TC TT

~

( )ETC T

2TeO

5x5x5 cm³

0 241 2 2.4 10T y

0.19 0.68m eV

• Next phase of CUORICINO, 750 kg

• 19 towers with 53 crystals each=988 crystals


Cryogenic Underground Observatory for Rare Events

2TeO

• 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

Enriched Xenon Observatory (EXO)


136 136 2 (Q=2.48 MeV)Xe Ba e

• 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

EXO-200


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


136Ba

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 80 70 5 1 2 x 10^27 33

10 80 70 10 1 4.1 x 10^28 7.3

01 2 [ ]T y

[ ]m meV

21 2T

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


Thank you

Determining the neutrino mass:

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