NEUTRINOLESS DOUBLE
BETA DECAY
DAVID SINCLAIR
ICFA 2013 BOGOTA
PHYSICS OF DOUBLE BETA DECAY
• Understanding Neutrinoless DBD is closely coupled
to understanding neutrino masses and mixing
• We therefore make a diversion to look at what we
know
ASSUMING 3 FAMILIES
Pontecorvo Maki Nakagawa Sakata Matrix
éU e1 U e 2 U e 3 ù
ê
ú
U = êU m1 U m 2 U m 3 ú
êëUt 1 Ut 2 Ut 3 úû
é1 0
0 ù é c13
ê
ú ê
U = ê0 c 23 s23 ú * ê 0
êë0 -s23 c 23 úû êë -s13e id
Atmospheric
Minos
T2K
LBNE
0 s13e -id ù é c12 s12 0 ù ée ia 1/ 2
0
0ù
ú ê
ú
ú ê
ia 2 / 2
1
0 ú * ê -s12 c12 0 ú * ê 0
e
0ú
0
c13 úû êë 0
0 1 úû êë 0
0
1 úû
Reactor
T2K
Minos
Solar
Solar
KAMLAND
bb 0n
WHAT DO WE KNOW ABOUT MASS
•
•
•
•
•
•
•
Neutrino oscillations tell us Dm2
From solar experiments, Kamland Dm212=8 x 10-5 eV2
From Atmospheric, Minos, T2K Dm223=2.4 x 10-3 eV2
We know the sign of Dm12 from matter effects
We don’t know the sign of Dm23 so we have
Normal hierarchy (m2 < m3) or
Inverted hierarchy (m2>m3)
OTHER CONSTRAINTS ON MASS
•
•
•
•
•
•
Tritium beta decay end point limits mne
Mainz measurement mne <2.2 eV
Similar limit from SN1987A
Cosmological Constraints
The number and mass of neutrinos impact the CMB
Also impact on large scale structure
Slide from Yvonne Wong
Taup 2011
Ofer
Lahav
WHAT DO WE KNOW ABOUT MIXING
ANGLES
•
•
•
•
•
•
With good accuracy
F12 = 33.8o from solar, kamland
F23 = 45o from SuperK, Minos…
F13 = 9o from reactors
d CP phase not known
a1, a2 Majorana phases not known
DIRECT MASS MEASUREMENT
• New tritium beta spectrum to be measured (Katrin)
• Sensitive to electron neutrinos to 200 meV
• New ideas being developed for lower energy beta
decays eg 187Re (E=2.47 keV) in cryogenic crystals
(eg MARE) future potential ~ 100 meV
• If electron neutrino mass is in this range then we
have degenerate masses
• Close to being ruled out by cosmological
observations
Katrin Vacuum Tank
SUMMARY
• Neutrino masses are not 0 but are very small
• Neutrino mixing is very large
• Leads to the possibility that the baryon asymmetry
in the universe arises from neutrino properties
• Sakharov conditions for matter-antimatter
asymmetry
• Baryon number violation
• C and CP violation
• Deviation form thermal equilibrium
NEUTRINO MASS IN THE STANDARD
MODEL
• In the standard model neutrino masses are 0
L = mD (fLfR + fRfL )
• Because we only observe left handed neutrinos we
cannot form a Dirac mass term this way
• Possible to form a Majorana mass term
1
c
*
c
L = (mM ff + mM f f )
2
MAJORANA AND DIRAC MASS
For mLM ~ 0 , mRM>>mD we have the ‘Seesaw model’
SEESAW MODEL
æ 0
M =ç
ç mD
è
mD
mR
ö
÷
÷
ø
mn i » mqi2 / mR
• Neutrino masses are very small because of mR in
denominator. mR is at the gut scale
• If mL is not zero it can dominate and give
degenerate neutrino masses
NEUTRINOS AND LEPTOGENESIS
• The only neutrinos which can impact the baryon
asymmetry are the very heavy right handed
neutrinos
• We would like to understand CP violation in this
sector
• This is far beyond the reach of experimental physics
• May be related to CP violation in light sector
• See e.g. Pascoli, Petcov and Riotto, CERN-PH-TH/2006-213
• This can come from either Dirac CP term d or from
the Majorana phases a or both
WHAT WOULD WE LIKE TO LEARN
ABOUT NEUTRINOS
• Precision measurements of oscillation parameters
including q13
• Determine the mass hierarchy critical
• Determine d
• Are neutrinos Majorana
• Determine the a parameters
• Show violation of total lepton number
NEUTRINO-LESS DOUBLE BETA DECAY
• Observation of neutrino-less double beta decay
would
• Demonstrate that neutrinos are Majorana particles
• Demonstrate DL=2 total lepton number violating process
• Set mass scale for the neutrino
• Rate is given by
1
0n
0n
2
= G | M | mbb
T1/ 2
DOUBLE BETA (CONT.)
• G is known, scales with E5
• M is a nuclear matrix element. Calculations are
converging (factor of 2)
• m2bb contains neutrino mixing information
mbb =| åU mi |=
2
ei
i
åU
i
2
ei
iai
e mi
Nucl. Phys. B659 359
Dark areas
Show variation
due to phases only
Light colours
include
experimental
errors
Assumed q13 =0
WHAT CAN WE LEARN?
• Any detection proves that neutrinos are Majorana
particles and that total lepton number is violated
• A crude measure sets the neutrino mass scale
• A precision measurement together with a precision
measure of the sum of the neutrino masses (or
measure of the electron neutrino mass) and a
precision nuclear matrix element may give the
Majorana phase
Heidelberg-Moscow Results for Ge
double beta decay
57 kg years of 76Ge data
Apply single site criterion
CANDIDATE ISOTOPES
Isotope
Energy (keV)
Abundance %
76Ge
2039.
7.8
136Xe
2462
8.9
130Te
2530
34.5
82Se
2996
9.2
100Mo
3035
9.6
150Nd
3367
5.6
48Ca
4274
0.19
PROJECTS UNDER CONSTRUCTION
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Majorana
30 kg 76Ge Ionization
Cuore
200 kg 130Te Bolometer
SNO+
44 kg 150Nd Scintillator
SuperNemo
200 kg 72Se Tracking calorimeter
NEXT
100 kg 136Xe Gas TPC
Many more R&D projects
OPERATING DETECTORS - EXO
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•
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200 kg enriched 136Xe
Ionization + scintilation
Tracking TPC
Lead shield
Aims to cover inverted hierarchy
OPERATING DETECTORS KAMLANDZEN
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•
•
•
400 kg enriched Xe dissolved into scintillator
Shielding by mineral oil
Almost ready to turn on
Aims to prove or reject Heidelberg Moscow
OPERATING DOUBLE BETA
EXPERIMENTS
• Gerda
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•
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76Ge,
15 -> 35 kg
Ionization detector
Liquid Ar + water shield, multisite selection
Direct test of Heidelberg-Moscow
Ultimate sensitivity is full inverted hierarchy
PROBLEM AT GERDA
• Shield is liquid argon
• Argon is activated by cosmic rays to produce 42K
• The 42K drifts to the germanium crystals and decays
giving a background. Rate higher than expected
• New designs to protect the Ge from ions
EXO FUTURE
• Next step will be nEXO
• 5 T liquid xenon enriched in 136Xe
• Location likely to be SNOLAB
nEXO
SNOlab, Cryopit Workshop, 21 Aug 2013
72
BARIUM TAGGING
Requires Ba+ ion
2P
1/2
Double beta decay
produces Ba++
650nm
493nm
4D
3/2
metastable 80s
2S
1/2
EXTRACTION OF IONS
Intensity
• Test process using atmospheric pressure
electrospray source and a quadrupole mass spec
3000
´106
2500
Ba++
2000
1500
1000
500
0
40
60
80
100
120
140
160
180
200
m/z
CONVERSION FROM BA++ TO BA+
• Pass ions through low pressure TEA
• TEA has low IP and can give up an electron to Ba++
but not to Ba+
• Use triple quadrupole system. First quad selects
Ba++, second contains the TEA, third analyses the
products
• Conversion efficiency looks very high and no
evidence fro molecular formation
Intensity
2500
´103
TEA +
2000
Ba+
1500
1000
500
0
0
(TEA - CH3)+
50
100
150
200
250
300
m/z
TIMESCALE FOR NEXT PHASE
• EXO is taking 0 neutrino search data now
• Will probably reach background limit in couple of
years
• DOE has indicated it wants to make a decision on
next generation detector in ~ 2 years
• We need to have a developed proposal on this
timescale
LIFE AT THE TONNE SCALE
• Xe is the easiest of the isotopes to separate
• But if we want 10 tonnes of 136Xe we need to start
with 100 tonnes of natural
• World production is 30 tonnes
• Producing 10 tonnes of 136Xe would take 5 years at
the Russian separators
• Clearly if this goes ahead it has to be a global effort
• Movement to coordinate astroparticle physics
globally
WHERE WILL THIS TAKE US IN A
DECADE? (PERSONAL GUESSES)
• Precision study of the PMNS matrix (except
Majorana phases) will move to the accelerators
• Masses and number of neutrinos will be tightly
constrained by cosmology
• We may be studying details of double beta decay
or we maybe building one or two global projects to
look for double beta decay with much greater
sensitivity than the current detectors