Additional Right-handed Neutrinos Research Articles

Bounds on lepton non-unitarity and heavy neutrino mixing

We present an updated and improved global fit analysis of current flavour and electroweak precision observables to derive bounds on unitarity deviations of the leptonic mixing matrix and on the mixing of heavy neutrinos with the active flavours. This new analysis is motivated by new and updated experimental results on key observables such as Vud, the invisible decay width of the Z boson and the W boson mass. It also improves upon previous studies by considering the full correlations among the different observables and explicitly calibrating the test statistic, which may present significant deviations from a χ2 distribution. The results are provided for three different Type-I seesaw scenarios: the minimal scenario with only two additional right-handed neutrinos, the next to minimal one with three extra neutrinos, and the most general one with an arbitrary number of heavy neutrinos that we parametrise via a generic deviation from a unitary leptonic mixing matrix. Additionally, we also analyze the case of generic deviations from unitarity of the leptonic mixing matrix, not necessarily induced by the presence of additional neutrinos. This last case relaxes some correlations among the parameters and is able to provide a better fit to the data. Nevertheless, inducing only leptonic unitarity deviations avoiding both the correlations implied by the right-handed neutrino extension as well as more strongly constrained operators is challenging and would imply significantly more complex UV completions.

From Peccei Quinn symmetry to mass hierarchy problem

We propose a non-universal U(1) X gauge extension to the standard model (SM) and an additional Peccei–Quinn (PQ) global symmetry to study the mass hierarchy and strong CP problem. The scheme allows us to distinguish among fermion families and to generate the fermionic mass spectrum of particles of the SM. The symmetry breaking is performed by two scalar Higgs doublets and two scalar Higgs singlets, where one of these has the axion which turns out to be a candidate for cold dark matter. The exotic sector is composed by one up-like T and two down-like J 1,2 heavy quarks, two heavy charged leptons , one additional right-handed neutrino per family , and an invisible axion a. In addition, the large energy scale associated to the breaking of the PQ-symmetry gives masses to the right-handed neutrinos in such a way that the active neutrinos acquire eV-mass values due to the see-saw mechanism. On the other hand, from the non-linear effective Lagrangian, the flavour changing of the down quarks and charged leptons with the axion are considered.

Scalar dark matter, GUT baryogenesis, and radiative neutrino mass

We investigate an interesting correlation among dark matter phenomenology, neutrino mass generation and GUT baryogenesis, based on the scotogenic model. The model contains additional right-handed neutrinos $N$ and a second Higgs doublet $\Phi$, both of which are odd under an imposed $Z_2$ symmetry. The neutral component of $\Phi$, i.e. the lightest of the $Z_2$-odd particles, is the dark matter candidate. Due to a Yukawa coupling involving $\Phi$, $N$ and the Standard Model leptons, the lepton asymmetry is converted into the dark matter asymmetry so that a non-vanishing $B-L$ asymmetry can arise from $(B-L)$-conserving GUT baryogenesis, leading to a nonzero baryon asymmetry after the sphalerons decouple. On the other hand, $\Phi$ can also generate neutrino masses radiatively. In other words, the existence of $\Phi$ as the dark matter candidate resuscitates GUT baryogenesis and realizes neutrino masses.

Natural SUSY at LHC with right-sneutrino LSP

We study an extension of the minimal supersymmetric standard model (MSSM) with additional right-handed singlet neutrino superfields. While such an extension incorporates a mechanism for the neutrino mass, it also opens up the possibility of having the right-sneutrinos ( tilde{nu} ) as the lightest supersymmetric particle (LSP). In this work, we focus on the viability of rather small (≲500 GeV) higgsino mass parameter (μ), an important ingredient for “naturalness”, in the presence of such a LSP. For simplicity, we assume that the bino and wino mass parameters are much heavier; thus we only consider (almost) pure and compressed higgsino-like states, with small mathcal{O}left(1{0}^{-2}right) gaugino admixture which nevertheless still affect the decay of the low-lying higgsino-like states, thus significantly affecting the proposed signatures at colliders. Considering only prompt decays of the higgino-like states, especially the lightest chargino, we discuss the importance of leptonic channels consisting of up to two leptons with large missing transverse energy to probe this scenario at the Large Hadron Collider (LHC). In addition we also comment on the dark matter predictions for the studied scenario.

Higgsino-like dark matter from sneutrino late decays

We consider Higgsino-like dark matter (DM) in the Minimal Supersymmetric Standard Model (MSSM) with additional right-handed neutrino chiral superfields, and propose a new non-thermal way of generating the right amount of relic DM via sneutrino late decays. Due to the large DM annihilation cross-section, decays must occur at lower temperatures than the freeze-out temperature Td≪TF,χ˜10∼μ/25, implying a mostly right-handed lightest sneutrino with very small Yukawa interactions. In that context, the right amount of Higgsino-like DM relic density can be accounted for if sneutrinos are produced via thermal freeze-in in the early Universe.

Hierarchy spectrum of SM fermions: from top quark to electron neutrino

In the SM gauge symmetries and fermion content of neutrinos, charged leptons and quarks, we study the effective four-fermion operators of Einstein-Cartan type and their contributions to the Schwinger-Dyson equations of fermion self-energy functions. The study is motivated by the speculation that these four-fermion operators are probably originated due to the quantum gravity that provides the natural regularization for chiral-symmetric gauge field theories. In the chiral-gauge symmetry breaking phase, as to achieve the energetically favorable ground state, only the top-quark mass is generated via the spontaneous symmetry breaking, and other fermion masses are generated via the explicit symmetry breaking induced by the top-quark mass, four-fermion interactions and fermion-flavor mixing matrices. A phase transition from the symmetry breaking phase to the chiral-gauge symmetric phase at TeV scale occurs and the drastically fine-tuning problem can be resolved. In the infrared fixed-point domain of the four-fermion coupling for the SM at low energies, we qualitatively obtain the hierarchy patterns of the SM fermion Dirac masses, Yukawa couplings and family-flavor mixing matrices with three additional right-handed neutrinos $\nu^f_R$. Large Majorana masses and lepton-symmetry breaking are originated by the four-fermion interactions among $\nu^f_R$ and their left-handed conjugated fields $\nu^{fc}_R$. Light masses of gauged Majorana neutrinos in the normal hierarchy ($10^{-5}-10^{-2}$ eV) are obtained consistently with neutrino oscillations. We present some discussions on the composite Higgs phenomenology and forward-backward asymmetry of $t\bar t$-production, as well as remarks on the candidates of light and heavy dark matter particles (fermions, scalar and pseudoscalar bosons).

A consistent model for leptogenesis, dark matter and the IceCube signal

We discuss a left-right symmetric extension of the Standard Model in which the three additional right-handed neutrinos play a central role in explaining the baryon asymmetry of the Universe, the dark matter abundance and the ultra energetic signal detected by the IceCube experiment. The energy spectrum and neutrino flux measured by IceCube are ascribed to the decays of the lightest right-handed neutrino $N_1$, thus fixing its mass and lifetime, while the production of $N_1$ in the primordial thermal bath occurs via a freeze-in mechanism driven by the additional $SU(2)_R$ interactions. The constraints imposed by IceCube and the dark matter abundance allow nonetheless the heavier right-handed neutrinos to realize a standard type-I seesaw leptogenesis, with the $B-L$ asymmetry dominantly produced by the next-to-lightest neutrino $N_2$. Further consequences and predictions of the model are that: the $N_1$ production implies a specific power-law relation between the reheating temperature of the Universe and the vacuum expectation value of the $SU(2)_R$ triplet; leptogenesis imposes a lower bound on the reheating temperature of the Universe at $7\times10^9\,\mbox{GeV}$. Additionally, the model requires a vanishing absolute neutrino mass scale $m_1\simeq0$.

ObservingCP-violating Higgs signals with heavy Majorana neutrinos

We demonstrate the ability to measure the CP-phase of a Higgs boson state by looking at the decays of heavy, Majorana neutrinos in the process $pp \to H \to N N$ at the LHC. This is shown for signals with both opposite-sign ($l^{\pm} l^{\mp} W^{\pm} W^{\mp}$) and same-sign ($l^{\pm} l^{\pm} W^{\mp} W^{\mp}$) leptonic final states. These signals are investigated in the general framework of a sequential fourth generation of fermions with an additional right-handed neutrino. Such a scenario would naturally give rise to heavy Majorana neutrinos as well as significant enhancements to Higgs production via gluon fusion due to the contributions of the new, heavy quark states running in the fermion loops. Combined with the low background inherent to a same-sign lepton signal, this could be a useful way to investigate the CP-nature of a Higgs boson state at the LHC.

Exclusive [formula omitted][formula omitted] decays and leptophobic [formula omitted] model

We consider the exclusive flavor changing neutral current processes B→Mνν¯(M=π,K,ρ,K∗) in the leptophobic Z′ model, in which the charged leptons do not couple to the extra Z′ boson. We find that these exclusive modes are very effective to constrain the leptophobic Z′ model. In the leptophobic Z′ model, additional right-handed neutrinos are introduced and they can contribute to the missing energy signal in B→M+E̸ decays. Through the explicit calculations, we obtain quite stringent bounds on the model parameters, |UsbZ′|⩽0.29 and |UdbZ′|⩽0.61, from the already existing experimental data. We also briefly discuss an interesting subject of massive right-handed neutrinos, which might be connected with the dark matter problem.

A heterotic standard model

Within the context of the E8×E8 heterotic superstring compactified on a smooth Calabi–Yau threefold with an SU(4) gauge instanton, we show the existence of simple, realistic N=1 supersymmetric vacua that are compatible with low-energy particle physics. The observable sector of these vacua has gauge group SU(3)C×SU(2)L×U(1)Y×U(1)B–L, three families of quarks and leptons, each with an additional right-handed neutrino, two Higgs–Higgs conjugate pairs, a small number of uncharged moduli and no exotic matter. The hidden sector contains non-Abelian gauge fields and moduli. In the strong coupling case there is no exotic matter, whereas for weak coupling there are a small number of additional matter multiplets in the hidden sector. The construction exploits a mechanism for “splitting” multiplets. The minimal nature and rarity of these vacua suggest the possible theoretical and experimental relevance of spontaneously broken U(1)B–L gauge symmetry and two Higgs–Higgs conjugate pairs. The U(1)B–L symmetry helps to naturally suppress the rate of nucleon decay.

Realistic neutrino masses from multi-brane extensions of the Randall-Sundrum model?

Scenarios based on the existence of large or warped (Randall-Sundrum model) extra dimensions have been proposed for addressing the long standing puzzle of gauge hierarchy problem. Within the contexts of both those scenarios, a novel and original type of mechanism generating small (Dirac) neutrino masses, which relies on the presence of additional right-handed neutrinos that propagate in the bulk, has arisen. The main objective of the present study is to determine whether this geometrical mechanism can produce reasonable neutrino masses also in the interesting multi-brane extensions of the Randall-Sundrum model. We demonstrate that, in some multi-brane extensions, neutrino masses in agreement with all relevant experimental bounds can indeed be generated but at the price of a constraint (stronger than the existing ones) on the bulk geometry, and that the other multi-brane models even conflict with those experimental bounds.

Unitarity triangles and geometrical description ofCPviolation with Majorana neutrinos

We generalize the geometrical description of CP violation in the Standard Model in terms of a unitarity triangle. For three left-handed Majorana neutrinos CP violation in the lepton sector is determined by three unitarity triangles. With three additional right-handed neutrinos 15 quadrangles are required to characterize CP violation. We show the relation of the unitarity polygons with physical observables.

HEAVY MAJORANA NEUTRINOS IN e-e- COLLISIONS

We discuss the process e-e-→W-W- mediated by heavy Majorana neutrino exchange in the t- and u channel. In our model the cross section for this reaction is a function of the masses (mN) of the heavy Majorana neutrinos and mixing parameters (UeN) originating from mixing between the ordinary left-handed standard model neutrinos and additional singlet right-handed neutrino fields. Taking into account the standard model background and constraints from low energy measurements, we present discovery limits in the [Formula: see text] plane. We also discuss how to measure in principle the CP violating phases, i.e., the relative phases between the mixing parameters.

Inverse neutrinoless double- beta decay in gauge theories with CP violation.

We investigate the reactions $e^-e^- \rightarrow W_i^-W_j^-$ for various gauge bosons production processes in the frame of the standard model with additional right-handed neutrinos and the Left-Right symmetric model. The present bounds on the various model parameters are taken into account. The question of the cross section behaviour for large energy and the CP violation problems are discussed.

On the possibility of detecting the e−e− → W−W− process in the Standard Model with additional right-handed neutrinos

We examine the e − e − → W − W − lepton number violating process in the frame of the Standard Model with additional right-handed neutrino singlets. We give results in the framework of the ‘see-saw’ as well as other models, where there is no relation between neutrino masses and the mixing matrix elements. The cross section for the ‘see-saw’ models is negligible because they preduct in a natural way very small electron-heavy neutrino mixing angles. However, there exist other models in which the electron-heavy neutrino mixing angles are free parameters and can be large. Taking into account the present experimental bounds on mixing angles the large cross section for the e − e − → W − W − is still acceptable.

CP violation in the lepton sector with Majorana neutrinos

We study CP violation in the lepton sector in extended models with right-handed neutrinos, without and with left-right symmetry, and with arbitrary mass terms. We find the conditions which must be satisfied by the neutrino and charged lepton mass matrices for CP conservation. These constraints, which are independent of the choice of weak basis, are proven to be also sufficient in simple cases. This invariant formulation makes apparent the necessary requirements for CP violation, as well as the size of CP violating effects. As an example, we show that CP violation can be much larger in left-right symmetric models than in models with only additional right-handed neutrinos, i.e., without right-handed currents.

Primodal nucleosynthesis, additional neutrinos and neutral currents from the superstring

The constraint on the number of neutrino species N v imposed by Big Bang nucleosynthesis is reviewed critically. It is argued that uncertainites in the neutron lifetime, systematic errors in the measurement of 4He abundance, and new astrophysical data on 3He which suggest extensive astration of D, permit N v = 5 or even 6. Some phenomenological models based on the superstring expect three additional right-handed neutrinos of low mass which are weakly coupled to ordinary matter through a new heavy Z′ gauge boson. Their compatibility with the relaxed nucleosynthesis limits quoted above imposes an interesting lower limit on m Z′ . As a byproduct of our analysis, we present the corrections to low energy v μ e scattering due to Z′ exchange.