Heavy Majorana Fermions Research Articles

Reconciling LSND and Super-Kamiokande data through the dynamical Lorentz symmetry breaking in a four-Majorana fermion model

We propose a model of Majorana fermions with quartic self-couplings. These Majorana fermions acquire masses via a type II seesaw mechanism in which the physical eigenstates are identified as a light Majorana fermion and another heavy Majorana fermion. On a physical basis, the quartic self-couplings involve axial currents of these Majorana fermions, and also the interaction of the axial current for the light particle with the heavy particle one. We introduce two auxiliaries gauge fields in this model, and we study the stability conditions of the correspondent effective potential of the model. The ground state of the effective potential introduces two 4-vectors as scales of vacuum expected values, and consequently, the dynamical Lorentz symmetry breaking (DLSB) emerges in the model. We use the expansion of the effective action to calculate the effective Lagrangian up to second order in the auxiliary fields as fluctuations around the ground state. This mechanism generates dynamics for the auxiliary gauge fields, mixed mass terms, longitudinal propagation, and Chern-Simons term through radiative corrections. After the diagonalization, the two gauge fields gain masses through an analogous type II seesaw mechanism in which a gauge boson has a light mass, and the other one acquires a heavy mass. In this scenario of Lorentz symmetry breaking, we obtain the correspondent dispersion relations for the Majorana fermions and the gauge boson fields. Posteriorly, we analyze the neutrino's oscillations in the presence of a DLSB parameter, in the transition $\nu_{e} \rightarrow \nu_{\mu}$. We discuss the parameter space of this transition and show that the DLSB can conciliate the LSND and super-Kamiokande results.

Low scale Dirac leptogenesis and dark matter with observable Delta N_{mathrm{eff}}

We propose a gauged B-L extension of the standard model (SM) where light neutrinos are of Dirac type by virtue of tiny Yukawa couplings. To achieve leptogenesis, we include additional heavy Majorana fermions without introducing any B-L violation by two units. An additional scalar doublet with appropriate B-L charge can allow such heavy fermion coupling with SM leptons so that out-of-equilibrium decay of the former can lead to generation of lepton asymmetry. Due to the B-L gauge interactions of the decaying fermion, the criteria of successful Dirac leptogenesis can also constrain the gauge sector couplings. The same B-L gauge sector parameter space can also be constrained from dark matter requirements if the latter is assumed to consist of SM singlet particles with non-zero B-L charges, which also keep the model anomaly free. The same B-L gauge interactions also lead to additional thermalised relativistic degrees of freedom Delta N_{mathrm{eff}} from light Dirac neutrinos which are tightly constrained by Planck 2018 data. While there exists parameter space from the criteria of successful leptogenesis, dark matter and Delta N_{mathrm{eff}} even after incorporating the latest collider bounds, the currently allowed parameter space can be probed by future measurements of Delta N_{mathrm{eff}}.

Three-body decays of heavy Dirac and Majorana fermions

Nonzero neutrino masses imply the existence of degrees of freedom and interactions beyond those in the Standard Model. A powerful indicator of what these might be is the nature of the massive neutrinos: Dirac fermions versus Majorana fermions. While addressing the nature of neutrinos is often associated with searches for lepton-number violation, there are several other features that distinguish Majorana from Dirac fermions. Here, we compute in great detail the kinematics of the daughters of the decays into charged-leptons and neutrinos of hypothetical heavy neutral leptons at rest. We allow for the decay to be mediated by the most general four-fermion interaction Lagrangian. We demonstrate, for example, that when the daughter charged-leptons have the same flavor or the detector is insensitive to their charges, polarized Majorana-fermion decays have zero forward-backward asymmetry in the direction of the outgoing neutrino (relative to the parent spin), whereas Dirac-fermion decays can have large asymmetries. Going beyond studying forward-backward asymmetries, we also explore the fully differential width of the three-body decays. It contains a wealth of information not only about the nature of the new fermions but also the nature of the interactions behind their decays.

Dirac neutrino mass generation from a Majorana messenger

The radiative type-I seesaw has been already implemented to explain the lightness of Majorana neutrinos with both Majorana and Dirac heavy fermions, and the lightness of Dirac neutrinos with Dirac heavy fermions. In this work we present a minimal implementation of the radiative type-I seesaw with light Dirac neutrinos and heavy Majorana fermions. An inert doublet and a complex singlet scalar complete the dark sector which is protected by an Abelian fermiophobic gauge symmetry that also forbids tree level mass contributions for the full set of light neutrinos. A fermion vector-like extension of the model is also proposed where the light right-handed neutrinos can thermalize in the primordial plasma and the extra gauge boson can be directly produced at colliders. In particular, the current upper bound on $\Delta N_{\text{eff}}$ reported by PLANCK points to large ratios $M_{Z'}/g'\gtrsim 40\ \text{TeV}$ which can be competitive with collider constraint for $g'$ sufficiently large in the ballpark of the Standard Model values, while future cosmic microwave background experiments may probe all the no minimal models presented here.

Hierarchy and decoupling

A large hierarchy between the electroweak scale and virtually any new scale of beyond-Standard-Model physics is often claimed to be unnatural. Sometimes, the apparent disparity between the measured Higgs mass and the size of the typical loop corrections encountered within such schemes is even interpreted as a profound indication that one should expect a remedy just behind the corner, probably in the form of a new physics such as low-scale supersymmetry, new strong dynamics etc. Without much of an aspiration to shed more light on the smallness of the electroweak scale we exploit the symmetry properties of the Coleman–Weinberg effective potential to make it very clear why to all orders in the perturbative expansion. With the method we present no inspection of the explicit form of the tadpole equations is required. These findings are exemplified on an extended series of specific simplified models in which the role of the heavy dynamics is played by all ‘reasonable’ types of fields (barring gravity), i.e. by a heavy scalar, a heavy (Majorana) fermion and a heavy vector. We show that the explicit dependence of mH on the heavy scale follows the expectation based on dynamical arguments such as the decoupling theorem.

Interference effects in LNV and LFV semileptonic decays: the Majorana hypothesis

In the case where the Standard Model is extended by one heavy Majorana fermion, the branching fractions of semileptonic meson decays into same-sign and opposite-sign dileptons are expected to be of the same order. As we discuss here, this need not be the case in extensions by at least two sterile fermions, due to the possible destructive and constructive interferences that might arise. Depending on the CP violating phases, one can have an enhancement of the lepton number violating modes and suppression of the lepton number conserving ones (and vice-versa). We explore for the first time the interference effects in semileptonic decays, and illustrate them for a future observation of kaon decays at NA62. We also argue that a non-observation of a given mode need not be interpreted in terms of reduced active-sterile mixings, but that it could instead be understood in terms of interference effects due to the presence of several sterile states; in particular, for different-flavour final state charged leptons, observing a lepton number conserving process and not a lepton number violating one does not rule out that the mediators are Majorana fermions.

NEUTRINO MASSES AND LEPTOGENESIS FROM EXTRA FERMIONS

Generation of the neutrino masses and leptogenesis (LG) in the standard model extended by the heavy Majorana fermions is considered. Classification of LG scenarios according to the new fermion mass spectra is given, where singlet–triplet LG is considered for the first time. The upper bound on the CP asymmetry relevant for LG with hierarchical heavy neutrinos (Davidson–Ibarra bound) is revised, and shown that in the case of one massless neutrino it essentially depends on the type of the light neutrino mass hierarchy. The resonant scenarios, which help to avoid the problem of extremely high reheating temperature in the early universe, are discussed. In particular, we present new simplified, generalized and detailed formulation of freed LG, which violates Davidson–Ibarra bound in a special class of models.

Effective field theory for a heavy Majorana fermion

We formulate an effective theory for systems containing a heavy Majorana fermion, such as bound states of a long-lived gluino. This "Majorana HQET" has the same degrees of freedom as the well-studied Dirac HQET. It respects an emergent U(1) symmetry despite the fundamental absence of a U(1) for Majorana fermions. Reparameterization invariance works identically in the two HQETs. Remarkably, while a Dirac HQET may or may not be charge conjugation symmetric, a charge conjugation symmetry emerges in all Majorana HQETs, potentially offering low energy probes to distinguish the two theories.

New solution for neutrino masses and leptogenesis in adjoint SU(5)

We investigate baryogenesis via leptogenesis and generation of neutrino masses and mixings through the Type I plus Type III seesaw plus an one-loop mechanism in the context of Renormalizable Adjoint SU(5) theory. One light neutrino remains massless, because the contributions of three heavy Majorana fermions \rho_0, \rho_3 and \rho_8 to the neutrino mass matrix are not linearly independent. However none of these heavy fermions is decoupled from the generation of neutrino masses. This opens a new range in parameter space for successful leptogenesis, in particular, allows for inverted hierarchy of the neutrino masses.

Distinguishing seesaw models at LHC with multi-lepton signals

We investigate the LHC discovery potential for electroweak scale heavy neutrino singlets (seesaw I), scalar triplets (seesaw II) and fermion triplets (seesaw III). For seesaw I we consider a heavy Majorana neutrino coupling to the electron or muon. For seesaw II we concentrate on the likely scenario where the new scalars decay to two leptons. For seesaw III we restrict ourselves to heavy Majorana fermion triplets decaying to light leptons plus gauge or Higgs bosons, which are dominant except for unnaturally small mixings. The possible signals are classified in terms of the charged lepton multiplicity, studying nine different final states ranging from one to six charged leptons. Using a fast detector simulation of signals and backgrounds, it is found that the trilepton channel ℓ ± ℓ ± ℓ ∓ is by far the best one for scalar triplet discovery, and for fermion triplets it is as good as the like-sign dilepton channel ℓ ± ℓ ± . For heavy neutrinos with a mass O ( 100 ) GeV , this trilepton channel is also better than the usually studied like-sign dilepton mode. In addition to evaluating the discovery potential, we make special emphasis on the discrimination among seesaw models if a positive signal is observed. This could be accomplished not only by searching for signals in different final states, but also by reconstructing the mass and determining the charge of the new resonances, which is possible in several cases. For high luminosities, further evidence is provided by the analysis of the production angular distributions in the cleanest channels with three or four leptons.

Pathways to Naturally Small Neutrino Masses

In the minimal standard electroweak gauge model, there is an effective dimension-five operator which generates neutrino masses, and it has only three tree-level realizations. One is the canonical seesaw mechanism with a right-handed neutrino. Another is having a heavy Higgs triplet as recently proposed. The third is to have a heavy Majorana fermion triplet, an example of which is presented here in the context of supersymmetric $\mathrm{SU}(5)$ grand unification. The three generic one-loop realizations of this operator are also discussed.

Resonant CP-Violating Scalar-Pseudoscalar Transitions at micro+ micro- Colliders.

A ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ collider is an appealing machine to probe resonant $\mathrm{CP}\ensuremath{-}\mathrm{v}\mathrm{i}\mathrm{o}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}$ transitions of a $\mathrm{CP}\ensuremath{-}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n}$ Higgs particle into the $Z$ boson or into another $\mathrm{CP}\ensuremath{-}\mathrm{o}\mathrm{d}\mathrm{d}$ Higgs scalar. These phenomena are studied within a manifestly gauge-invariant approach implemented by the pinch technique. The $\mathrm{CP}$ invariance of an extended Higgs sector motivated by supersymmetric ${\mathrm{E}}_{6}$ models is assumed to be broken radiatively by the presence of heavy Majorana fermions. $\mathrm{CP}$ violation originating from Higgs- $Z$ mixing is found to be very modest, whereas $\mathrm{CP}$-number-violating transitions involving Higgs scalars only can be resonantly enhanced up to order of unity.

Design of a Liquid Xenon Calorimeter for the Observation of Cosmic Gamma Ray Lines in the 1∼100 GeV Region

A liquid xenon calorimeter has recently been designed. The main purpose is to show the capability of observing monochromatic cosmic gamma ray lines in the 1∼100 GeV region. The calorimeter has a geometric factor of ∼1 m2·sr, an energy resolution of ∼0.4% and an angular resolution of about 1 degree for GeV-region gamma rays. These characteristics will meet the conditions required for the observation of gamma ray lines produced by the pair annihilation of hypothetical SUSY heavy Majorana fermions which might exist in the galactic halo as dark matter. The calorimeter is also applicable to the search for point sources emitting gamma rays as well as the investigation of diffuse gamma rays at high energies.

Liquid xenon calorimeter for observation of cosmic line gamma-rays in the GeV region

Abstract A liquid xenon calorimeter designed on the basis of a new principle has been proposed to be used for observing cosmic line gamma-rays in the GeV region. This calorimeter has a geometric factor of 1 m 2 sr or more, an energy resolution of the order of 0.1 % for GeV gamma-rays; and an angular resolution of about 1 degree. These characteristics satisfy most of the conditions theoretically required for observation of line gamma-rays produced by pair annihilation of heavy majorana fermions in the galactic halo.

Cosmic Production of Quarkonium?

It has been suggested that pair annihilation of heavy Majorana fermions in the galactic halo into quarkonium plus a monochromatic photon could occur at an observable rate. Here we show that a calculation of Srednicki, Theisen, and Silk seriously overestimates the rate for this process, by an ordre of magnitude or more, as a result of neglecting the bound-state structure of quarkonium. It may nevertheless still be possible to resolve the correspondingly smaller gamma-ray line flux over the diffuse cosmic background by use of the currently planned space-borne superconducting magnet spectrometer faciltiy with an energy resolution of 1% or better.