1. Double beta decay and Leptogenesis
Nov
연세대학교 원주캠퍼스
Sin Kyu Kang
(Seoul National University of Technology)

2. Prologue
With the discovery that neutrinos are not massless, there is intense interest in neutrinoless double-beta decay (0nbb) measurements.
0nbb decay probes fundamental questions :
Lepton number violation : leptogenesis might be the
explanantion for the observed matter-antimatter
asymmetry.
Neutrino properties : the only practical technique to
determine if neutrinos are their own anti-particle :
Majorana particles.

3. Establishing that neutrinos are Majorana particles would be as important as the discovery of neutrino oscillations
If neutrinos are Majorana particles
Neutrino oscillations :
- not sensitive to the nature of neutrinos
- provide information on , but not on
the absolute values of neutrino masses.

4. If 0nbb decay observed : Violates lepton number :
Neutrino is a Majorana particle.
Provides a promising lab. 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 processes.

5. Implication of 0nbb on baryogenesis

6. Non-zero neutrino masses Cosmological Baryon Asymmetry
Physics beyond the SM (New Physics Scale)
SEESAW MODEL
Two or more singlet neutrinos with Majorana masses M~ GeV
Baryogenesis via Leptogenesis (Fukugita,Yanagida,1986)
B-L and CP violation and out-of-equilibrium

7. Current observation of baryon asymmetry
Why do we exist ?
Current observation of baryon asymmetry
What created this tiny excess matter?
Baryogenesis
Necessary requirements for baryogenesis:
B number non-conservation
CP violation
Non-equilibrium
Sakharov’s conditions

8. Leptogenesis
Generate L from the direct CP violation in right-handed neutrino decay (Type I seesaw model)
Two generations enough for CP violation because of Majorana nature (choose 1 & 3)
L gets converted to B via EW anomaly
 More matter than anti-matter
 We have survived “The Great Annihilation”

9. In Type II seesaw model :

10. Can we prove it experimentally?
Unfortunately, no: it is difficult to reconstruct relevant CP-violating phases from neutrino data
But: we will probably believe it if
0nbb found
CP violation found in neutrino oscillation
EW baryogenesis ruled out

11. Neutrinoless double beta decay
Lepton number violation
Baryon asymmetry  Leptogenesis due to
violation of B-L number

12. The half-life time, , of the 0nbb decay can be factorized as :
: phase space factor
: Nuclear matrix element
:depends on neutrino mass hierarchy

13. Consistent with cosmological bound
Best present bound :
Heidel-Moscow
Half-life
Consistent with cosmological bound

14. Neutrino mass spectrum

15. Normal hierarchy: Inverted hierarchy
(Bilenky et al. ’01, Pascoli & Petcov ’04)
Normal hierarchy:
Inverted hierarchy

16. Quasi-degenerate
Estimate by using the best fit values of parameters including uncertainties in Majorana phases

17. For inverted hierarchy, a lower limit on <mn> obtained
( Hirsch et al. , hep-ph/ )
For inverted hierarchy, a lower limit on <mn> obtained
8 meV

18. In principle, a measurement of |<m>| combined with a measurement of m1(mass scale)
(in tritium beta-decay exp. and/or cosmology)
would allow to establish if CP is violated.
To constrain the CPV phases,
once the neutrino mass spectrum is known

19. Establishing CPV due to Majorana CP phases requires
Due to the experimental errors on the parameters and nuclear matrix elements uncertainties, determining that CP is violated in the lepton sector due to Majorana CPV phases is challeging.
Given the predicted values of , it might be possible only for IH or QD sepctra. In these two cases, the CPV region is inversely proportional to
Establishing CPV due to Majorana CP phases requires
Small experimental errors on and neutrino
masses
Small values of
depends on the CPV phases :

20. Connection between low energy CPV and leptogenesis
High energy parameters Low energy parameters
9 parameters are lost, of which 3 phases.
In a model-independent way there is no direct connection between the low-energy phases and the ones entering leptogenesis.

21. Using the biunitary parameterization,
depends only on the mixing in RH sector.
mn depends on all the parameters in Yn .
If there is CPV in VR, we can expect to have CPV in mn .
In models in which there is a reduced number of parameters, it is possible to link directly the Dirac and Majorana phases to the leptogenesis one.
Additional information can be obtained in LFV charged lepton decays which depend on VL.

22. Existence of a correlation between
In minimal seesaw with two heavy Majorana neutrinos
(Glashow, Frampton, Yanagida,02)
 mD contains 3 phases
(Endo,Kaneko,Kang,Morozumi,Tanimoto)
(2002)

23. Constraints on leptogenesis
Type I Seesaw (for MR1 << MR2, MR3) (S. Davidson etal. 02)
Bound on lepton asymmetry
for neutrino mass scale
For successful thermal leptogenesis :
MR for neutrino mass scale
Lower bound on MR1 :

24. Type II Seesaw (for MR1 << MR2, MR3 , MD ) (S.F.King 04)
Bound on lepton asymmetry
for neutrino mass scale
(in sharp contrast to type I)
For successful thermal leptogenesis :
MR for neutrino mass scale
Bound on type II MR 1 lower than Type I bound

25. Leptogenesis takes place in the context of see-saw models, which
Conclusions
Establishing that neutrinos are Majorana is a fundamental and
challenging task.
Leptogenesis takes place in the context of see-saw models, which
explain the origin of neutrino masses.
The observation of neutrinoless double beta decay (L violation)
and of CPV in the lepton sector would be an indication, even if
not a proof, of leptogenesis as the explanation for the observed
baryon asymmetry of the Universe.
Constraints on leptogensis indicate that type I leptogenesis bad
prospects for observing 0nbb-decay whereas type II leptogenesis
good prospects for observing 0nbb-decay.