Neutrino Physics - Double-Beta Decay Steve Elliott Los ...June 2007 Steve Elliott, FNAL Neutrino Summer School 2 Lecture Outline •Double Beta Decay –Basic physics –General experimental

Neutrino Physics - Double-Beta Decay

Steve ElliottLos Alamos National

Laboratory

June 2007 Steve Elliott, FNAL Neutrino Summer School 2

Lecture Outline

• Double Beta Decay– Basic physics– General experimental techniques– The various experiments


Example Decay Scheme

2+

0+

0+

0+

2-

Ge76

As76

Se76

!!

In many even-even nuclei, β decay is energetically forbidden. This leaves ββ

as the allowed decay mode.

EndpointEnergy


ββ(2ν): Allowed weak decay

!

2n" 2p+ 2e#

+ 2$ e

e-

e-

Z+2Z+1

Zνe

νe


ββ(0ν): requires massive Majorana νOnly practical way to address the particle-antiparticle question

e-

e-

Z+2

Z+1

Z νe

n! p + e"+ #e

!e + n" p + e#

!

(RH "e )

!

(LH "e)


Energy Spectrum for the 2 e-

2.01.51.00.50.0

Sum Energy for the Two Electrons (MeV)

Two Neutrino Spectrum Zero Neutrino Spectrum

1% resolution!(2") = 100 * !(0")

EndpointEnergy


ββ History

• ββ(2ν) rate first calculated by Maria Goeppert-Mayer in 1935.

• First observed directly in 1987.• Why so long? Background

τ1/2(U, Th) ~ Tuniverse

τ1/2(ββ(2ν)) ~ 1010 Tuniverse

• But next we want to look for a process with:

τ1/2(ββ(0ν)) ~ 1017 Tuniverse


ββ Candidates

There are a lot of them!


How to choose a ββ isotope?

• Detector technology exists

• High isotopic abundance or an enrichedsource exists.

• High energy = fast rate

• High energy = above background


ββ CandidatesAbundance > 5%,Trans. Energy > 2 MeV

Frequently studied isotope.


ββ Decay Rates

!2" = G2" M2"

2

!0" = G0"M0"

2

m"2

G are calculable phase space factors.G0ν ~ Q5

|M| are nuclear physics matrix elements.Hard to calculate.

mν is where the interesting physics lies.


“Because Its Not There”

Larson


Neutrino Mass: What do we want to know?

Mas

sAbsolute

MassScale

RelativeMassScale

!"

!#

!#

!"

$

%

& & & & &

'

(

) ) ) ) )

or

!"

!#

$

% & &

'

( ) )

Dirac or Majoranaνe ν1 ν2 ν3

Ue1

2Ue2

2Ue3

2

Mixing


Neutrino Mass: How do we learn what we want toknow?

Absolute Mass Scale

Relative Mass Scale

Mixing Matrix

Elements

CP nature

of

, cosm. Oscil.

Need all 3 types of experiments.


Neutrino Masses: What do we know?

• The results of oscillation experiments indicate νdo have mass!, set the relative mass scale, and aminimum for the absolute scale.

• β decay experiments set a maximum for theabsolute mass scale.

50 meV < mν < 2200 meV


We also know ν mix.

!e!µ

!"

#

$

% %

&

'

( ( =

Ue1 Ue2 Ue3

Uµ1 Uµ2 Uµ3

U"1 U"2 U"3

#

$

% %

&

'

( (

!1!2!3

#

$

% %

&

'

( (

The weak interaction produces νe, νµ, ντ.

These are not pure mass states but a linearcombination of mass states.

Oscillation experiments indicate that ν mix and constrain Uαi.


Oscillations and HierarchyPossibilities

mass

Normal Invertedmsmallest1

2

312

3

49 meV

9 meV

νe is composed of a large fraction of ν1.


What about mixing, mν & ββ(0ν)?

m!! = Uei

2

mi

i=1

3

" #i

ε = ±1, CP cons.

Compare to β decay result:

m! = Uei

2

mi

2

i=1

3

"

virtual νexchange

real νemission

No mixing: m!! = m"e= m

1


Why does the CP parity appear in <mββ>?

Look at the critical part of this diagram.

A0ν ∼ Σin

n

p

p

W-

W-

νi

e-

Uei

Uei

e-


The crossed channel.

e+ e-

W- W+

νi

A! Uei

2e+W

"HSM

#i

i

$ #iHSMe"W

+

The 1st vertex creates the CP partnerof the particle needed by the 2nd vertex.

But CP !i= "

i!i

Upon substitution, the factor εi appears.

Vertex 1

Vertex 2


What can be learned from Oscillations & ββ?

• From oscillations, we have:Information on UeiInformation on δm2

• With <mββ> constraints, we can constrain m1:(2 flavor example)

!

m"" = Ue1

2m1+#

21Ue2

2m

1

2+$m

21

2


Min. <mββ> as a vector sum.General Case

m!! = Ue1

2

m1+ e

i!Ue2

2

m2+ e

i"Ue3

2

m3

!

"

Ue3

2m3

Ue2

2m2

Ue1

2m1

<mββ>

<mββ> is the modulus of the resultant.In this example, <mββ> has a min. It cannot be 0.

min


PlotThanks toPetr Vogel

More General: 3 ν

50 meV orfew x 1027 yr

msmallest

<mββ

>


More General

PlotThanks toPetr Vogel

50 meV orfew x 1027 yr


50 meVOr ~ 1027 yr

Normal

Inverted

0.1

1

10

100

1000Eff

ecti

ve !! M

ass

(m

eV

)

12 3 4 5 6 7

102 3 4 5 6 7

1002 3 4 5 6 7

1000

Minimum Neutrino Mass (meV)

Ue1

= 0.866 "m2

sol = 70 meV

2

Ue2

= 0.5 "m2

atm = 2000 meV

2

Ue3 = 0

Inverted

Normal

Degenerate

Solar Scale

Atmospheric Scale

KKDC Claim

An exciting time for ββ!

< mββ > in the range of10 - 50 meV is very interesting.

For the next experiments:

For at leastone

neutrino:

!

mi > "matmos2

# 50meV

!

m"" # 50meV


The 1st Observation


The Heidelberg-Moscow Experiment

Foun

datio

ns o

f Phy

sics

, 32,

(200

2)

8”

~10 kg of 76Ge13 years of data


Elliott & VogelAnnu. Rev. Part. Sci. 2002 52:115

102

103

104

105

Mass L

imit

(m

eV

)

20202000198019601940

Year

Ge-76Ge-76

Te-128

Te-128

Se-82

Ge-76

Ge-76

Ca-48Nd-150

Nd-150

EXO-200

GERDA

MJ-120

CUORE

>3.6x1021 y150Nd

<(0.8-5.6) eV>4.5x1023 y136Xe

<(0.41-1.) eV>3.0x1024 y130Te

<(1.1-1.5) eV>7.7x1024 y128Te

<1.7 eV>1.7x1023 y116Cd

<(0.6-2.7) eV>5.8x1023 y100Mo

<(1.2-3.2) eV>2.1x1023 y82Se

=0.44 eV=1.2x1025 y76Ge

<(0.33-1.35) eV>1.6x1025 y76Ge

<0.35 eV>1.9x1025 y76Ge

<(7.2-44.7) eV>1.4x1022 y48Ca

m!! "1

M G0#$1 / 2

%

& ' '

(

) * *


An Ideal ExperimentMaximize Rate/Minimize Background

• Large Mass (~ 1 ton)• Good source radiopurity

• Demonstrated technology• Natural isotope

• Small volume, source = detector• Good energy resolution

• Ease of operation• Large Q value, fast ββ(0ν)

• Slow ββ(2ν) rate• Identify daughter

• Event reconstruction• Nuclear theory

m!! "b#E

Mtlive

$

% & &

'

( ) )

1

4


Great Number of Proposed Experiments

• Calorimeter– Semi-conductors– Bolometers– Crystals/nanoparticles immersed in scintillator

• Tracking– Liquid or gas TPCs– Thin source with wire chamber or scintillator

“Found” PeaksPR

D45

, 254

8 (1

992)

A 2527-keV Ge-det. peak that was an electronic artifact.

A ~2528-keV Te-det.peak that was a 2σStatistical flucuation. N

P B3

5 (P

roc.

Sup

p.),

366

(199

4).

Need more thanone experiment


A Recent Claim for ββ(0ν)

Mod

. Phy

s. Le

tt. A

16, 2

409

(200

1)

20

15

10

5

0

Counts

20802060204020202000

Energy (keV)


The ROI

The “feature” at 2038keV is arguablypresent. This willprobably requireexperimental testing.

Background leveldepends on intensity fitto other peaks.


Future Data Requirements

Why wasn’t this claim sufficient to avoidcontroversy?

• Low statistics of claimed signal - hard torepeat measurement• Background model uncertainty• Unidentified lines• Insufficient auxiliary handles

Result needs confirmation or repudiation


Various Levels of Confidence

• A preponderance of the evidence: a combination of– Correct peak energy– Single-site energy deposit– Proper detector distributions (spatial, temporal)– Rate scales with isotope fraction

• Beyond a reasonable doubt: include the following– Observe the two-electron nature of the event– Measure kinematic dist. (energy sharing, opening angle)– Observe the daughter– Observe the excited state decay

• Open and shut case: the smoking gun– See the process in several isotopes


Classes of Background for ββ(0ν)

• ββ(2ν) tailNeed good energy resolution.

• Natural U, Th in source and shieldingPure materials, segmentation, pulse shape.

• Cosmic ray activationStore and prepare materials underground.


ββ(2ν) as a Background.Sum Energy Cut Only

next generationexperimentalgoal

10-4

10-3

10-2

10-1

100

101

102

103

<m

!!>

Sen

siti

vity

(meV

)

54321Resolution (%)

100Mo

136Xe

76Ge

130Te

2.0

1.5

1.0

0.5

0.0

dN

/d(K

e/Q

)

1.00.80.60.40.20.0Ke/Q

30

20

10

0

x1

0-6

1.101.000.90

Ke/Q

!

S

B=me

7Q" 6T1/ 2

2#

T1/ 2

0#


Natural Activity

• The Problem: τ(U, Th) ~ 1010 yearsGoal: τ(ββ(0ν)) ~ 1027 years

• Detector: Intrinsic Ge is very pure• Cryostat: Electro-formed Cu• Shielding: Roman Pb• Front End Electronics: behind shield


Cosmic Ray Induced Activity

• Material dependent.Lots of experience with Ge.

• Need for depth to avoid activation.

• Need for storage to allow activationto decay.


Pb(n,n’γ) and 76Ge: an example

206Pb 207Pb

3744 keV 3633 keV

1703 keV1467 keV1167 keV

803 keV

g.s.571 keV

g.s.

2041 keV

3062 keV

DEP of 3062line is at 2040

keV!!

Am

Be

stud

y at

LA

NL

n,n’

stu

dy a

t TU

NL

Documents

Neutrino Physics - Double-Beta Decay Steve Elliott Los ...June 2007 Steve Elliott, FNAL Neutrino Summer School 2 Lecture Outline •Double Beta Decay –Basic physics –General experimental