Distinguishable RGE Running Effects Between Dirac Neutrinos and Majorana Neutrinos with Vanishing Majorana CP-Violating Phases

In a novel parametrization of neutrino mixing and in the approximation of τ-lepton dominance, we show that the one-loop renormalization-group equations （RGEs） of Dirac neutrinos are different from those of Majorana neutrinos even if two Majorana CP-violating phases vanish. As the latter can keep vanishing from the electroweak scale to the typical seesaw scale, it makes sense to distinguish between the RGE running effects of neutrino mixing parameters in Dirac and Majorana cases. The differences are found to be quite large in the minimal supersymmetric standard model with sizable tan β, provided the masses of three neutrinos are nearly degenerate or have an inverted hierarchy.

Active-Sterile Neutrino Oscillations and Leptogenesis

We study coherent active-sterile neutrino oscillations as a possible source of leptogenesis. To this end, we add 3 gauge invariant Weyl_R neutrinos to the Standard Model with both Dirac and Majorana type mass terms. We find that the measured active neutrino masses and mixings, and successful baryogenesis via leptogenesis, may be achieved with fine-tuning, if at least one of the sterile neutrinos has a mass in the approximate range 0.14 to 1.1 GeV.

Massive and massless neutrinos on unbalanced seesaws

The observation of neutrino oscillations requires new physics beyond the standard model （SM）. A SM-like gauge theory with p lepton families can be extended by introducing q heavy right-handed Majorana neutrinos but preserving its SU（2）L × U（1）Y gauge symmetry. The overall neutrino mass matrix M turns out to be a symmetric （p＋q） × （p＋q） matrix. Given p 〉 q, the rank of M is in general equal to 2q, corresponding to 2q non-zero mass eigenvalues. The existence of （p-q） massless left-handed Majorana neutrinos is an exact consequence of the model, independent of the usual approximation made in deriving the Type-I seesaw relation between the effective p x p light Majorana neutrino mass matrix Mv and the q × q heavy Majorana neutrino mass matrix MR. In other words, the numbers of massive left- and right-handed neutrinos are fairly matched. A good example to illustrate this ＂seesaw fair play rule＂ is the minimal seesaw model with p = 3 and q = 2, in which one massless neutrino sits on the unbalanced seesaw.

Search for Majorana Neutrino Signal in B_c Meson Rare Decay

We study the Bc meson rare decay in order to search for the Majorana neutrino signal. It is found that the corresponding decay rate is sensitive to the Majorana neutrino mass and mixing angles. The signal of B~ --~ lil l~ Mm induced by the Majorana neutrino within the mass region mr 〈 mn 〈mB may be observed at LHCb.

Lepton number violation in D meson decay

The lepton number violation （LNV） process can be induced by introducing a fourth generation heavy Majorana neutrino, which is coupled to the charged leptons of the Standard Model （SM）. There have been many previous studies on the leptonic number violating decay processes with this mechanism. We follow the trend to study the process： D→Kllπ with the same-sign dilepton final states. We restrict ourselves to certain neutrino mass regions, in which the heavy neutrino could be on-shell and the dominant contribution to the branching fraction comes from the resonance enhanced effect. Applying the narrow width approximation （NWA）, we found that the upper limit for the branching fractions for D^0 → K^- 1＋1＋π- are generally at the order of 10-12 to 10-9, if we take the most stringent upper limit bound currently available in the literature for the mixing matrix elements. We also provide the constraints, which is competitive compared to the LNV B decays, on the mixing matrix element ｜VeN ｜^2 based on the upper limit of D^0→K^-e＋e＋π- estimated from the Monte-Carlo （MC） study at BESⅢ. Although the constraints are worse than the ones from （0vββ） decay in the literature, the future experiment at the charm factory may yield more stringent constraints.