Peter Rosen and Neutrinoless Double-Beta...

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).


Early Understanding


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+


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


Double-Beta Decay Modes

W-

e- e-

νe νe

W-


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)


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),


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


50 years ago:Revelations & Revolution


1956 The ν is first observed (in SC)Reines, Cowan, Harrison, McGuire, and Kruse

Science, July 1956


Weak Interaction maximally violates parity


1958 Goldhaber-Grodzins-Sunyar

• Weak Interactionmaximally violates parityV-A natureneutrinos emitted in beta-decay have intrinsic handedness


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


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


Prospect for 0νββ ? Nuovo Cimento 17, Suppl. I, 132 (1960).


ββ-decay searches (1959)

Rep. Prog. Phys. 22 121 (1959)

geo-chemical

tracking(cloud chamber

or emulsion)

e1, e2 time coincidence


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


Towards a contemporary view


W- W-

e- e-

νi (R) νi (L)Uei Uei


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



>> 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)



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)


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)


1/20νT

−1=G0ν M0ν

2η2


W- W-

e- e-

νi (R) νi (L)Uei UeiFound limits on η of ~10-4


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”


Any process that allows 0νββ to occur requires Majorana neutrinos with non-zero mass.

Schechter and Valle (1982)


1/20νT

−1=G0ν M0ν

2η2


W- W-

e- e-

νi (R) νi (L)Uei UeiFound limits on η of ~10-4


The rise of GUTs

Primakoff and Rosen, Ann. Rev. Nucl. Part. Sci. 1981


Motivates new searches for 0νββ

T1/2 (0νββ) ≥ 1.7 x 1022 years


0νββ Decay - Current Understanding


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


Reflections on the future


0νββ Decay Sensitivity to

58

9≡ 0

ν3Atm.

ν2ν1

Solar

ν2ν1

ν3Atm.

Solar

≡ 0

5049

Assumes LV mechanism is light Majorana neutrino exchange


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


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


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


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


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.


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.

Peter Rosen and Neutrinoless Double-Beta...

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