Lepton Mass Research Articles

Low scale type I seesaw model for lepton masses and mixings

In contrast to the original type I seesaw mechanism that requires right-handed Majorana neutrinos at energies much higher than the electroweak scale, the so-called low scale seesaw models allow lighter masses for the additional neutrinos. Here we propose an alternative low scale type I seesaw model, where neither linear nor inverse seesaw mechanisms take place, but the spontaneous breaking of a discrete symmetry at an energy scale much lower than the model cutoff is responsible for the smallness of the light active neutrino masses. In this scenario, the model is defined with minimal particle content, where the right-handed Majorana neutrinos can have masses at the $\sim 50\mbox{ GeV}$ scale. The model is predictive in the neutrino sector having only four effective parameters that allow to successfully reproduce the experimental values of the six low energy neutrino observables.

Characteristics of Hot Quark Matter with Neutrino Confinement

The MIT quark bag model is used to determine the thermodynamic characteristics of hot strange quark matter that is opaque to neutrinos. Three different variants of the lepton content of hot quark matter are considered. The numerical calculations are done for different temperatures and lepton charge densities. It is shown that in hot (kT~100 MeV) quark matter there are roughly as many neutrinos as massive leptons, the total energy of the leptons is 4-20 percent of the energy of the quarks, and during cooling an energy on the order of 30-40 percent of the total energy is released.

Modular invariant models of lepton masses at levels 4 and 5

We explore alternative descriptions of the charged lepton sector in modular invariant models of lepton masses and mixing angles. In addition to the modulus, the symmetry breaking sector of our models includes ordinary flavons. Neutrino mass terms depend only on the modulus and are tailored to minimize the number of free parameters. The charged lepton Yukawa couplings rely upon the flavons alone. We build modular invariant models at levels 4 and 5, where neutrino masses are described both in terms of the Weinberg operator or through a type I seesaw mechanism. At level 4, our models reproduce the hierarchy among electron, muon and tau masses by letting the weights play the role of Froggatt-Nielsen charges. At level 5, our setup allows the treatment of left and right handed charged leptons on the same footing. We have optimized the free parameters of our models in order to match the experimental data, obtaining a good degree of compatibility and predictions for the absolute neutrino masses and the C P violating phases. At a more fundamental level, the whole lepton sector could be correctly described by the simultaneous presence of several moduli. Our examples are meant to make a first step in this direction.

Lepton-proton two-photon exchange in chiral perturbation theory

We use heavy baryon chiral perturbation theory to evaluate the two-photon exchange corrections to the low-energy elastic lepton-proton scattering at next-to-leading order accuracy, i.e., ${\mathcal O}(\alpha, M^{-1})$, including a non-zero lepton mass. We consider the elastic proton intermediate state in the two-photon exchange together in the soft photon approximation. The infrared singular contributions are projected out using dimensional regularization. The resulting infrared singularity-free two-photon exchange contribution is in good numerical agreement with existing predictions based on standard diagrammatic soft photon approximation evaluations.

A modular A4 symmetry realization of two-zero textures of the Majorana neutrino mass matrix

We show how to realize two-zero textures of the Majorana neutrino mass matrix Mν based on modular A4 invariant models without flavons. In these models, all matter fields are assigned to three inequivalent singlets, 1, 1′ and 1″, of the finite modular group Γ3≃A4. Considering tensor products of the A4 group, it is easy to make the charged lepton mass matrix Mℓ diagonal. Since not all modular forms of a specific weight and level 3 can be arranged into three inequivalent singlets of A4 simultaneously, we can always make some entries in Mν vanish by properly assigning the representations and modular weights for the matter fields. We consider two cases where neutrino masses originate from the Weinberg operator and the type-I seesaw mechanism, respectively. For the former case, all seven viable two-zero textures of Mν (A1,2, B1,2,3,4 and C) can be realized successfully. For the latter case, only five of them (namely A1,2, B3,4 and C) can be achieved due to the intrinsic structure of the right-handed neutrino mass matrix MR in our assumption for simplicity.

Towards a minimal SU(5) GUT

We propose a simple $SU(5)$ model that connects the neutrino mass generation mechanism to the observed disparity between the masses of charged leptons and down-type quarks. The model is built out of $5$-, $10$-, $15$-, $24$-, and $35$-dimensional representations of $SU(5)$ and comprises two (three) $3 \times 3$ ($3 \times 1$) Yukawa coupling matrices to accommodate all experimentally measured fermion masses and mixing parameters. The gauge coupling unification considerations, coupled with phenomenological constraints inferred from experiments that probe neutrino masses and mixing parameters and/or look for proton decay, fix all relevant mass scales of the model. The proposed scenario places several multiplets at the scales potentially accessible at the LHC and future colliders and correlates this feature with the gauge boson mediated proton decay signatures. It also predicts that one neutrino is massless.

Standard Model Particles with Mass Treated as Spheres with Finite Radii

The particle physics Standard Model involves three charge 0 neutrinos, three charge e leptons, three charge (2/3)e quarks, and three charge −(1/3)e quarks, where e is electron charge. However, the Standard Model cannot explain why there are three generations of particles in each charge state and makes no predictions relating to quark and lepton masses. This analysis, treating Standard Model particles as spheres with radii 1/4 the particle Compton wavelength, explains three, and only three fermions are in each charge state and relates first generation quark masses to the electron mass.

Flavor moonshine

Abstract The flavor moonshine hypothesis is formulated to suppose that all particle masses (leptons, quarks, Higgs, and gauge particles—more precisely, their mass ratios) are expressed as coefficients in the Fourier expansion of some modular forms just as, in mathematics, dimensions of representations of a certain group are expressed as coefficients in the Fourier expansion of some modular forms. The mysterious hierarchical structure of the quark and lepton masses is thus attributed to that of the Fourier coefficient matrices of certain modular forms. Our intention here is not to prove this hypothesis starting from some physical assumptions but rather to demonstrate that this hypothesis is experimentally verified and, assuming that the string theory correctly describes the natural law, to calculate the geometry (Kähler potential and the metric) of the moduli space of the Calabi–Yau manifold, thus providing a way to calculate the metric of the Calabi–Yau manifold itself directly from the experimental data.

A Novel Model of Charged Leptons

A novel model of charged leptons is presented, which contains two basics hypotheses. The first hypothesis is that the Yukawa coupling between Higgs field and charged leptons is the weak interaction, the Higgs field is a scalar intermediate boson which changes the chirality of charged leptons in the weak interaction. The other hypothesis is that the flavor eigenstates of charged leptons are the superposition states of left-handed and right-handed elementary Weyl spinors before the electroweak symmetry breaking. According to this model, the Yukawa coupling constants between Higgs field and three generations of charged leptons are considered to be a universal constant, and the difference of the masses of different charged leptons is due to the different left-right mixing angles of their flavor eigenstates.

Electron and its symmetry as basic elements of particle mass spectrum and discreteness in nuclear data

An analysis of empirical correlations in particle masses and nuclear data confirmed the great role of lepton masses (me, mμ), parameters of the Nonrelativistic Constituent Quark Model and the QED radiative corrections in the Standard Model development. Integer relations (1:13:16:17:18:54) between doubled value of the pion β-decay energy (close to 16me=δ=8.176 MeV), the muon mass, the pion parameters (fπ=130.7 MeV, mπ) and the NRCQM parameters ΔMΔ=147 MeV=(mΔ−mN)/2 and Mq=mΞ/3 are compared with the masses of quarks and fundamental fields and with the number of fermions in the central field. Confirmations of the period 16me were obtained from the analysis of CODATA relations (exact ratios of the masses of nucleons to the electron mass) and global analysis of nuclear data.

NeoMinkowskian Cosmological Black Hole, Poincaré’s Gravific Electron and Density of CBR

In the previous paper (JMP 2014) we showed that there exists a NeoMinkowskian Gravitational Expanding Solution of GR (General Relativity) with CC (Cosmological Constant). We prove now that NeoMinkowskian Vacuum (non-baryonic Fluid), with gravitational (first) density (dark energy) and gravitational waves (at light speed), corresponds to the Gravitation Field of a Cosmological Black Hole (CBH). The latter predicts furthermore a basic emission of Radiation (CBR) from Hubble spherical singular Horizon to the inside of CBH (unlike Hawking’s emission) at an initial singular time. Our solution is then compatible with a well-tempered Big Bang and Expanding Universe (Escher’s Figure, see Penrose, 3) but incompatible with inflation. The latter is based on Hypothesis of a so-called Planck’s particle (Lemaitre’s primitive atom) characterized by a so-called Planck length. We prove that we can short-circuit this unstable particle with a stable cosmological Poincare’s electron with gravific pressure. It is well known that electron is a stranger in usual Minkowskian vacuum (dixit Einstein). The stranger electron can be perfectly integrated in NeoMinkowskian Radiation fluid and then also (with its mass, charge and wavelength) in (second density of) CBR. Everything happens as if the leptonic mass of the electron were induced by our cosmological field. The unexpected cosmological model proposed here is the only one that predicts numerical values of (second) density and temperature of CBR very close to the observed (COBE) values.

Baryon asymmetry from left-right phase transition

We extend the standard model fermions by a mirror copy to realize a left-right symmetry. During a strongly first order phase transition of the spontaneous left-right symmetry breaking, the CP-violating reflections of the mirror fermions off the mirror Higgs bubbles can generate a mirror lepton asymmetry and an equal mirror baryon asymmetry. We then can obtain an ordinary baryon asymmetry through the mirror fermion decays where a dark matter scalar plays an essential role. Benefitted from a parity symmetry for solving the strong CP problem, the cosmic baryon asymmetry can be well described by the ordinary lepton mass matrices up to an overall factor. In this scenario, the Dirac CP phase in the Majorana neutrino mass matrix can provide a unique source for the required CP violation. Furthermore, the Higgs triplet for type-II seesaw as well as the first generation of mirror charged fermions can be allowed at the TeV scale.

Fitting neutrino masses in a realistic intersecting D-braneworld

The correct quark and charged lepton mass matrices along with a nearly correct CKM matrix may be naturally accommodated in a Pati-Salam model constructed from intersecting D6 branes on a $T^6/(\Z_2 \times \Z_2)$ orientifold. Furthermore, near-tribimaximal mixing for neutrinos may arise naturally due to the structure of the Yukawa matrices. Consistency with the quark and charged lepton mass matrices in combination with obtaining near-tribimaximal mixing fixes the Dirac neutrino mass matrix completely. Then, applying the seesaw mechanism for different choices of right-handed neutrino masses and running the obtained neutrino parameters down to the electroweak scale via the RGEs, we are able to make predictions for the neutrino masses and mixing angles. We obtain lepton mixing angles which are close to the observed values, $\theta_{12} =33.8^{\circ}\pm1.2^{\circ}$, $\theta_{23}=46.9^{\circ}\pm0.9^{\circ}$, and $\theta_{13}=8.56^{\circ}\pm0.20^{\circ}$. In addition, the neutrino mass-squared differences are found to be $\Delta m^2_{32} = 0.0025\pm0.0001$~eV$^2$ and $\Delta m^2_{21} = 0.000075\pm0.000003$~eV with $m_1=0.0150\pm0.0002$~eV, $m_2=0.0173\pm0.0002$~eV, and $m_3=0.053\pm 0.002$~eV so that $\sum_i m_i = 0.085\pm0.002$~eV, consistent with experimental observations.

A4 lepton flavor model and modulus stabilization from S4 modular symmetry

We study the modulus stabilization in an $A_4$ model whose $A_4$ flavor symmetry is originated from the $S_4$ modular symmetry. We can stabilize the modulus so that the $A_4$ invariant superpotential leads to the realistic lepton masses and mixing angles. We also discuss the phenomenological aspect of the present model as a consequence of the modulus stabilization.

Dirac neutrinos in the 2HDM with restrictive Abelian symmetries

Recently, there has been a growing interest in extensions of the Standard Model in which naturally small Dirac neutrino masses arise due to existence of a symmetry which protects neutrino's Diracness. Motivated by this, we consider an extension of the Standard Model with a second Higgs doublet (2HDM) and three right-handed neutrinos where lepton number is conserved and, thus, neutrinos are Dirac particles. In this framework, we identify the most restrictive texture-zero combinations for the Dirac-neutrino and charged-lepton mass matrices that lead to masses and mixings compatible with current experimental data. We then investigate, in a systematic way, which of these combinations can be realized by Abelian continuous U(1) or discrete $\mathbb{Z}_N$ symmetries. We conclude that, from the 28 initially possible sets of maximally-restricted lepton mass matrices, only 5 have a symmetry realization in the 2HDM. For these cases, one-to-one relations among the Yukawa couplings and the neutrino mass and mixing parameters are established, and the fermion interactions with the neutral and charged scalars of the 2HDM are also determined. Consequences for lepton universality in $\tau$ decays and rare lepton-flavor-violating processes are also discussed.

Lepton mass effects for beam-normal single-spin asymmetry in elastic muon-proton scattering

We estimate the beam-normal single-spin asymmetry in elastic lepton-proton scattering without employing the ultrarelativistic approximation. Our calculation is relevant for analyses of muon scattering at energies of few hundred MeV and below -- when effects of the muon mass become essential. At such energies, the transverse polarization of the muon beam is expected to contribute significantly to the systematic uncertainty of precision measurements of elastic muon-proton scattering. We evaluate such systematics using an example of the MUSE experiment at PSI. The muon asymmetry is estimated at about 0.1\% in kinematics of MUSE and it is the largest for scattering into a backward hemisphere.

The number of sufficient and necessary conditions for CP conservation with Majorana neutrinos: Three or four?

As is well-known, there exist totally three CP-violating phases in the leptonic sector if three ordinary neutrinos are massive Majorana particles. In this short note, we raise the question whether the number of sufficient and necessary conditions for CP conservation in the leptonic sector with massive Majorana neutrinos is three or four. An intuitive answer to this question would be three, which is also the total number of independent CP-violating phases. However, we give a counter example, in which three conditions are in general not sufficient for CP conservation. Only for all the lepton masses and mixing angles within their experimentally allowed ranges can we demonstrate that it is possible to find out three weak-basis invariants, which should be vanishing to guarantee leptonic CP conservation.

Neutrino Masses from the Point of View of Economy and Simplicity

In the framework of such basic principles as local gauge invariance, unification of the weak and electromagnetic interactions and spontaneous symmetry breaking in the Standard Model the most economical and simplest possibilities are realized. We discuss the problem of neutrino masses from the point of view of economy and simplicity. It is unlikely that neutrino masses are of the same SM origin as masses of leptons and quarks. The Weinberg effective Lagrangian is the simplest and the most economical, beyond the Standard Model mechanism of the generation of small Majorana neutrino masses. The resolution of the sterile neutrino anomaly and observation of the neutrinoless double $\beta$-decay would be crucial tests of this mechanism.

Higgs Parity grand unification

The vanishing of the Higgs quartic coupling of the Standard Model at high energies may be explained by spontaneous breaking of Higgs Parity. Taking Higgs Parity to originate from the Left-Right symmetry of the SO(10) gauge group, leads to a new scheme for precision gauge coupling unification that is consistent with proton decay. We compute the relevant running of couplings and threshold corrections to allow a precise correlation among Standard Model parameters. The scheme has a built-in solution for obtaining a realistic value for mb/mτ , which further improves the precision from gauge coupling unification, allowing the QCD coupling constant to be predicted to the level of 1% or, alternatively, the top quark mass to 0.2%. Future measurements of these parameters may significantly constrain the detailed structure of the theory. We also study an SO(10) embedding of quark and lepton masses, showing how large neutrino mixing is compatible with small quark mixing, and predict a normal neutrino mass hierarchy. The strong CP problem may be explained by combining Higgs Parity with space-time parity.

Lepton flavor violation and collider searches in a type I + II seesaw model

Neutrinos are massless in the Standard Model. The most popular mechanism to generate neutrino masses are the type I and type II seesaw, where right-handed neutrinos and a scalar triplet are augmented to the Standard Model, respectively. In this work, we discuss a model where a type I + II seesaw mechanism naturally arises via spontaneous symmetry breaking of an enlarged gauge group. Lepton flavor violation is a common feature in such setup and for this reason, we compute the model contribution to the mu rightarrow egamma and mu rightarrow 3e decays. Moreover, we explore the connection between the neutrino mass ordering and lepton flavor violation in perspective with the LHC, HL-LHC and HE-LHC sensitivities to the doubly charged scalar stemming from the Higgs triplet. Our results explicitly show the importance of searching for signs of lepton flavor violation in collider and muon decays. The conclusion about which probe yields stronger bounds depends strongly on the mass ordering adopted, the absolute neutrino masses and which much decay one considers. In the 1–5 TeV mass region of the doubly charged scalar, lepton flavor violation experiments and colliders offer orthogonal and complementary probes. Thus if a signal is observed in one of the two new physics searches, the other will be able to assess whether it stems from a seesaw framework.