We consider a class of models in which the neutrinos acquire Majorana masses through mixing with singlet neutrinos that emerge as composite states of a strongly coupled hidden sector. In this framework, the light neutrinos are partially composite particles that obtain their masses through the inverse seesaw mechanism. We focus on the scenario in which the strong dynamics is approximately conformal in the ultraviolet, and the compositeness scale lies at or below the weak scale. The small parameters in the Lagrangian necessary to realize the observed neutrino masses can naturally arise as a consequence of the scaling dimensions of operators in the conformal field theory. We show that this class of models has interesting implications for a wide variety of experiments, including colliders and beam dumps, searches for lepton flavor violation and neutrinoless double beta decay, and cosmological observations. At colliders and beam dumps, this scenario can give rise to striking signals involving multiple displaced vertices. The exchange of hidden sector states can lead to observable rates for flavor violating processes such as $μ\rightarrow e γ$ and $μ\rightarrow e$ conversion. If the compositeness scale lies at or below a hundred MeV, the rate for neutrinoless double beta decay is suppressed by form factors and may be reduced by an order of magnitude or more. The late decays of relic singlet neutrinos can give rise to spectral distortions in the cosmic microwave background that are large enough to be observed in future experiments.
Assuming the existence of a heavy Majorana neutral particle arising from a composite model scenario we discuss the constraints imposed by present experimental limits of half-life neutrinoless double beta decay $(0νββ) $ measurements on the coupling of the heavy composite neutrinos to the gauge bosons. For neutrino masses $M_N = 1 $ TeV we obtain a rather weak lower bound on the compositeness scale: $Λ\ge 0.23 $ TeV.
Neutrinos can have naturally small Dirac masses if the Standard Model singlet right-handed neutrinos are light composite fermions. Theories which produce light composite fermions typically generate many of them, three of which can marry the left-handed neutrinos with small Dirac masses. The rest can serve as sterile states which can mix with the Standard Model neutrinos. We present explicit models illustrating this idea.
Probing Neutrino Compositeness with Invisible and Displaced Signals
INFN, Sezione di Firenze（意大利国家核物理研究所佛罗伦萨分部）；Department of Physics and Astronomy, University of Florence（佛罗伦萨大学物理与天文学系）；Perimeter Institute for Theoretical Physics（理论物理珀米特研究所）；Galileo Galilei Institute for Theoretical Physics（伽利略·伽利莱理论物理研究所）
Partial compositeness as a theory of flavor in the lepton sector is assessed. We begin presenting the first systematic analysis of neutrino mass generation in this context, and identifying the distinctive mass textures. We then update the bounds from charged lepton flavor and CP violating observables. We put forward a $U(1)^3\times CP$ symmetry of the composite sector, in order to allow the new physics to be not far above the TeV scale. This hypothesis effectively suppresses the new contributions to the electron EDM and $μ\to eγ$, by far the most constraining observables, and results in a nove…
Providing an Ultra-Violet completion valid up to the Planck scale is of paramount importance to validate the composite Higgs paradigm, at par with supersymmetry. We propose the first complete and feasible framework, based on partial unification of a confining hypercolor gauge group, where couplings of the standard model fermions are mediated by both gauge and scalar bosons. We demonstrate our approach by providing an explicit model based on a Techni-Pati-Salam unification, $SU(8)_{\rm PS}\times SU(2)_L\times SU(2)_R$, able to generate masses for all fermion generations, including neutrinos, vi…
We analyze signals in the Cosmic Microwave Background (CMB) in theories where the small Dirac neutrino masses arise as a consequence of the compositeness of right-handed neutrinos. In such theories, the right-handed neutrinos are massless ``baryons'' of a new strong gauge interaction. We find that the results crucially depend on whether or not the new strong sector undergoes chiral symmetry breaking. In the case with chiral symmetry breaking, we find that there are indeed signals in the CMB, but none of them is a direct consequence of neutrino compositeness. In contrast, if the underlying theo…
In models of electroweak symmetry breaking in which the SM fermions get their masses by mixing with composite states, it is natural to expect the top quark to show properties of compositeness. We study the phenomenological viability of having a mostly composite top. The strongest constraints are shown to mainly come from one-loop contributions to the T-parameter. Nevertheless, the presence of light custodial partners weakens these bounds, allowing in certain cases for a high degree of top compositeness. We find regions in the parameter space in which the T-parameter receives moderate positive …
Recently, the LHCb and CDF collaborations reported the measure of an unexpectedly large direct CP asymmetry in D meson decays. In this paper we ask if new physics associated with Partial Compositeness could plausibly explain this result. We find that Composite Higgs models with mass scale around 10 TeV can account for it, while marginally satisfying all other flavor constraints in the quark sector. The minimal framework is however inadequate in the lepton sector due to the strong constraint from μ to e γ. This tension can be efficiently alleviated by realizing Partial Compositeness within Supe…
Within a chiral $SU(15)$ gauge theory in which the Standard Model fermions are bound states of massless preons, we show that proton-decay operators are likely induced at the compositeness scale, $Λ_{\rm pre}$. Our estimate of the limit imposed by searches for proton decays is $Λ_{\rm pre} > 10^4 \, {\rm TeV}^{1/2} \, C_8^{1/4} \, m_{\cal Q}^{1/2}$, where $C_8$ is a rescaled coefficient of an 8-prebaryon operator induced by $SU(15)$ dynamics, and $m_{\cal Q}$ is the mass of a composite vectorlike quark. The latter has a lower limit related to the mass of a composite vectorlike lepton, which in …
A class of phenomena either not contained in the Standard Model or in its perturbative treatment (multiple W/Z production, compositeness of quarks and leptons) leads to a large energy deposition in neutrino telescopes within a rather small volume. The shape of the particle distribution arising from such phenomena is estimated.
We scrutinize the recently further strengthened hints for new physics in semileptonic $B$-meson decays, focusing on the 'clean' ratios of branching fractions $R_K$ and $R_{K^\ast}$ and examining to which pattern of new effects they point to. We explore in particular the hardly considered, yet fully viable, option of new physics in the right-handed electron sector and demonstrate how a recently proposed framework of leptons in composite Higgs setups naturally solves both the $R_K$ and $R_{K^\ast}$ anomalies via a peculiar structure of new physics effects, predicted by minimality of the model an…
We show that conventional Higgs compositeness conditions can be achieved by the running of large Higgs-Yukawa couplings involving right-handed neutrinos that become active at $\sim 10^{13}- 10^{14}$ GeV. Together with a somewhat enhanced quartic coupling, arising by a Higgs portal interaction to a dark matter sector, we can obtain a Higgs boson composed of neutrinos. This is a "next-to-minimal" dynamical electroweak symmetry breaking scheme.
We present a mechanism that naturally produces light Dirac neutrinos. The basic idea is that the right-handed neutrinos are composite. Any realistic composite model must involve `hidden flavor' chiral symmetries. In general some of these symmetries may survive confinement, and in particular, one of them manifests itself at low energy as an exact $B-L$ symmetry. Dirac neutrinos are therefore produced. The neutrinos are naturally light due to compositeness. In general, sterile states are present in the model, some of them can naturally be warm dark matter candidates.
We construct UV completions of bottom-up models with a pseudo Nambu-Goldstone Boson (NGB) composite Higgs and partial compositeness, admitting a weakly coupled description of the composite sector. This is identified as the low energy description of an SO(N) supersymmetric gauge theory with matter fields in the fundamental of the group. The Higgs is a NGB associated to an SO(5)/SO(4) coset of a global symmetry group and is identified with certain components of matter fields in a Seiberg dual description of the theory. The Standard Model (SM) gauge fields are obtained by gauging a subgroup of th…
Composite Higgs models with anarchic partial compositeness require a scale of new physics ${\cal O}(10-100)$ TeV, with the bounds being dominated by the dipole moments and $ε_K$. The presence of anarchic bilinear interactions can change this picture. We show a solution to the SM flavor puzzle where the electron and the Right-handed quarks of the first generation have negligible linear interactions, and the bilinear interactions account for most of their masses, whereas the other chiral fermions follow a similar pattern to anarchic partial compositeness. We compute the bounds from flavor and CP…
We present a composite scotogenic model for neutrino masses, which are generated via loops of $\mathbb{Z}_2$-odd composite scalars. We consider three different approaches to the couplings of the neutrinos (including three right-handed singlets) and the composite sector: ETC-like four-fermion interactions, fundamental partial compositeness and fermion partial compositeness. In all cases, the model can feature sizeable couplings and remain viable with respect to various experimental constraints if the three $ \mathbb{Z}_2 $-odd right-handed neutrinos have masses between the TeV and the Planck sc…
Partial compositeness is a mechanism for the generation of fermion masses which replaces a direct Higgs coupling to the fermions by a linear mixing with heavy composite partners. We present the first calculation of the relevant matrix element in a lattice model which is very close to a candidate theory containing a composite Higgs boson and a partially composite top quark. Specifically, our model is an SU(4) gauge theory coupled to dynamical fermions in the fundamental and two-index antisymmetric (sextet) representations. The matrix element we obtain is small and hence our result disfavors the…
The top quark can be naturally singled out from other fermions in the Standard Model due to its large mass, of the order of the electroweak scale. We follow this reasoning in models of pseudo Nambu Goldstone Boson composite Higgs, which may derive from an underlying confining dynamics. We consider a new class of flavour models, where the top quark obtains its mass via partial compositeness, while the lighter fermions acquire their masses by a deformation of the dynamics generated at a high flavour scale. One interesting feature of such scenario is that it can avoid all the flavour constraints …
We study in detail the contribution of heavy composite Majorana neutrinos to neutrino-less double beta decay. Our analysis confirms the result of a previous estimate by two of the authors. Excited neutrinos couple to the electroweak gauge bosons through a magnetic type effective Lagrangian. The relevant nuclear matrix element is related to matrix elements available in the literature and current bounds on the half-life of neutrino-less double beta decay are converted into bounds on the compositeness scale and/or the heavy neutrino mass. Our bounds are of the same order of magnitude as those ava…
We give a brief review of the existing bounds on effective couplings for excited states ($e^*, ν^*, u^*, d^* $) of the ordinary quarks and leptons arising from a composite model scenario. We then explore the phenomenological implications of the hypothesis that the excited neutrino $ν^*$ might be of Majorana type. Recent bounds on the half-life of the $ΔL =2 $ neutrinoless double beta decay ($0νββ$) are used to constraint the compositeness effective couplings. We show that the bounds so obtained are roughly of the same order of magnitude as those available from high energy experiments.
We discuss a test of the Standard Model fermion mass origin in models of dynamical electroweak symmetry breaking. The couplings of composite pseudoscalar resonances to top quarks allow to distinguish high-scale Extended-Technicolor-type fermion mass generation from fermion partial compositeness and low-scale mass generation via an induced vacuum expectation value of a doublet coupled to the composite sector. These different possible origins of fermion masses are thus accessible via weak-scale physics searched for at the LHC.
Leptogenesis is an appealing framework to account for the baryon asymmetry in the universe. To this end physics beyond the Standard Model is demanded. In this paper we investigate the possibility to attain successful leptogenesis with composite Majorana neutrinos. We work in the framework of effective gauge mediated and contact interactions without any reference to an underlying compositeness theory. This approach is the one adopted in all current experimental searches for composite fermions at colliders. In the case of gauge mediated interactions, we calculate the CP asymmetry in heavy compos…
We review recent work on realistic models that break supersymmetry dynamically and give rise to composite quarks and leptons, all in a single sector. These models have a completely natural suppression of flavor-changing neutral currents, and the hierarchy of Yukawa couplings is explained by the dimensionality of composite states. The generic signatures are unification of scalar masses with different quantum numbers at the compositeness scale, and lighter gaugino, Higgsino, and third-generation sfermion masses.
The texture zero mass matrices for the leptons and the seesaw mechanism are used to derive relations between the matrix elements of the lepton mixing matrix and the ratios of the neutrino masses.
We present a realization of the idea that the Higgs boson is mainly a bound state of neutrinos induced by strong four-fermion interactions. The conflicts of this idea with the measured values of the top quark and Higgs boson masses are overcome by introducing, in addition to the right-handed neutrino, a new fermion singlet, which, at low energies, implements the inverse see-saw mechanism. The singlet fermions also develop a scalar bound state which mixes with the Higgs boson. This allows us to obtain a small Higgs boson mass even if the couplings are large, as required in composite scalar scen…
A framework for predicting charged fermion masses in supersymmetric grand unified theories is extended to make predictions in the neutrino sector. Eight new predictions are made, and their relevance to neutrino oscillation experiments and the solar neutrino problem are discussed.
The possible mechanisms of generation of the neutrino of small masses and sterile neutrino are considered. In the supposition of stability of tracks of a left-right symmetry concerning masses of constituent quarks, the angles of mixing of quarks are evaluated. With usage of experimental data on neutrino oscillation parameters the neutrino masses on order of magnitude are computed.
The energies of neutrinos in a neutrinosphere, the dense matter created after the gravitational collapse of a massive star, are estimated. Cubic equations for energy eigenvalues of neutrinos are used, with the effective masses found by taking the neutrinos at rest in neutrinosphere matter. Large differences in the effective mass of some neutrino species in a neutrinosphere compared to vacuum are found.
A large hierarchy of the Dirac masses can result in a small hierarchy for the low energy masses of the active neutrinos. This can happen even if the Majorana masses of right-handed neutrinos are all equal. A realistic description of the observed neutrino masses and mixing can be obtained starting from a large hierarchy in the Dirac masses. A large mixing for solar neutrinos results from the neutrino sector. The small value of the MNS matrix element U_e3 is a natural consequence of the scheme. The masses of the two lighter neutrinos are related to the solar neutrino mixing angle: mu_1/mu_2 = ta…
We discuss which information on neutrino masses and mixing can be obtained from the results of all neutrino oscillation experiments in the cases of three and four massive neutrinos. We show that in the three-neutrino case the neutrino oscillation data are not compatible with a hierarchy of couplings. In the case of four neutrinos, a hierarchy of masses is disfavored by the data and only two schemes with two pairs of neutrinos with close masses separated by a gap of the order of 1 eV can accommodate the results of all experiments.
We discuss first the flavor mixing of the quarks, using the texture zero mass matrices. Then we study a similar model for the mass matrices of the leptons. We are able to relate the mass eigenvalues of the charged leptons and of the neutrinos to the mixing angles and can predict the masses of the neutrinos. We find a normal hierarchy - the masses are 0.004 eV, 0.01 eV and 0.05 eV. The atmospheric mixing angle is given by the mass ratios of the charged leptons and the neutrinos. we find about 40 degrees, consistent with the experiments. The mixing element, connecting the first neutrino wit the …
An alternative to the famous see-saw mechanism is proposed to explain the smallness of the neutrino masses (if present). This model involves a fourth family which mixes very little with the other three. It contains one heavy neutrino (mN > mZ /2) and three very light neutrinos whose masses are radiatively induced. In contrast with the see-saw mechanism, all neutrino masses are Dirac masses. In one particular scenario, the three light neutrinos are almost degenerate in mass and are found to be consistent with fits to the Solar and Atmospheric neutrino deficits. They might even account for the H…
We discuss the possibility of using experiments timing the propagation of neutrino beams over large distances to help determine the absolute masses of the three neutrinos.
The phenomenology of absolute neutrino masses is reviewed, focusing on tritium beta decay, cosmological measurements and neutrinoless double-beta decay.
We investigate a possibility of a composite Z' vector boson. For the compositeness, the required gauge coupling g in low energy is not so big, g^2/(4π) > 0.015 in the case of the U(1)_{B-L} model. We show that the Stueckelberg model is effectively induced in low energy via the fermion loop from the Nambu-Jona-Lasinio (NJL) model having the vectorial four-fermion interaction. In terms of the renormalization group equations (RGE's), this situation is expressed by the compositeness conditions. We find that the solutions of the RGE's with the compositeness conditions are determined by the infrared…
We briefly summarise the current status of neutrino masses and mixing, paying special attention to the prospects for observing new leptonic interactions.
We propose a novel theoretical understanding of neutrino masses and mixings, which is attributed to the intrinsic vector-like feature of the regularized Standard Model at short distances. We try to explain the smallness of Dirac neutrino masses and the decoupling of the right-handed neutrino as a free particle. Neutrino masses and mixing angles are completely related to each other in the Schwinger-Dyson equations for their self-energy functions. The solutions to these equations and a possible pattern of masses and mixings are discussed.
The experimental evidences in favor of oscillations of solar (and KamLAND) and atmospheric (and K2K) neutrinos are briefly reviewed and accommodated in the framework of three-neutrino mixing. The implications for the values of neutrino masses are discussed and the bounds on the absolute scale of neutrino masses from Tritium beta-decay and cosmological data are reviewed. Finally, we discuss the implications of three-neutrino mixing for neutrinoless double-beta decay.
In this talk I review what we know and don't know about neutrinos, neutrino masses and lepton mixing. I also discuss the importance of the discovery that neutrinos have nonzero masses, and illustrate how little is currently known about the physics behind them.
Neutrino masses are zero in the minimal supersymmetric standard model. I discuss how they may become nonzero with new interactions which may or may not violate R-parity conservation.
We propose a model for neutrino mass generation in wich no physics beyond a TeV is required. We extend the standard model by adding two charged singlet fields with lepton number two. Dirac neutrino masses $m_{ν_D} \leq MeV$ are generated at the one loop level. Small left handed majorana neutrino masses can be generated via the seesaw mechanism with right handed neutrino masses $M_R$ are of order TeV scale.
Cosmology yields the most restrictive limits on neutrino masses and conversely, massive neutrinos would contribute to the cosmic dark-matter density and would play an important role for the formation of structure in the universe. Neutrino oscillations may well solve the solar neutrino problem and can have a significant impact on supernova physics. The neutrino signal from a future galactic supernova could provide evidence for cosmologically interesting neutrino masses or set interesting limits.
A new mechanism is proposed to explain neutrino masses and their mixing via SU(1,1) horizontal symmetry breaking. Based on this mechanism smallness of neutrino masses is naturally realized and inverted hierarchical spectrum with large mixing is given.
New effects related to refraction of neutrinos in different media are reviewed and implication of the effects to neutrino mass and mixing are discussed. Patterns of neutrino masses and mixing implied by existing hints/bounds are described. Recent results on neutrino mass generation are presented. They include neutrino masses in SO(10) GUT's and models with anomalous $U(1)$, generation of neutrino mass via neutrino-neutralino mixing, models of sterile neutrino.
We discuss the neutrino mixing, using the texture 0 mass matrices, which work very well for the quarks. The solar mixing angle is directly linked to the mass ratio of the first two neutrinos. The neutrino masses are hierarchical, but the mass ratios turn out to be much smaller than for quarks. The atmospheric mixing angle is 38$^{\circ}$. The CP violation for leptons should be much smaller than for quarks.
The simplest possibility to generate small Majorana neutrino masses is the seesaw mechanism. However, the smallness of the observed neutrino masses can also be understood, if neutrino masses are generated by higher-dimensional operators and/or at higher loop level. In this talk recent work on systematic classifications of higher-dimensional and radiative neutrino mass models is summarized. Two particular classes of special genuine loop diagrams, i.e. diagrams which can lead to genuine neutrino mass models only under some specific, well-defined conditions, are also discussed.
Absolute neutrino masses provide a key to physics beyond the standard model. We discuss the impact of absolute neutrinos masses on physics beyond the standard model, the experimental possibilities to determine absolute neutrinos masses, and the intriguing connection with the Z-burst model for extreme-energy cosmic rays.
If the cosmological limits on the sum of the neutrino masses are taken seriously we have first measurements of the masses of the neutrinos. Using the Planck experiment's limit $\sum_{i=1}^3{m_i} < 230 ~meV$ and some simple assumptions on measurement uncertainties the mass of the heaviest neutrino is $63\pm11 ~meV$ and the lightest $40\pm18 ~meV$ for either hierarchy.
We discuss a model for neutrino masses and mixings based on R parity violating Yukawa couplings. The model requires no right-handed neutrinos. It accommodates the SuperKamiokande data on atmospheric neutrinos and successfully relates this data to the mass difference for solar neutrinos in the small-angle MSW solution. We obtain an unexpected testable pattern for the three neutrino flavors.
We study a model for the mass matrices of the leptons. We are ablte to relate the mass eigenvalues of the charged leptons and of the neutrinos to the mxiing angles and can predict the masses of the neutrinos. We find a normal hierarchy -the masses are 0.004 eV, 0.01 eV and 0.05 eV. The atmospheric mixing angle is given by the mass ratios of the charged leptons and of the neutrinos. We find 38 degrees, consistent with the experiments. The mixing element, connecting the first neutrino with the electron, is found to be 0.05.
We briefly review the recent activity on neutrino masses and mixings which was prompted by the confirmation of neutrino oscillations by the Superkamiokande experiment.
I briefly review the arguments which in the 1970's convinced many theoretical physicists to anticipate lepton number violation, and to guess a range of neutrino masses.
We study a model with Majorana neutrino masses generated through doubly charged scalars at two-loop level. We give explicit relationships between the neutrino masses and the same sign dilepton decays of the doubly charged scalars. In particular, we demonstrate that at the tribimaximal limit of the neutrino mixings, the absolute neutrino masses and Majorana phases can be extracted through the measurements of the dilepton modes at colliders.
We examine some patterns of Majorana neutrino mass matrix which is compatible with the phenomenological lepton flavor mixing matrix and non-observation of neutrinoless double beta decay. Imposing $(M_ν)_{ee}=0$ for the Majorana neutrino mass matrix in the leading order, we obtain a relationship between the solar mixing angle and the neutrino masses $m_1$ and $m_2$. Additional possible texture zeros are assigned to the mass matrix so as for the nonvanishing $θ_{13}$ to be predictable in terms of neutrino masses. We also show how three neutrino masses can be predicted from the solar mixing angle…
Using recent data on neutrino oscillations, we argue that a hierarchical solution for neutrino masses in a three-family context is possible, and that the masses of the tau and mu neutrinos are very nearly determined within that possibility. We also examine the predictions of a model that determines neutrino and charged lepton mass matrices as well as its consistency with data.
