Double beta decay and Leptogenesis International workshop on double beta decay searches Oct SNU Sin Kyu Kang (Seoul National University of Technology)

Prologue With the discovery that neutrinos are not massless, there is intense interest in neutrinoless double-beta decay (0  measurements. 0  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.

 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.

 If 0  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.

Implication of 0  on baryogenesis

Non-zero  neutrino masses 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 Physics beyond the SM (New Physics Scale) Cosmological Baryon Asymmetry

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

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”

In Type II seesaw model :

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 –0  found –CP violation found in neutrino oscillation –EW baryogenesis ruled out

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

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

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

Neutrino mass spectrum

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

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

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

 In principle, a measurement of | | combined with a measurement of m 1 (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

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 :

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.

 Using the biunitary parameterization,  depends only on the mixing in RH sector.  m  depends on all the parameters in Y.  If there is CPV in V R, we can expect to have CPV in m.  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 V L.

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

Constraints on leptogenesis  Type I Seesaw (for M R1 << M R2, M R3 ) (S. Davidson etal. 02) Bound on lepton asymmetry for neutrino mass scale For successful thermal leptogenesis : M R1 for neutrino mass scale Lower bound on M R1 :

 Type II Seesaw (for M R1 << M R2, M R3, M   S.F.King 04  Bound on lepton asymmetry for neutrino mass scale For successful thermal leptogenesis : M R1 for neutrino mass scale Bound on type II M R 1 lower than Type I bound (in sharp contrast to type I)

 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 0  -decay whereas type II leptogenesis good prospects for observing 0  -decay.