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CENPACENPACenter for Experimental Nuclear Physics and AstrophysicsUniversity of Washington
Peter Rosen and Neutrinoless Double-Beta Decay
A legacy of insights and advocacy
J.F. WilkersonCarolina International Symposium
on Neutrino PhysicsMay 16, 2008
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Impetus for 0νββ Our view of neutrinoless double beta decay (0νββ-decay) has dramatically evolved and shifted over the years. With the discovery that neutrinos are not massless, there is intense interest in neutrinoless double-beta decay (0νββ) measurements.
0νββ decay probes fundamental questions:• Lepton number conservation — might Leptogenesis be the
explanation for the observed matter - antimatter asymmetry?• Neutrino properties — the only practical technique to determine if
neutrinos are their own anti-particles — Majorana particles.
If 0νββ is observed:• Provides a promising laboratory method for determining the absolute
neutrino mass scale that is complementary to other measurement techniques.
• Measurements in a series of different isotopes potentially can reveal the underlying interaction process(es).
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Early Understanding
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Double-Beta Decay
48Ca, 76Ge, 82Se, 96Zr 100Mo, 116Cd 128Te, 130Te, 136Xe, 150Nd
In a number of even-even nuclei, β-decay is energetically forbidden or strongly disfavored, while double-beta decay, from a nucleus of (A,Z) to (A,Z+2), is energetically allowed.
A, Z+2
A, Z+3
A, Z+1
A, Z
A, Z-1
ββ
0+
0+
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Double-Beta Decay Modes
W-
e- e-
νe νe
W-
>> Nuclear Process(A, Z) (A, Z+2)
0ν double-beta decay (0νββ): Nucleus (A, Z) → Nucleus (A, Z+2) + e- + e-
2ν double-beta decay (2νββ): Nucleus (A, Z) → Nucleus (A, Z+2) + e- + νe + e- + νe
Allowed second-order weak process
Maria Goeppert-Mayer (1935)
Ettore Majorana (1937) realized symmetry properties of Dirac’s theory allowed the
possibility for electrically neutral spin-1/2 fermions to be their own anti-particle
2νββ observed for48Ca, 76Ge, 82Se, 96Zr 100Mo, 116Cd 128Te, 130Te, 150Nd
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Double-Beta Decay Modes
W-
e- e-
νe νe
W-
>> Nuclear Process(A, Z) (A, Z+2)
0ν double-beta decay (0νββ): Nucleus (A, Z) → Nucleus (A, Z+2) + e- + e-
2ν double-beta decay (2νββ): Nucleus (A, Z) → Nucleus (A, Z+2) + e- + νe + e- + νe
Allowed second-order weak process
Maria Goeppert-Mayer (1935)
>> Nuclear Process(A, Z) (A, Z+2)
W- W-
e- e-
ν ν
2νββ observed for48Ca, 76Ge, 82Se, 96Zr 100Mo, 116Cd 128Te, 130Te, 150Nd
ν + n → p + e-n → p + e- + ν Racah (1937),
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Early Estimates of ββ Decay Rates0ν double-beta decay (0νββ) 2ν double-beta decay (2νββ)
Maria Goeppert-Mayer (1935)using Fermi Theory
Furry (1939), assuming Parityconserved, so no preferential handedness
0νββ mode highly favored over 2νββ
If observe 0νββ ⇒
neutrinos are Majorana
If observe 2νββ ⇒
neutrinos are Dirac
1/20νββT
−1∝Phase Space (2-body) ∝Q5
Avignone, Elliott, Engle
1/22νββT ≈ 1022 years 1/20νββT ≈ 1016 years
1/22νββT
−1∝Phase Space (4-body) ∝Q11
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
50 years ago:Revelations & Revolution
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
1956 The ν is first observed (in SC)Reines, Cowan, Harrison, McGuire, and Kruse
Science, July 1956
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Weak Interaction maximally violates parity
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
1958 Goldhaber-Grodzins-Sunyar
• Weak Interactionmaximally violates parityV-A natureneutrinos emitted in beta-decay have intrinsic handedness
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Implications for ββ-decayPrimakoff and Rosen Rep. Prog. Phys. 22 121 (1959)
• Initially considered most general, Lorentz invariant Hamiltonian -- included the possibility of both L-conserving and L-violating interactions, P-conserving and P-violating terms, along with potential contributions from S ,T, V, and A interactions.
• Recognized that with a “two-component” neutrino coupling (emitted in pure left or right handed helicity states) then no neutrinoless double-beta decay.
• Calculated both 2νββ and 0νββ-decay rates. For 0νββ assumed a predominately V-A and P-violating interaction, but also some amount of parity-conserving, lepton-violating scalar interaction. (Allowed by the existing experimental constraints.)
• Utilized closure method to account for the Nuclear Processes in calculating rates - nuclear structure, average separation between neutrons
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Implications for ββ-decay
Primakoff and Rosen Rep. Prog. Phys. 22 121 (1959)
Conclusions
•Observation of 2νββ tells one nothing about Dirac/Majorana nature of ν. Possible to have Majorana neutrinos, but a pure “two component” parity-violating interaction would not allow one to observe 0νββ.
•Calculated T1/2 for 2νββ-decays on order of 1020 to 1022 years
• Calculated T1/2 for 0νββ-decays on order of 1015 to 1016 years, with assumption that |Fs/Fv |2 ~ 1/3
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Prospect for 0νββ ? Nuovo Cimento 17, Suppl. I, 132 (1960).
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
ββ-decay searches (1959)
Rep. Prog. Phys. 22 121 (1959)
geo-chemical
tracking(cloud chamber
or emulsion)
e1, e2 time coincidence
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
ββ-decay - New Expt. Methods- CaF2 scintillating crystal- Source == Detector- T1/2 (0νββ) ≥ 2 x 1020 years
48Ca
76Ge - Ge(Li) detector (HPGe crystal)- Source == Detector- T1/2 (0νββ) ≥ 3 x 1020 years
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Towards a contemporary view
>> Nuclear Process(A, Z) (A, Z+2)
W- W-
e- e-
νi (R) νi (L)Uei Uei
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
ν Helicity and 0νββ-decay
>> Nuclear Process
(A, Z) (A, Z+2)
W- W-
e- e-
νi νi
ν + n → p + e-n → p + e- + ν
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
ν Helicity and 0νββ-decay
>> Nuclear Process
(A, Z) (A, Z+2)
W- W-
e- e-
νi νi(R) (L)
ν + n → p + e-n → p + e- + ν (R)(L)
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
ν Helicity and 0νββ-decay
The transition can occur if neutrinos have mass via a reference frame boost“wrong-handed” helicity admixture ~ mi/Eνi
>> Nuclear Process
(A, Z) (A, Z+2)
W- W-
e- e-
νi νi(R) (L)
0νββ requires in addition to Majorana neutrinos and a lepton violating interaction, a mechanism to “flip” from right handed anti-neutrino to left-handed neutrino
ν + n → p + e-n → p + e- + ν (R)(L)
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Opposite-helicity lepton violating interaction
“... we parameterized a possible lepton nonconservation by inclusion in the lepton weak current of an “opposite-helicity” term (η) ... however we kept our “Majorana” neutrino massless; even though a nonvanishing η in general implies a nonvanishing mν proportional to η”
Primakoff and Rosen Phys. Rev. 184 1925 (1969)
Primakoff and Rosen Phys. Rev. 184 1925 (1969)
1/20νT
−1=G0ν M0ν
2η2
>> Nuclear Process(A, Z) (A, Z+2)
W- W-
e- e-
νi (R) νi (L)Uei UeiFound limits on η of ~10-4
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Opposite-helicity lepton violating interaction
“... we parameterized a possible lepton nonconservation by inclusion in the lepton weak current of an “opposite-helicity” term (η) ... however we kept our “Majorana” neutrino massless; even though a nonvanishing η in general implies a nonvanishing mν proportional to η”
Primakoff and Rosen Phys. Rev. 184 1925 (1969)
Any process that allows 0νββ to occur requires Majorana neutrinos with non-zero mass.
Schechter and Valle (1982)
Primakoff and Rosen Phys. Rev. 184 1925 (1969)
1/20νT
−1=G0ν M0ν
2η2
>> Nuclear Process(A, Z) (A, Z+2)
W- W-
e- e-
νi (R) νi (L)Uei UeiFound limits on η of ~10-4
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
The rise of GUTs
Primakoff and Rosen, Ann. Rev. Nucl. Part. Sci. 1981
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Motivates new searches for 0νββ
T1/2 (0νββ) ≥ 1.7 x 1022 years
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
0νββ Decay - Current Understanding
>> Nuclear Process(A, Z) (A, Z+2)
W- W-
e- e-
νi (R) νi (L)Uei Uei
0νββ Requires:• neutrino must have non-zero mass
• Rate goes as (mi/Eνi)2 so 0νββ strongly suppressed compared to 2νββ• lepton number violation
• No experimental evidence that Lepton number is conserved (Allowed by SM principles, such as electroweak-isospin conservation and renormalizability)
Amp[0νββ]∝ miUei2
i∑ ≡ mββ
1/20νT
−1=G0ν M0ν
2η2
⇓
1/20νT
−1=G0ν M0ν
2mββ
2
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Reflections on the future
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
0νββ Decay Sensitivity to
58
9≡ 0
ν3Atm.
ν2ν1
Solar
ν2ν1
ν3Atm.
Solar
≡ 0
5049
Assumes LV mechanism is light Majorana neutrino exchange
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
0νββ Decay Sensitivity to
58
9≡ 0
ν3Atm.
ν2ν1
Solar
ν2ν1
ν3Atm.
Solar
≡ 0
5049
KKDC 76Ge claim
Disfavored by C
osmology
KATR
IN expected sensitivity
Disfavored by ββ-decay
Assumes LV mechanism is light Majorana neutrino exchange
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Underlying 0νββ Decay Mechanisms
There are many possible underlying mechanisms for 0νββ decay (with Lepton violating interactions (η))
• light Majorana neutrino exchange• heavy Majorana neutrino exchange• right-handed currents (RHC)• exchange mechanisms arising from R-Parity violating
supersymmetry models.• • •
1/20νT
−1=G0ν M0ν
2mββ
2=G0ν M0ν (η)
2η2
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
0νββ-decay as a Probe of LV InteractionsIf 0νββ is observed, then measurements on 3-4 multiple isotopes might be able to distinguish potential physics mechanisms
Comparison assumes a single dominant mechanism.
Requires results from 3-4 isotopes & calculation of NME to ~20%
Gehman & Elliott J.Phys.G34:667, 2007arXi:hep-ph/0701099
Also seeDeppisch & PäsPhys.Rev.Lett.98:232501,2007arXi:hep-ph/0612165
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Discovery of 0νββ-decay• Strong evidence : a combination of
• Correct peak energy• Single-site energy deposit• Proper detector distributions (spatial, temporal)• Rate scales with isotope fraction• Full energy spectrum understood
• Further confirmation: more difficult• Observe the two-electron nature of the event• Measure kinematic dist. (energy sharing, opening angle)• Observe the daughter• Observe the excited state decay
• Irrefutable• Observe the process in several isotopes, using a variety of
experimental techniques
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
Summary• The observation of 0νββ-decay would demonstrate Lepton
number violation and indicate that neutrinos are Majorana particles - constituting a major discovery.- Needs to be confirmed from independent experiments using
different isotopes and measurement techniques.
• If 0νββ-decay is observed then it opens an exquisitely sensitive window to search for physics beyond the Standard model. - Measurements in different isotopes may provide insights into
the underlying physics process(es) (η).- Extraction of requires understanding NME and of the
underlying lepton violating interaction.
Peter Rosen and 0νββ-Decay - Insights and Advocacy CINSP, May 16, 2008
0νββ-decay and Peter Rosen
When Peter passed away in the fall of 2006, we lost an eloquent spokesman and tireless advocate for 0νββ who is greatly missed by his
friends and colleagues.
Special thanks to Boris Kayser and Lincoln Wolfenstein.
Over the past 50 years there have been dramatic changes in our understanding of the
framework of nuclear and particle physics. And yet 0νββ remains extremely relevant as
we endeavor to elucidate the underlying framework of our universe.
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