Double beta decay and Leptogenesis Nov. 6 -8 2009 연세대학교 원주캠퍼스 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 (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.

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 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.

Implication of 0nbb on baryogenesis

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~ 109-1015 GeV Baryogenesis via Leptogenesis (Fukugita,Yanagida,1986) B-L and CP violation and out-of-equilibrium

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

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 0nbb 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 0nbb decay can be factorized as : : phase space factor : Nuclear matrix element :depends on neutrino mass hierarchy

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

Neutrino mass spectrum

Normal hierarchy: Inverted hierarchy (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

For inverted hierarchy, a lower limit on <mn> obtained ( Hirsch et al. , hep-ph/0609146 ) For inverted hierarchy, a lower limit on <mn> obtained 8 meV

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

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 :

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. 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.

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)

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 : MR1 for neutrino mass scale Lower bound on MR1 :

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 : MR1 for neutrino mass scale Bound on type II MR 1 lower than Type I bound

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.