M

Novosibirsk 2/7/2012

The search for neutrinoless Double Beta Decay: implications, status t t and d prospects t Andrea Giuliani CSNSM Orsay, CNRS/INP23

France

Outline  Introduction  Why Double Beta Decay is important  Challenges in front of us  Precursors P  Overv Overview ew of the present exper experimental mental methods  Some relevant experiments  A critical comparison of technologies  Conclusions C l i


Decay modes for Double Beta Decay 

((A,Z) , )  ((A,Z+2) , ) + 2e- + 2e



((A,Z) , )  ((A,Z+2) , ) + 2e-



2e-

(A,Z)  (A,Z+2) +

2 Double Beta Decay allowed by the Standard Model already observed –  ~1018 – 1021 y neutrinoless Double Beta Decay (0-DBD) never observed ((except p a discussed claim))  > 1025 y

+

Double Beta Decay with Majoron j ((light g neutral boson)) never observed –  > 1022 y

Processes  and  would imply new physics beyond the Standard Model

violation of total lepton number conservation They are very sensitive Th iti tests t t to t new physics h i since i th phase the h space term t is much larger for them than for the standard process (in particular for ) interest for 0-DBD lasts for 70 years !

Goeppert-Meyer proposed the standard process in 1935 Racah proposed the neutrinoless process in 1937

Double Beta Decay and elementary nuclear physics Even-even Weiszaecker’s formula for the binding energy of a nucleus

Odd-odd

MASS

Nuclear mass as a function of Z, with fixed A (even) Odd-odd

 Q-value

X DBD

 ECEC

Even-even

Z

How many y nuclei in this condition?

Q-va alue [M MeV]

Limit of the 222Rn induced radiactivity Limit of the  natural radioactivity

A


Neutrino flavour oscillations Flavour eigenstates  Mass eigenstates Weak interactions

Sun Atmosphere Acceleratore Reactors

Propagation

Detectors

Flavour oscillations

I f f Info from neutrino t i oscillations ill ti 

oscillations do occur



neutrinos are massive

given the three  mass eigenvalues M1, M2, M3 we have approximate measurements of two Mij2 (Mij2  Mi2 – Mj2) M122 ~ (9 meV)2 Solar

 measurements

  

0.026

|M232 | ~ (50 meV)2

of the 3 angles which parametrize Ulj

e  

Atmospheric elements of the  mixing matrix

What oscillations don don’tt tell us 

absolute neutrino mass scale



neutrino mass hierarchy

degeneracy ?

direct

(M1~M2~M3 )

inverted

Future oscillation experiments will have access to this parameter

 DIRAC or MAJORANA nature of neutrinos

 

   small masses (see-saw)  matter-antimatter asymmetry (leptogenesis)

More in detail: double beta decay y and neutrino u d W-

d

W-

u

ee e e-

a LH neutrino (L=1) is absorbed at this vertex

a RH antineutrino (L=-1) is emitted at this vertex

With massless neutrinos, the process is forbidden because neutrino has no correct helicity / lepton number to be absorbed at the second vertex

 IF neutrinos are massive DIRAC particles:

Helicities can be accommodated thanks to the finite mass, mass BUT Lepton number is rigorously conserved

 IF neutrinos are massive MAJORANA particles: Helicities can be accommodated thanks to the finite mass, AND Lepton number is not relevant

0-DBD is forbidden 0-DBD 0 DBD is allowed

Other possible mechanisms (V+A) current ,,

SUSY ’111,’113’131,…..

However, independently of the mechanism, However mechanism the Schechter-Valle theorem states that Observation of

0-DBD

m  0 

even if the light neutrino exchange should not provide the dominant contribution

More on the mass mechanism how 0-DBD is connected to neutrino mixing matrix and masses in case of p process induced by y light g  exchange g (mass mechanism) neutrinoless D bl Beta Double B D Decay rate

Phase space

Nuclear matrix elements

Effective Majorana mass

1/ = G(Q G(Q,Z) Z) |Mnucll|2M 2 what the experimentalists try to measure

0.026

parameter containing the physics: what the nuclear theorists Effective Majorana j mass t tto calculate try l l t

i | U | 2M + ei i | U | 2M | M M = ||Ue1 | 2M1 + ei e2 2 e3 3 1

2

M [eV]

Cosmology, single and double  decay Cosmology, single and double  decay measure different combinations Cosmology of the neutrino mass eigenvalues, constraining the neutrino mass scale In a standard three active neutrino scenario:

3

   Mi i=1 3

2 |U |2  M M M  i ei

cosmology m l simple sum pure kinematical effect

1/2

i=1 3

M   Mi |Uei|2 ei  i=1

i

 decay incoherent sum real neutrino double  decay coherent sum virtual neutrino Majorana phases

The three constrained parameters can be plot as a function of the li ht t neutrino lightest t i mass Two bands appear in each plot, corresponding to inverted and direct hierarchy

[eV V]

Present bounds

M [eV]

M [[eV]

The two bands merge in the degenerate case (the only one presently probed)


Three hurdles to leap over M [eV] ~300 meV

100-1000 counts/y/ton Klapdor’s claim

0.5-5 counts/y/ton 20 meV

0 1 1 counts/y/(100 0.1-1 t / /(100 tton)) 1 meV

The first leap p look lookss easy y but one year ago nobody did it… 1990’s 2000’s

2000’s

Adapted from

A. Faessler et al., Phys. Rev. D79,053001(2009)

Ge experimental p range (Klapdor)

…but something g new is appearing pp g at the horizon: Xe is coming

KamLAND-ZEN K LAND ZEN EXO Adapted from

A. Faessler et al., Phys. Rev. D79,053001(2009)

Background demands Present generation experiments, under commissioning or construction, aim at scrutinizing Klapdor Klapdor’ss claim and possibly attacking the inverted hierarchy region

To start to explore the inverted hierarchy region

Sensitivity at the level of 1-10 counts / y ton To cover the inverted hierarchy region

Sensitivity at the level of 0.1 -1 counts / y ton

The order of magnitude of the target bakground is ~ 1 counts / y ton

Signal and Background sources

 Natural radioactivity of materials (source itself, surrounding structures)

100Mo

 7.1x1018 136Xe  2.2x1021

 Neutrons (in particular muon-induced)  Cosmogenic induced activity (long living)  2  Double Beta Decay

=E/Q

Experimental approaches Two approaches:

 constraints on detector materials  very large masses are possible demonstrated: up to ~ 50 kg proposed: up to ~ 1000 kg

e-



e-

Source  Detector (calorimetric technique)    

scintillation phonon-mediated detection solid-state devices gaseous/lquid detectors

 with proper choice of the detector, very high energy resolution Ge diodes Ge-diodes bolometers

 in gaseous/liquid xenon detector, indication of event topology

e- detector



source

e- detector

Source  Detector    

scintillation gaseous TPC gaseous drift chamber magnetic field and TOF

 it is difficult to get large source mass  neat reconstruction of event topology  several candidates can be studied with the same detector

The sensitivity sensitivity F: lifetime corresponding to the minimum detectable number of events over background at a given confidence level b  0

b: specific background coefficient [[counts/(keV ( kg g y)]

live time source mass

b = 0

energy resolution

F  (MT / bE)1/2

F  MT

importance of the nuclide choice

(but large uncertainty due to nuclear physics)

sensitivity y to

1    M Q1/2 |Mnucl|

bE MT

1/4

Choice of the nuclide Transition energy (MeV)

5

Isotopic abundance (%) 40

4

20

3

0 Nuclear Matrix Element

2 48

Ca

76

Ge 82Se

96

Zr 100Mo116Cd 130Te136Xe150Nd

48

Ca

76

Ge 82Se

96

Zr 100Mo 116Cd 130Te136Xe150Nd

No super-favoured isotope ! Sign S gn of con convergence! ergence!

But do not forget the phase space! G [y-1]

Suhonen et al., R0 =1.2 fm & gA = 1.25

10-12

10-13

R0=1 2fm & gA=1 R0=1.2fm gA=1.25 25

10-14

10-15 48Ca 76Ge 82Se 96Zr 100Mo 116Cd 128Te 130Te 136Xe 150Nd 124Sn

Enrichment


Precursors of the present generation searches I will comment about three crucial experiments:

 Heidelberg – Moscow (HM) (closed in 2003) dominated d i t d DBD scenario i over a decade d d The basic detecting element preludes to a present generation experiment GERDA

 NEMO3 (recently stopped) it was tracking experiment capable to study different candidate nuclides It is also a precursor of the present generation experiment SuperNEMO

 CUORICINO (closed in 2008) it was a calorimetric experiment with the potential to approach the HM result it is i also l a prelude l d tto a presentt generation ti experiment, i t CUORE (Cryogenic Underground Observatory for Rare Events),

ee-

High energy resolution (2%) Tracking / topology capability Easy to control bakground (with the exception of 2 DBD component)

The Heidelberg Moscow experiment Source = detector Well established technology of Ge diodes    

This technique has been dominating the field for decades and is still one of the most promising for the future E. Fiorini – 60s

Five Ge diodes for an overall mass of 10.9 kg isotopically enriched ( 86%) in 76Ge Underground operation in the Gran Sasso laboratory (Italy) Lead box and nitrogen flushing of the detectors Digital Pulse Shape Analysis (PSA) (factor 5 reduction) identification of Multi-site events (gamma background)

7.6  1025 76Ge nuclei

Background in the region of DBD: b = 0.17 counts/(keV kg y)

T0 < 1.9 x 1025 y

M < 0.3 – 2.5 eV

similar results obtained by IGEX experiment

HM: claim of evidence of 0 0-DBD Suddenly, in December 2001, 4 authors (KDHK) of the HM collaboration announce the discovery of neutrinoless DBD

2001

2004

Re-analysis of data Larger statistic

54.98 kg•y

2.2 

71.7 kg•y

4

M = 0.24 - 0.58 eV /20 (y) = (0.69 – 4.18)  1025 y (1.19  1025 y most probable)

Klapdor et al., Physics Letters B 586 (2004) 198-212 New analysis 2 years later: 2.23+0.44−0.31 × 1025 y central value for M ~0.3 eV Klapdor et al., Mod. Phys. Lett. A, 21 (2006)1547

CUORICINO Source = detector Bolometric technique: young (born in ~ 1985) but now firmly established Nuclide under study: 130Te CUORICINO source

The bolometric technique for the study of DBD was proposed by E. Fiorini and T.O. Niinikoski in 1983

 0 DBD is a factor 5-10 faster than in 76Ge  A.I.: A I 34%  enrichment i h t nott necessary experiments can be expanded at low cost

5  1025 130Te nuclei Bolometric technique: the nuclear energy is measured as a temperature increase of a single crystal

T = E/C

thanks to a proper thermometer thermometer,

T  V

In order to get low specific heat, the temperature must be very low (5 – 10 mK) Typical signal sizes: 0.1 mK / MeV, converted to about 1 mV / MeV

The CUORICINO detectors Thermometer (doped Ge chip)

Energy absorber single g TeO2 crystal y  790 g  5 x 5 x 5 cm

Cuoricino results (2010) data: 2003 — 2008 19.75 kg-yr 130Te exposure

Q=2527.5 keV

No evidence of neutrinoless double beta decay in 130Te. 0 (130Te) > (−0.25 ± 1.44stat ± 0.30syst) × 10−25 y−1

Cuoricino results (2010) data: 2003 — 2008 19.75 kg-yr 130Te exposure

Q=2527.5 keV

Background:

0.169 ± 0.006 counts/keV/kg/y

Lower limit, limit half half-life: life: T102 (130Te) ≥ 2.8 2 8 × 1024 y (90% Upper limit, Majorana mass: C.L.) mνe < 300 – 710 meV

Expected signal

HYPOTHETICAL O CA

The signature for 0νββ in 130Te would be a small peak above background at the reaction reaction’ss Q-value Q value of 2527 keV.


Experiment classification criteria ee-

Source  Detector

High energy resolution (