Spontaneous Violation Of Lepton Number Research Articles

The Type-I Seesaw family

We provide a comprehensive analysis of the Type-I Seesaw family of neutrino mass models, including the conventional type-I seesaw and its low-scale variants, namely the linear and inverse seesaws. We establish that all these models essentially correspond to a particular form of the type-I seesaw in the context of explicit lepton number violation. We then focus into the more interesting scenario of spontaneous lepton number violation, systematically categorizing all inequivalent minimal models. Furthermore, we identify and flesh out specific models that feature a rich majoron phenomenology and discuss some scenarios which, despite having heavy mediators and being invisible in processes such as μ → eγ, predict sizable rates for decays including the majoron in the final state.

Linear seesaw mechanism from dark sector

We propose a minimal model where a dark sector seeds neutrino mass generation radiatively within the linear seesaw mechanism. Neutrino masses are calculable, since tree-level contributions are forbidden by symmetry. They arise from spontaneous lepton number violation by a small Higgs triplet vacuum expectation value. Lepton flavour violating processes e.g. μ → eγ can be sizeable, despite the tiny neutrino masses. We comment also on dark-matter and collider implications.

Dark matter in the scotogenic model with spontaneous lepton number violation

Scotogenic models constitute an appealing solution to the generation of neutrino masses and to the dark matter mystery. In this work we consider a version of the Scotogenic model that breaks lepton number spontaneously. At this scope, we extend the particle content of the Scotogenic model with an additional singlet scalar which acquires a non-zero vacuum expectation value and breaks a global lepton number symmetry. As a consequence, a massless Goldstone boson, the majoron, appears in the particle spectrum. We discuss how the presence of the majoron modifies the phenomenology, both in flavor and dark matter observables. We focus on the fermionic dark matter candidate and analyze its relic abundance and prospects for both direct and indirect detection.

Observable flavor violation from spontaneous lepton number breaking

We propose a simple model of spontaneous lepton number violation with potentially large flavor violating decays, including the possibility that majoron emitting decays, such as μ → eJ, saturate the experimental bounds. In this model the majoron is a singlet-doublet admixture. It generates a type-I seesaw for neutrino masses and contains also a vector-like lepton. As a by-product, the model can explain the anomalous (g − 2)μ in parts of its parameter space, where one expects that the branching ratio of the Higgs to muons is changed with respect to Standard Model expectations. However, the explanation of the muon g − 2 anomaly would lead to tension with recent astrophysical bounds on the majoron coupling to muons.

Electroweak symmetry breaking in the inverse seesaw mechanism

We investigate the stability of Higgs potential in inverse seesaw models. We derive the full two-loop RGEs of the relevant parameters, such as the quartic Higgs self-coupling, taking thresholds into account. We find that for relatively large Yukawa couplings the Higgs quartic self-coupling goes negative well below the Standard Model instability scale ∼ 1010 GeV. We show, however, that the “dynamical” inverse seesaw with spontaneous lepton number violation can lead to a completely consistent and stable Higgs vacuum up to the Planck scale.

Consistency of the dynamical high-scale type-I seesaw mechanism

We analyze the consistency of electroweak breaking within the simplest high-scale Standard Model type-I seesaw mechanism. We derive the full two-loop RGEs of the relevant parameters, including the quartic Higgs self-coupling of the Standard Model. For the simplest case of bare "right-handed" neutrino mass terms we find that, with large Yukawa couplings, the Higgs quartic self-coupling becomes negative much below the seesaw scale, so that the model may be inconsistent even as an effective theory. We show, however, that the "dynamical" type-I high-scale seesaw with spontaneous lepton number violation has better stability properties.

Electroweak breaking and Higgs boson profile in the simplest linear seesaw model

We examine the simplest realization of the linear seesaw mechanism within the Standard Model gauge structure. Besides the standard scalar doublet, there are two lepton-number-carrying scalars, a nearly inert SU(2)L doublet and a singlet. Neutrino masses result from the spontaneous violation of lepton number, implying the existence of a Nambu-Goldstone boson. Such “majoron” would be copiously produced in stars, leading to stringent astrophysical constraints. We study the profile of the Higgs bosons in this model, including their effective couplings to the vector bosons and their invisible decay branching ratios. A consistent electroweak symmetry breaking pattern emerges with a compressed spectrum of scalars in which the “Standard Model” Higgs boson can have a sizeable invisible decay into the invisible majorons.

Massive Majorons and constraints on the Majoron-neutrino coupling

We revisit a singlet Majoron model in which neutrino masses arise from the spontaneous violation of lepton number. If the Majoron obtains a mass of order MeV, it can play the role of dark matter. We discuss constraints on the couplings of the massive Majoron with masses of order MeV to neutrinos from supernova data. In the dense supernova core, Majoron-emitting neutrino annihilations are allowed and can change the signal of a supernova. Based on the observation of SN1987A, we exclude a large range of couplings from the luminosity and the deleptonization arguments, taking the effect of the background medium into account. If the Majoron mass does not exceed the Q-value of the experiment, the neutrino-Majoron couplings allow for neutrinoless double beta decay with Majoron emission. We derive constraints on the couplings for a Majoron mass of order MeV based on the phase space suppression and the diminishing signal-to-background ratio due to the Majoron mass. The combination of constraints from astrophysics and laboratory experiments excludes a large range of neutrino-Majoron couplings in the mass range of intererest for Majoron dark matter.

Vacuum stability with spontaneous violation of lepton number

The vacuum of the Standard Model is known to be unstable for the measured values of the top and Higgs masses. Here we show how vacuum stability can be achieved naturally if lepton number is violated spontaneously at the TeV scale. More precise Higgs measurements in the next LHC run should provide a crucial test of our symmetry breaking scenario. In addition, these schemes typically lead to enhanced rates for processes involving lepton flavour violation .

Leptogenesis with a dynamical seesaw scale

In the simplest type-I seesaw leptogenesis scenario right-handed neutrino annihilation processes are absent. However, in the presence of new interactions these processes are possible and can affect the resulting B-L asymmetry in an important way. A prominent example is provided by models with spontaneous lepton number violation, where the existence of new dynamical degrees of freedom can play a crucial role. In this context, we provide a model-independent discussion of the effects of right-handed neutrino annihilations. We show that in the weak washout regime, as long as the scattering processes remain slow compared with the Hubble expansion rate throughout the relevant temperature range, the efficiency can be largely enhanced, reaching in some cases maximal values. Moreover, the B-L asymmetry yield turns out to be independent upon initial conditions, in contrast to the ''standard'' case. On the other hand, when the annihilation processes are fast, the right-handed neutrino distribution tends to a thermal one down to low temperatures, implying a drastic suppression of the efficiency which in some cases can render the B-L generation mechanism inoperative.

Fermiophobia in a Higgs triplet model

A fermiophobic Higgs boson can arise in models with an extended Higgs sector, such as models with scalars in an isospin triplet representation. In a specific model with a scalar triplet and spontaneous violation of lepton number induced by a scalar singlet field, we show that fermiophobia is not a fine-tuned situation, unlike in two higgs doublet models. We study distinctive signals of fermiophobia which can be probed at the LHC. For the case of a small Higgs mass, a characteristic signal would be a moderate $B(H\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma})$ accompanied by a large $B(H\ensuremath{\rightarrow}JJ)$ (where $J$ is a Majoron), the latter being an invisible decay. For the case of a large Higgs mass there is the possibility of dominant $H\ensuremath{\rightarrow}ZZ$, $WW$ and suppressed $H\ensuremath{\rightarrow}JJ$ decay modes. In this situation, $B(H\ensuremath{\rightarrow}ZZ)$ is larger than $B(H\ensuremath{\rightarrow}WW)$, which differs from the SM prediction.

Reappraisal of spontaneousR-parity violation

In this short reappraisal of spontaneous lepton number violation in a supersymmetric scenario implemented through singlet sneutrino vacuum expectation value (VEV), we contribute with two new things in the context where the lepton number symmetry is global: (i) provide explicit expressions of $R$-parity violating couplings in terms of the neutrino Yukawa couplings and the singlet sneutrino VEV, and (ii) estimate the limit on this VEV using the current knowledge of the light neutrino mass and the astrophysical constraint on the Majoron-electron coupling. Besides, we put updated constraints on the VEV and Yukawa coupling of the singlet superfield when lepton number is gauged.

The seesaw mechanism at TeV scale in the 3-3-1 model with right-handed neutrinos

We implement the seesaw mechanism in the 3-3-1 model with right-handed neutrinos. This will be accomplished by the introduction of a scalar sextet into the model and the spontaneous violation of lepton number. The main result of this work is that the seesaw mechanism can work already at the TeV scale with the consequence that the right-handed neutrino masses lie in the electroweak scale, in the range from MeV to tens of GeV. This window provides a great opportunity to test their appearance at current detectors, though when we contrast our results with some previous analyses concerning the detection sensitivity at LHC, we conclude that further work is needed in order to validate this search.

Bulk majorons at colliders

Lepton number violation may arise via the spontaneous breakdown of a global symmetry. In extra dimensions, spontaneous lepton number violation in the bulk implies the existence of a Goldstone boson, the majoron J (0), as well as an accompanying tower of Kaluza–Klein (KK) excitations, J ( n) . Even if the zero-mode majoron is very weakly interacting, so that detection in low-energy processes is difficult, the sum over the tower of KK modes may partially compensate in processes of relevance at high-energy colliders. Here we consider the inclusive differential and total cross sections for e − e −→ W − W − J, where J represents a sum over KK modes. We show that allowed parameter choices exist for which this process may be accessible to a TeV-scale electron collider.

PossibleZ-width probe of a “brane-world” scenario for neutrino masses

The possibility that the accurately known value of the Z width might furnish information about the coupling of two neutrinos to the Majoron (Nambu-Goldstone boson of spontaneous lepton number violation) is proposed and investigated in detail. Both the "ordinary" case and the case in which one adopts a "brane" world picture with the Majoron free to travel in extra dimensions are studied. Bounds on the dimensionless coupling constants are obtained, allowing for any number of extra dimensions and any intrinsic mass scale. These bounds may be applied to a variety of different Majoron models. If a technically natural see-saw model is adopted, the predicted coupling constants are far below these upper bounds. In addition, for this natural model, the effect of extra dimensions is to decrease the predicted partial Z width, the increase due to many Kaluza-Klein excitations being compensated by the decrease of their common coupling constant.

Phenomenology of the neutrino-mass-giving Higgs triplet and the low-energy seesaw violation of lepton number

Small realistic Majorana neutrino masses can be generated via a Higgs triplet ( ξ ++, ξ +, ξ 0) without having energy scales larger than M ∗= O(1) TeV in the theory. The large effective mass scale Λ in the well-known seesaw neutrino-mass operator Λ −1( LLΦΦ) is naturally obtained with Λ∼M ∗ 2/μ, where μ is a small scale of lepton-number violation. In theories with large extra dimensions, the smallness of μ is naturally obtained by the mechanism of “shining” if the number of extra dimensions n⩾3. We study here the Higgs phenomenology of this model, where the spontaneous violation of lepton number is treated as an external source from extra dimensions. The observable decays ξ ++→ l i + l j + will determine directly the magnitudes of the { ij} elements of the neutrino mass matrix. The decays ξ +→ W + J 0 and ξ 0→ ZJ 0, where J 0 is the massless Goldstone boson (Majoron), are also possible, but of special importance is the decay ξ 0→ J 0 J 0 which provides stringent constraints on the allowed parameter space of this model. Based on the current neutrino data, we also predict observable rates of μ–e conversion in nuclei.

Seesaw majoron model of neutrino mass and novel signals in Higgs boson production at LEP

We perform a careful study of the neutral scalar sector of a model which includes a singlet, a doublet, and a triplet scalar field under SU(2). This model is motivated by neutrino physics, since it is simply the most general version of the seesaw model of neutrino mass generation through spontaneous violation of lepton number. The neutral Higgs sector contains three CP-even and one massive CP-odd Higgs boson A, in addition to the massless CP-odd mojoron J. The weakly interacting majoron remains massless if the breaking of lepton number symmetry is purely spontaneous. We show that the massive CP-odd Higgs boson may invisibly decay to three majorons, as well as to a CP-even Higgs H boson plus a majoron. We consider the associated Higgs production e + e − → Z → HA followed by invisible decays A → JJJ and H → JJ and derive the corresponding limits on masses and coupling that follow from LEP I precision measurements of the invisible Z width. We also study a novel b bb bp T signal predicted by the model, analyze the background and perform a Monte Carlo simulation of the signal in order to illustrate the limits on Higgs boson mass, couplings and branching ratios that follow from that.

Single-photon Z decays and small neutrino masses

We discuss some rare Z decay signatures associated with extensions of the Standard Model with spontaneous lepton number violation close to the weak scale. We show that single-photon Z decays such as Z → γH and Z → γJJ where H is a CP-even Higgs boson and J denotes the associated CP-odd majoron may occur with branching ratios accessible to LEP sensitivities, even though the corresponding neutrino masses can be very small, as required in order to explain the deficit of solar neutrinos.

Invisible Higgs bosons at present and future colliders

In many extensions of the standard model the Higgs boson can have substantial invisible decay modes, for example, into light or massless weakly interacting Goldstone bosons associated to the spontaneous violation of lepton number below the weak scale. In this work, we first report on the model independent limits on the Higgs boson from the analysis of the present LEP samples after including the possibility of invisible decays and study the prospects for LEP II. Next, we review the detectability prospects for such invisible Higgs boson at the Next Linear Collider.

Limits on associated production of visibly and invisibly decaying Higgs bosons from Z decays

Many extensions of the standard electroweak model Higgs sector suggest that the main Higgs decay channel is “invisible”, for example, h → JJ where J denotes the majoron, a weakly interacting pseudoscalar Goldstone boson associated to the spontaneous violation of lepton number. In many of these models the Higgs boson may also be produced in association to a massive pseudoscalar boson ( HA), in addition to the standard Bjorken mechanism ( HZ). We describe a general strategy to determine limits from LEP data on the masses and couplings of such Higgs bosons, using the existing data on acoplanar dijet events as well as data on four and six b jet event topologies. For the sake of illustration, we present constraints that can be obtained for the ALEPH data.