Leptonic flavor-changing Z decays in SU(2)U(1) theories with right-handed neutrinos

We analyze possible lepton-flavor-violating decays of the Z⁰ particle in a minimal extension of the standard model, in which one right-handed neutral field for each family has been introduced. Such rare leptonic decays are induced by Majorana neutrinos at the first electroweak loop level and are generally not suppressed by the ordinary “see-saw” mechanism. In particular, we find that experimental bounds on branching ratios of the order of 10⁻⁵–10⁻⁶ attainable at LEP may impose constraints on lepton-flavor-mixing parameters and the masses of the heavy Majorana neutrinos.     

The self-dual Chern-Simons CP(N) models

We study the Chern-Simons CP(N) models with a global U(1) symmetry and found the self-dual models among them. The Bogomolnyi-type bound in these self-dual models is a nontrivial generalization of that in the pure CP(N) models. Our models have quite a rich vacuum and soliton structure and approach the many known gauged self-dual models in an appropriate limit.     

CP violation in multi-Higgs supersymmetric models

We consider supersymmetric extensions of the standard model with two pairs of Higgs doublets. We study the possibility of spontaneous CP violation in these scenarios and present a model where the origin of CP violation is soft, with all the complex phases in the Lagrangian derived from complex masses and vacuum expectation values (VEVs) of the Higgs fields. The main ingredient of the model is an approximate global symmetry, which determines the order of magnitude of Yukawa couplings and scalar VEVs. We assume that the terms violating this symmetry are suppressed by powers of the small parameter ε_PQ = O(m_b/m_t). The tree-level flavor-changing interactions are small due to a combination of this global symmetry and a flavor symmetry, but they can be the dominant source of CP violation. All CP-violating effects occur at order ε_PQ² as the result of exchange of almost decoupled extra Higgs bosons and/or through the usual mechanisms with an almost real CKM matrix. On dimensional grounds, the model gives ε_K ≈ ε_PQ² and predicts for the neutron electric dipole moment (and possibly also for ε_K¹) a suppression of order ε_PQ² with respect to the values obtained in standard and minimal supersymmetric scenarios. The predicted CP asymmetries in B decays are generically too small to be seen in the near future. The mass of the lightest neutral scalar, the strong CP problem, and possible contributions to the Z decay into b quarks (the R_b puzzle) are also briefly addressed in the framework of this model.     

Energy crisis or a new soliton in the noncommutative CP(1) model?

The non-commutative (NC) CP(1) model is studied from field theory perspective. Our formalism and definition of the NC CP(1) model differs crucially from the existing one [Phys. Lett. B 498 (2001) 277, hep-th/0203125, hep-th/0303090].

Due to the U(1) gauge invariance, the Seiberg–Witten map is used to convert the NC action to an action in terms of ordinary spacetime degrees of freedom and the subsequent theory is studied. The NC effects appear as (NC parameter) θ-dependent interaction terms. The expressions for static energy, obtained from both the symmetric and canonical forms of the energy momentum tensor, are identical, when only spatial noncommutativity is present. Bogomolny analysis reveals a lower bound in the energy in an unambiguous way, suggesting the presence of a new soliton. However, the BPS equations saturating the bound are not compatible to the full variational equation of motion. This indicates that the definitions of the energy momentum tensor for this particular NC theory (the NC theory is otherwise consistent and well defined), are inadequate, thus leading to the “energy crisis”.

A collective coordinate analysis corroborates the above observations. It also shows that the above mentioned mismatch between the BPS equations and the variational equation of motion is small.     

Family symmetry, gravity, and the strong CP problem

We show how in a class of models Peccei-Quinn symmetry can be realized as an automatic consequence of a gauged U(1) family symmetry. These models provide a solution to the strong CP problem either via a massless u-quark or via the DFSZ invisible axion. The local family symmetry protects against potentially large corrections to induced by quantum gravitational effects. In a supersymmetric extension, the “μ-problem” is shown to have a natural solution in the context of gravitationally induced operators. We also present a plausible mechanism which can explain the integ-generational mass hierarchy in such a context.     

Same-sign dileptons as a signature for heavy Majorana neutrinos in hadron-hadron collisions

We discuss the possibility of same-sign dileptons as a signature for Majorana neutrinos. The production mechanism is given by a single heavy neutrino production and decay pp → l^±NX → l^±l^±X′. Cross section and distributions are presented for the LHC energies.     

On the penguin phases and missing CP asymmetries in exclusive hadronic B decays

Within the standard model, we elucidate the picture of exclusive hadronic B-meson decays, which occur through both the tree and penguin channels. We emphasize that the weak penguin phase is entangled with the strong one and they can be parted only in special approximation. Ignoring this point will lead to significant missing of direct CP violation in either charged-B or neutral-B decays. The effects of these phases on CP-violating partial rate asymmetries are examined, and the relevant ambiguities in the previous literature are clarified.     

Light neutrinos from a mini-seesaw mechanism in warped space

The seesaw mechanism provides a simple explanation for the lightness of the known neutrinos. Under the standard assumption of a weak scale Dirac mass and a heavy sterile Majorana scale the neutrino mass is naturally suppressed below the weak scale. However, Nature may employ Dirac and Majorana scales that are much less than typically assumed, possibly even far below the weak scale. In this case the seesaw mechanism alone would not completely explain the lightness of the neutrinos. In this work we consider a warped framework that realizes this possibility by combining naturally suppressed Dirac and Majorana scales together in a mini-seesaw mechanism to generate light neutrino masses. Via the AdS/CFT correspondence the model is dual to a 4D theory with a hidden strongly coupled sector containing light composite right-handed neutrinos.     

Majorana neutrinos and CP violation in the leptonic sector

A set of weak-basis-independent necessary conditions is derived for CP conservation in the leptonic sector of the SU(2) × U(1) gauge theory with an arbitrary number of generations, when both Majorana and Dirac neutrino mass terms are present. In the case of two and three generations of left-handed Majorana neutrinos, weak-basis-independent conditions are given which are necessary and sufficient for CP invariance in the leptonic sector. It is also shown that for three or more generations of Majorana neutrinos, there may be CP violation even in the limit of complete neutrino-mass degeneracy.     

Neutrino flavor conversion in random magnetic fields

If massive neutrinos possess magnetic moments, a magnetic field can cause a spin flip. In the case of Dirac neutrinos the spin flip converts an active neutrino into a sterile one and vice versa. By constrast, if neutrinos are Majorana particles, a spin flip converts them to a neutrino of a different flavor. We examine the behavior of neutrinos in a random magnetic field as it occurs, for instance, in certain astronomical objects, such as an active galactic nucleus. Both Dirac and Majorana neutrinos behave ergodically: independently of their initial density matrix, they tend towards an equipartition of the helicity states. As a result, about half of the Dirac neutrinos produced becomes sterile. For Majorana neutrinos, there will be an approximate equipartition of flavors, independently of the production mechanism.     

Search for heavy Majorana neutrinos at future lepton colliders

A nonzero neutrino mass may be a sign of new physics beyond the standard model (SM). To explain the small neutrino mass, we can extend the SM using right-handed Majorana neutrinos in a low-scale seesaw mechanism, and the CP violation effect can be induced due to the CP phase in the interference of heavy Majorana neutrinos. The existence of heavy Majorana neutrinos may lead to lepton number violation processes, which can be used to search for the signals of heavy Majorana neutrinos. In this paper, we focus on the CP violation effect related to two generations of heavy Majorana neutrinos at \begin{document}$ 15 $\end{document} GeV \begin{document}$ <m_{N_1}< 70 $\end{document} GeV in the pair production of W bosons and rare decays. It is valuable to investigate Majorana neutrino production signals and the related CP violation effects in rare W boson decays at future lepton colliders.     

Probing vector-like top partner from same-sign dilepton events at the LHC

The standard model is a successful theory but is lacking a mechanism for neutrino mass generation as well as a solution to the naturalness problem. In the models that are proposed to simultaneously solve the two problems, heavy Majorana neutrinos and top partners are usually predicted to lead to a new decay mode of the top partner mediated by the heavy Majorana neutrinos: $T\to b\,{W}^{+}\to b\,{{\ell }}^{+}{{\ell }}^{+}q\bar{q^{\prime} }$. In this paper, we will study the observability of such a new signature via the pair production process of a top partner ${pp}\to T\bar{T}\to 2b+{{\ell }}^{\pm }{{\ell }}^{\pm }+4j$ in a model independent way. By performing Monte Carlo simulations, we present the 2σ exclusion limits of the top partner mass and mixing parameters at the HL-LHC.     

Signatures for Majorana neutrinos at hadron colliders

The Majorana nature of neutrinos may only be experimentally verified via lepton-number violating processes involving charged leptons. We explore the Delta L = 2 like-sign dilepton production at hadron colliders to search for signals of Majorana neutrinos. We find significant sensitivity for resonant production of a Majorana neutrino in the mass range of 10-80 GeV at the current run of the Tevatron with 2 fb(-1) integrated luminosity and in the range of 10-400 GeV at the CERN LHC with 100 fb(-1).     

Leptogenesis from first principles in the resonant regime

The lepton asymmetry generated by the out-of-equilibrium decays of heavy Majorana neutrinos with a quasi-degenerate mass spectrum is resonantly enhanced. In this work, we study this scenario within a first-principle approach. The quantum field theoretical treatment is applicable for mass splittings of the order of the width of the Majorana neutrinos, for which the enhancement is maximally large. The non-equilibrium evolution of the mixing Majorana neutrino fields is described by a formal analytical solution of the Kadanoff–Baym equations, that is obtained by neglecting the back-reaction. Based on this solution, we derive approximate analytical expressions for the generated asymmetry and compare them to the Boltzmann result. We find that the resonant enhancement obtained from the Kadanoff–Baym approach is smaller compared to the Boltzmann approach, due to additional contributions that describe coherent transitions between the Majorana neutrino species. We also discuss corrections to the masses and widths of the degenerate pair of Majorana neutrinos that are relevant for very small mass splitting, and compare the approximate analytical result for the lepton asymmetry with numerical results.     

Majorana neutrinos and same-sign dilepton production at LHC and in rare meson decays

We discuss same-sign dilepton production mediated by Majorana neutrinos in high-energy proton–proton collisions for at the LHC energy TeV, and in the rare decays of the , and B mesons of the type . For the pp reaction, assuming one heavy Majorana neutrino of mass , we present discovery limits in the plane where are the mixing parameters. Taking into account the present limits from low-energy experiments, we show that at LHC one has sensitivity to heavy Majorana neutrinos up to a mass TeV in the dilepton channels , and , but the dilepton states will not be detectable due to the already existing constraints from neutrinoless double beta decay. We work out a large number of rare meson decays, both for the light and heavy Majorana neutrino scenarios, and argue that the present experimental bounds on the branching ratios are too weak to set reasonable limits on the effective Majorana masses. Received: 24 April 2001 / Published online: 29 June 2001     

Constraint on the heavy sterile neutrino mixing angles in the SO(10) model with double see-saw mechanism

Constraints on the heavy sterile neutrino mixing angles are studied in the framework of a minimal supersymmetric SO(10) model with the use of the double see-saw mechanism. A new singlet matter in addition to the right-handed neutrinos is introduced to realize the double see-saw mechanism. The light Majorana neutrino mass matrix is, in general, given by a combination of those of the singlet neutrinos and the active neutrinos. The minimal SO(10) model is used to give an example form of the Dirac neutrino mass matrix, which enables us to predict the masses and the mixing angles in the enlarged 9×9 neutrino mass matrix. Mixing angles between the light Majorana neutrinos and the heavy sterile neutrinos are shown to be within the LEP experimental bound on all ranges of the Majorana phases.     

Allowed excitation of unnatural-parity states in the 208Pb(4He, 6He) reaction

We consider, in the context of simple gauge models, the possibility that lepton number is a spontaneously broken global symmetry, leading to massive Majorana neutrinos. We find that the usual objections associated with global symmetry breakdown may be moot in this case since the Goldstone boson associated with lepton number breakdown - the majoron - is so weakly coupled to matter that it may have well escaped detection.     

Is the magic texture of Majorana neutrinos immanent in Dirac nature?

Magic textures are successful candidates of the correct texture for Majorana neutrinos. In this study, we demonstrate that several types of magic textures of Majorana neutrinos are approximately immanent in the flavor mass matrix of Dirac neutrinos. In addition, the normal mass ordering of Dirac neutrino masses is slightly preferable to inverted mass ordering in the context of magic textures.     

Leptogenesis with linear,inverse or double seesaw

The left-right symmetric model with doublet and bi-doublet Higgs scalars can accommodate linear, inverse or double seesaw for generating small neutrino masses in the presence of three singlet fermions. If the singlet fermions have small Majorana masses, they can form three pairs of quasi-degenerate Majorana fermions with three right-handed neutrinos. The decays of the quasi-degenerate Majorana fermions can realize the resonant leptogenesis. Alternatively, the right-handed neutrinos can obtain seesaw suppressed Majorana masses if the singlet fermions are very heavy. In this case leptogenesis, with or without resonant effect, is allowed in the decays of the right-handed neutrinos.