Observation of lepton number ($L$) violation by two units at colliders would provide evidence for the Majorana nature of neutrinos. We study signals of $L$-violation in the context of two popular models of neutrino masses, the type-II seesaw model and the Zee model, wherein small neutrino masses arise at the tree-level and one-loop level, respectively. We focus on $L$-violation signals at the LHC arising through the process $pp \to \ell^{\pm}\ell^{\prime \pm}$ + jets within these frameworks. We obtain sensitivity to $L$-violation in the type-II seesaw model for triplet scalar masses up to 700 GeV and in the Zee model for charged scalar masses up to 4.8 TeV at the high-luminosity LHC with an integrated luminosity of $3~\text{ab}^{-1}$.
We discuss the various incarnations of a gauged B-L symmetry: 1) unbroken, it features Dirac neutrinos, neutrinogenesis to create the baryon asymmetry of our Universe, and a potentially light Z' boson; 2) broken by two units, we obtain the standard case of Majorana neutrinos, seesaw and thermal leptogenesis; 3) broken by four units, we find Dirac neutrinos with lepton-number-violating interactions, which can give rise to a new Dirac leptogenesis mechanism. We review and discuss the signatures distinguishing the three scenarios.
Heavy Majorana neutrinos are predicted in addition to ordinary active neutrinos in the models with the seesaw mechanism. We investigate the lepton number violation (LNV) in $B$ decays induced by such a heavy neutrino $N$ with GeV-scale mass. Especially, we consider the decay channel $B^+ \to μ^+ \, N \to μ^+ μ^+ π^-$ and derive the sensitivity limits on the mixing angle $Θ_μ$ by the future search experiments at Belle II and in $e^+ e^-$ collisions at the Future Circular Collider (FCC-ee).
Given the best-fit results of six neutrino oscillation parameters, we plot the Dirac and Majorana unitarity triangles (UTs) of the 3\times 3 lepton flavor mixing matrix to show their real shapes for the first time. The connections of the Majorana UTs with neutrino-antineutrino oscillations and neutrino decays are explored, and the possibilities of right or isosceles UTs are discussed. In the neutrino mass limit of m_1 \to 0 or m_3 \to 0, which is allowed by current experimental data, we show how the six triangles formed by the effective Majorana neutrino masses \langle m\rangle_{αβ} (for α, β=…
Motivated by the recent investigation of neutrinoless $τ$-lepton decays by the CLEO collaboration, we perform a systematic analysis of such decays in a possible new-physics scenario with heavy Dirac/Majorana neutrinos, including heavy-neutrino nondecoupling effects, finite quark masses, and quark as well as meson mixings. We find that $τ$-lepton decays into an electron or muon and a pseudoscalar or vector meson can have branching ratios close to the experimental sensitivity. Numerical estimates show that the predominant decay modes of this kind are $τ^-\to e^-ϕ$, $τ^-\to e^-ρ^0$, and $τ^-\to e…
We revisit the time evolution of the lepton family number for a SU(2) doublet consisting of a neutrino and a charged lepton. The lepton family number is defined through the weak basis of the SU(2) doublet, where the charged lepton mass matrix is real and diagonal. The lepton family number carried by the neutrino is defined by the left-handed current of the neutrino family. For this work we assume the neutrinos have Majorana mass. This Majorana mass term is switched on at time $t=0$ and the lepton family number is evolved. Since the operator in the flavor eigenstate is continuously connected to…
Two powerful and straightforward methods are presented for probing the Majorana nature and CP violation of neutralinos at future $e^+e^-$ linear colliders.
We discuss the potential to observe lepton number violation (LNV) in displaced vertex searches for heavy neutral leptons (HNLs) at future lepton colliders. Even though a direct detection of LNV is impossible for the dominant production channel because lepton number is carried away by an unobservable neutrino, there are several signatures of LNV that can be searched for. They include the angular distribution and spectrum of decay products as well as the HNL lifetime. We comment on the perspectives to observe LNV in realistic neutrino mass models and argue that the dichotomy of Dirac vs Majorana…
We investigate purely leptonic decays of leptons $ł^{\prime-}\to l^-\barν_lν_{l^{\prime}}$ to distinguish Dirac or Majorana neutrinos. We derive the differences of the decay width (and associated quantities) between the two neutrino hypotheses by the effect of identical particles. Evidence from experimental data makes the hypothesis of Majorana neutrinos very unlikely and our results strongly favor the hypothesis of Dirac neutrinos (in the standard three-neutrino theory). Moreover, our exploration for decay widths can be extended to the polarization angles of leptons, which will inspire new in…
Two important features of TeV-scale radiative seesaw models, in which tiny neutrino masses are generated at the quantum level, are an extended scalar (Higgs) sector and the Majorana nature. We study phenomenological aspects of these models at the ILC. It is found that the Majorana nature of the models can be tested directly via the electron-positron and electron-electron collision experiments.
We discuss the one-loop electroweak renormalization of the leptonic mixing matrix in the case of Majorana neutrinos, and establish its relationship with the renormalization group evolution of the dimension five operator responsible for the light Majorana neutrino masses. We compare our results in the effective theory with those in the full seesaw theory.
In this talk, we have reviewed the recent development on the time evolution of lepton family number carried by Majorana neutrinos \cite{Adam:2021qiq}. This article focuses on the subtle points of the derivation of the lepton family numbers and their time evolution. We also show how the time evolution is sensitive to $m_{ee}$ and $m_{eμ}$ components of the effective Majorana mass matrix by applying the formula to the two family case. The dependence on the Majorana phase is clarified and the implication on CNB (cosmic neutrino background) is also discussed.
In the light of the recent measurement of the leptonic mixing angle $θ_{13}$, implications of the latest mixing data have been investigated for non-minimal textures of lepton mass matrices assuming the neutrinos to be Majorana like. Large number of possible texture specific lepton mass matrices have been examined for their compatibility with the lepton mixing data in the case of normal hierarchy, inverted hierarchy and degenerate scenario of neutrino masses. Specifically, apart from other phenomenological quantities the implications of the lepton mixing angle $θ_{13}$ have been investigated on…
The electric dipole moment of the electron, $d_e$, is known to vanish up to thre e-loops in the standard model with massless neutrinos. However, if neutrinos are massive Majorana particles, we obtain the result that $d_e$ induced by leptonic CKM mechanism is non-vanishing at two-loop order, and it applies to all massive Majorana neutrino models.
Neutrinoless double beta decay (NDBD) is the only experiment that could probe the Majorana nature of the neutrino. Here we study the theoretical implications of NDBD for models yielding tri-bimaximal lepton mixing like A4 and S4.
The notion of U(1) lepton number (which may only be softly broken) is applied to models of dark matter which interacts with leptons. Previous scotogenic models of Majorana or Dirac neutrino mass are shown to be derivable in this framework without additional symmetries. Only complete renormalizable theories are considered. An explicit class of models with $Z_n$ ($n \geq 5$) lepton and dark symmetry for Dirac neutrinos is derived, as well as an example of $Z_3$ dark symmetry.
The Majorana nature of neutrinos may only be experimentally verified via lepton-number violating processes involving charged leptons. We explore the $ΔL=2$ 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 LHC with 100 fb$^{-1}$.
We consider a gauged B$-$L (Baryon number minus Lepton number) extension of the Standard Model (SM), which is anomaly free in the presence of three SM singlet Right Handed Neutrinos (RHNs). Associated with the $U(1)_{\rm B-L}$ gauge symmetry breaking, the RHNs acquire Majorana masses and then with the electroweak symmetry breaking, tiny Majorana masses for the SM(-like) neutrinos are naturally generated by the seesaw mechanism. As a result of the seesaw mechanism, the heavy mass eigenstates which are mainly composed of the SM-singlet RHNs obtain suppressed electroweak interactions through smal…
If new leptons exist close to the electroweak scale, they can be produced in pairs at LHC through standard or new interactions. We study the production of heavy lepton pairs in SM extensions with: (i) a Majorana or Dirac lepton triplet, as those appearing in type-III seesaw; (ii) a lepton isodoublet (N,E)_{L,R}; (iii) a charged isosinglet E_{L,R}; (iv) a Majorana or Dirac neutrino singlet N and an additional Z' gauge boson. It is shown that the trilepton final state l+-l+-l-+, which has a small SM background, constitutes the golden channel for heavy neutrino searches, being very sensitive to M…
We relate the observed pattern of leptonic mixing to the quaidegeneracy of three Majorana neutrinos. We show how lifting the degeneracy may lead to the measured value of $|U_{13}|$ and to sizeable CP violation of Dirac-type. We show some of the correlations obtained among physical observables, starting from some of the most interesting schemes proposed in the literature.
If the neutrinos are Majorana fermions, there are at least three new, potentially observable CP-odd phases that parameterize CP-invariance violating phenomena. We currently have no information regarding any of them and know that two out of the three, the so-called Majorana phases, are very hard to access experimentally. Here, we discuss the effect of Majorana phases on charged-lepton electric dipole moments (EDM), and explicitly show that neutrino Majorana phases induce EDMs even in the absence of other sources of CP-invariance violation. We also argue that while the Majorana neutrino contribu…
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 i…
In the Standard Model minimally extended to include massive neutrinos, we compute the leading CP-violating zero temperature contributions to the one-loop effective action induced by integration of the leptons. Such contributions start at operators of dimension six and they are P even for Dirac neutrinos and P even or odd for Majorana neutrinos. Dimension four operators are allowed in the mixed Dirac-Majorana case. It is verified by explicit calculation that CP can be violated in two generation settings for Majorana neutrinos. Using different neutrino scenarios we give upper bounds for the coup…
The nonzero neutrino mass can be a signal for new physics beyond the standard model. To explain the tiny neutrino mass, we can extend the standard model with right-handed Majorana neutrinos in a low-scale seesaw mechanism, while 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 as a probe to search for the signal of heavy Majorana neutrinos. In this paper, we focus on the CP violation effect related to two generations of heavy Majo…
B meson semi-leptonic rare decays are sensitive to new physics beyond standard model. We study the $B^{-}\to π^{-}μ^{+}μ^{-}$ process and investigate the Majorana neutrino contribution to its decay width. The constraints on the Majorana neutrino mass and mixing parameter are obtained from this decay channel with the latest LHCb data. Utilizing the best fit for the parameters, we study the lepton number violating decay $B^{-}\to π^{+}μ^{-}μ^{-}$, and find its branching ratio is about $6.4\times10^{-10}$, which is consistent with the LHCb data reported recently.
We construct a scotogenic Majorana neutrino mass model in a gauged $U(1)_X$ extension of the standard model, where the mass of the gauge boson and the unbroken gauge symmetry, which leads to a stable dark matter (DM), can be achieved through the Stueckelberg mechanism. It is found that the simplest version of the extended model consists of the two inert-Higgs doublets and one vector-like singlet fermion. In addition to the Majorana neutrino mass, we study the lepton flavor violation (LFV) processes, such as $\ell_i \to \ell_j γ$, $\ell_i \to 3 \ell_j$, $μ-e$ conversion rate in nucleus, and muo…
A Majorana type of the neutrino mass matrix induces a class of lepton number violating processes. Cross sections of these reactions are given in terms of the neutrino mass matrix element, and a semi-realistic event rate is estimated. These processes provide mass and mixing parameters not directly accessible by the neutrino oscillation experiments. If these processes are discovered with a larger rate than given here, it would imply a new physics of the lepton number violation not directly related to the Majorana neutrino mass, such as R-parity violating operators in SUSY models.
I analyze a fourth generation lepton sector in which the lightest particle is a stable Majorana neutrino. In this scenario fourth generation neutrinos have both a Dirac and Majorana mass, resulting in two Majorana neutrino mass eigenstates. A reanalysis of LEP's lower mass bound is performed on stable Majorana neutrinos from the Z width and the lower mass bound is loosened. I also extrapolate LEP's SUSY squark search with a 2 jet plus missing missing energy final state to the production and decay of a pair of heavy Majorana neutrinos; here it is expected that significant regions of the neutrin…
The existence of Majorana neutrinos must lead to lepton-number violating processes, and the Majorana nature of neutrinos can only be experimentally verified via lepton-number violating processes. We propose a new approach to search for Majorana neutrinos at future electron-positron colliders by exploiting this feature. We investigate the $ΔL = 2$ like-sign dilepton production and find that lepton colliders with different center-of-mass energies have comparative advantages in resonant production of a Majorana neutrino in either light neutrino mass range or heavy mass range. At the future Circul…
In this talk, we present a recent investigation of the sufficient and necessary conditions for CP conservation in the leptonic sector with massive Majorana neutrinos in terms of CP-odd weak-basis invariants. The number of weak-basis invariants to guarantee CP conservation in the leptonic sector is clarified and a new set of invariants are advocated for the description of CP conservation, given the physical parameters in their experimentally allowed regions.
We propose a new strategy for detecting the CP-violating phases and the effective mass of muon Majorana neutrinos by measuring observables associated with neutrino-antineutrino oscillations in $π^{\pm}$ decays. Within the generic framework of quantum field theory, we compute the non-factorizable probability for producing a pair of same-charged muons in $π^{\pm}$ decays as a distinctive signature of $ν_μ-\bar{ν_μ}$ oscillations. We show that an intense neutrino beam through a long baseline experiment is favored for probing the Majorana phases. Using the neutrino-antineutrino oscillation probabi…
We point out unique relations between the decay widths for leptoquarks and Higgs bosons that can be used to test the unification of quarks and leptons at the TeV scale. We discuss the main predictions of the minimal theory for quark-lepton unification and show how the different decays for leptoquarks and Higgses are related by the symmetry of the theory.
Heavy neutral leptons (HNLs) are hypothetical particles able to explain several puzzles of fundamental physics, first and foremost - neutrino oscillations. Being sterile with respect to Standard Model interactions, these particles admit Majorana masses, allowing for violation of the total lepton number. Lepton number violating (LNV) processes thus become a key signature of HNLs, pursued by many experiments. In this work, we demonstrate that if HNLs are the sole origin of neutrino masses, destructive interference between Majorana states suppresses the same-sign di-lepton signal. In the phenomen…
The high scale mixing unification hypothesis recently proposed by three of us (R. N. M., M. K. P. and G. R.) states that if at the seesaw scale, the quark and lepton mixing matrices are equal then for quasi-degenerate neutrinos, radiative corrections can lead to large solar and atmospheric mixings and small reactor angle at the weak scale in agreement with data. Evidence for quasi-degenerate neutrinos could, within this framework, be interpreted as a sign of quark-lepton unification at high scale. In the current work, we extend this model to show that the hypothesis works quite successfully in…
A mechanism is presented, in which the mixing of right handed heavy Majorana neutrinos creates a $CP-$asymmetric universe. When these Majorana neutrinos subsequently decay more leptons than anti-leptons are produced. The lepton asymmetry created by this new mechanism can exceed by a few orders of magnitude any lepton asymmetry originating from direct decays. The asymmetry is finally converted into a baryon asymmetry during the electroweak phase transition.
A basis independent formulation of quark-lepton complementarity is implemented at a high scale for quasidegenerate Majorana neutrinos. It is shown that even with the renormalization group evolution in the minimal supersymmetric standard model, the scenario can be consistent with the data provided a nontrivial role is played by the Majorana phases. Correlated constraints are found on these phases and the neutrino mass scale using the current data. We also indicate how future accurate measurements of the mixing angles can serve as tests of this scenario and restrict the values of the Majorana ph…
The possible extension of generation structure is investigated on the basis of a composite model, in which leptons and quarks are composed of rishons $T$ with charge 1/3 and purely neutral $V$. In this sub-system, we regard $V$ as the origin of Majorana particles. The seesaw mechanism implies the possible existence of heavy $V$, denoted as $V_B$ in addition to ordinary $V$, denoted as $V_s$. If we regard $V_B$ as a constitute block of leptons and quarks, a new generation scheme arises. This new scheme satisfies the anomaly-free condition.
A previous short analysis of the seesaw mechanism, based on quark-lepton symmetry, experimental data and hierarchical neutrino spectrum, is enlarged to include small but not zero U_{e3}, inverted mass hierarchy, and the qualitative effect of Majorana phases. The structure of the heavy neutrino mass matrix obtained in several cases is discussed. We find two leading forms for this matrix. One is diagonal and stands at the unification scale or above. The other is off-diagonal and stands at the intermediate scale.
We suppose that the observed neutrino masses can be parametrised by a lepton number violating dimension-five operator, and calculate the mixing of double insertions of this operator into lepton flavour changing dimension-six operators of the standard model effective theory. This allows to predict the log-enhanced, but $m_ν^2$-suppressed lepton flavour violation that is generic to high-scale Majorana neutrino mass models. We also consider the Two Higgs Doublet Model, where the second Higgs allows the construction of three additional dimension-five operators, and evaluate the corresponding anoma…
Non-Abelian discrete symmetries provide an interesting opportunity to address the flavor puzzle in the lepton sector. However, the number of currently viable models based on such symmetries is rather large. High-precision measurements of the leptonic mixing parameters by future neutrino experiments, including ESSnuSB, T2HK, DUNE, and JUNO, will be crucial to test such models. We show that the complementarity among these experiments offers a powerful tool for narrowing down this broad class of lepton flavor models.
We explore generic features of the leptonic CP violation in the framework of the seesaw type I mechanism with similarity of the Dirac lepton and quarks mass matrices $m_D$. For this, we elaborate on the standard parametrization conditions which allow to simultaneously obtain the Dirac and Majorana phases. If the only origin of CP violation is the left-handed (LH) transformation which diagonalizes $m_D$ (similar to quarks), the leptonic CP violation is suppressed and the Dirac phase is close to $π$ or to $0$ with $\sin δ_{CP} \approx (\sin θ_{13}^q /\sin θ_{13}) \cos θ_{23} \sin δ_q \sim λ^2 \s…
We discuss the possibility of observing multi-lepton signals at the Large Hadron Collider (LHC) from the production and decay of heavy Standard Model (SM) singlet neutrinos added in extensions of SM to explain the observed light neutrino masses by seesaw mechanism. In particular, we analyze two `smoking gun' signals depending on the Dirac or Majorana nature of the heavy neutrino: (i) for Majorana case, the same-sign di-lepton signal which can be used as a probe of lepton-number violation, and (ii) for Dirac case, the tri-lepton signal which conserves lepton number but may violate lepton flavor…
The possibility of discovering heavy Majorana neutrinos and lepton number violation via the like sign dilepton signal at hadron super- colliders is investigated. The cross-sections for the production of these neutrinos singly as well as in pairs are computed both in three and four generation scenarios within the framework of the gauge group $SU(2)_L \otimes U(1)_Y$ and the dominant processes are identified. The suppression of the Standard model background by suitable kinematical cuts is also discussed.
A general feature of TeV-scale radiative seesaw models, in which tiny neutrino masses are generated via loop corrections, is an extended scalar (Higgs) sector. Another feature is the Majorana nature; e.g., introducing right-handed neutrinos with TeV-scale Majorana masses under the discrete symmetry, or otherwise introducing some lepton number violating interactions in the scalar sector. We study phenomenological aspects of these models at collider experiments. We find that, while properties of the extended Higgs sector of these models can be explored to some extent, the Majorana nature of the …
Enlightened by the idea of the 3 times 3 CKM angle matrix proposed recently by Harrison et al., we introduce the Dirac angle matrix Phi and the Majorana angle matrix Psi in the lepton sector for Dirac and Majorana neutrinos respectively. We show that in presence of the CP violation, the angle matrix Phi or Psi is entirely equivalent to the complex MNS matrix V itself, but has the advantage of being real, phase rephasing invariant, directly associated to the leptonic unitarity triangles (UTs) and do not depend on any particular parametrization of V. In this paper, we further analyzed how the an…
We calculate the two-loop diagrams that give a non-zero contribution to the electric dipole moment d_l of a charged lepton l due to possible Majorana masses of neutrinos. Using the example with one generation of the Standard Model leptons and two heavy right-handed neutrinos, we demonstrate that the non-vanishing result for d_l first appears in order O(m_l m_ν^2 G_F^2), where m_νis the mass of the light neutrino and the see-saw type relation is imposed. This effect is beyond the reach of presently planned experiments.
Several beyond the Standard Model scenarios introduce new heavy neutrinos, whose Dirac or Majorana nature could be tested by comparing the rates of lepton number violating and lepton number conserving processes: a Dirac fermion induces only the latter, while a Majorana one predicts the same rate for both of them. Nevertheless, in the presence of more than one Majorana fermion, this picture may change drastically due to interference effects. We focus on lepton number violating and lepton flavour violating semileptonic meson decays induced by two heavy Majorana fermions, exploring the necessary …
We discuss the possibility to distinguish between Dirac and Majorana neutrinos in the context of the minimal gauge theory for neutrino masses, the B-L gauge extension of the Standard Model. We revisit the possibility to observe lepton number violation at the Large Hadron Collider and point out the importance of the decays of the new gauge boson to discriminate between the existence of Dirac or Majorana neutrinos.
We show that a detector placed near a beta-beam storage ring can probe lepton number violating interactions, as predicted by supersymmetric theories with R-parity non-conservation. In the presence of such interactions, nu_tau can be produced during beta-decay leading to tau leptons through weak interactions. Alternatively, electron neutrinos from beta-decay of radioactive ions can produce tau leptons in a nearby detector through these interactions. The muons from the decay of these tau leptons can be readily identified in a small iron calorimeter detector and will signal violation of R-parity.
We perform realistic simulations of the current and future long baseline experiments such as T2K, NO$ν$A, DUNE and T2HK in order to determine their ultimate potential in probing neutrino oscillation parameters. We quantify the potential of these experiments to underpin the octant of the atmospheric angle $θ_{23}$ as well as the value and sign of the CP phase $δ_{CP}$.
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 fo…
An observation of any lepton number violating process will undoubtedly point towards the existence of new physics and indirectly to the clear Majorana nature of the exchanged fermion. In this work, we explore the potential of a minimal extension of the Standard Model via heavy sterile fermions with masses in the $[ 0.1 - 10]$ GeV range concerning an extensive array of "neutrinoless" meson and tau decay processes. We assume that the Majorana neutrinos are produced on-shell, and focus on three-body decays. We conduct an update on the bounds on the active-sterile mixing elements, $|U_{\ell_α4} U_…
We study 2 --> n inelastic fermion-(anti)fermion scattering into multiple longitudinal weak gauge bosons and derive universal upper bounds on the scales of fermion mass generation by imposing unitarity of the S-matrix. We place new upper limits on the scales of fermion mass generation, independent of the electroweak symmetry breaking scale. We find that the strongest 2 --> n limits fall in a narrow range, 3-170 TeV (with n=2-24), depending on the observed fermion masses.
We computed the kinematics of Z-boson decay into a heavy-light neutrino pair when the Z-boson is produced at rest in electron-positron collisions, including the subsequent decay of the heavy neutrino into a visible final state containing a charged-lepton. We concentrated on heavy-neutrino masses of order dozens of GeV and the issue of addressing the nature of the neutrinos - Dirac fermions or Majorana fermions. We find that while it is not possible to tell the nature of the heavy and light neutrinos on an event-by-event basis, the nature of the neutrinos can nonetheless be inferred given a lar…
