Dipole Moments Of Charged Leptons Research Articles

Electric dipole moments of charged leptons in models with pseudo-Dirac sterile fermions

In this work, we address the impact of a small lepton number violation on charged lepton electric dipole moments — EDMs. Low-scale seesaw models protected by lepton number symmetry and leading to pseudo-Dirac pairs in the neutrino heavy spectrum provide a natural explanation for the smallness of neutrino masses with potentially testable consequences. Among which, it was thought that the small mass gap in each pair of pseudo-Dirac neutrinos may induce important contribution to the charged lepton EDMs. Recently, it has been shown that the contribution from some of the Feynman diagrams to charged lepton EDMs exactly cancel by virtue of the Ward-Takahashi identity in quantum electrodynamics. We thus consider here the Standard Model minimally extended with pairs of pseudo-Dirac sterile fermions and derive the complete analytical formula at two loops for the charged lepton EDMs. In addition, we numerically evaluate the order of the predicted EDMs consistent with the experimental bounds and constraints such as neutrino oscillation data, charged lepton flavour violating processes, sterile neutrino direct searches, meson decays, sterile neutrino decays, and cosmological and astrophysical observations. We find that, in the minimal setup accommodating neutrino data (masses and mixings) with only two pseudo-Dirac pairs, the predicted electron EDM is O10−36\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O}\\left({10}^{-36}\\right) $$\\end{document}e cm, at most, which is much smaller than the current experimental bound and even future sensitivity. Hopefully, the electron EDM might reach future sensitivity, once extra pseudo-Dirac neutrinos are taken into account. The analytical formulae we derive are generic to any model involving pseudo-Dirac pairs in the heavy neutrino spectrum.

Contribution of BSM Particles on Electric Dipole Moment of Charged Leptons in the Minimal Left-Right Symmetric Model

In this paper, the contribution of Beyond the Standard Model (BSM) particles on the Electric Dipole Moment (EDM) of charged lepton is calculated in a minimal Left-Right Symmetric Model. First-order quantum correction i.e., one loop correction, from relevant diagrams in the unitary gauge using Feynman parameterization is used because there is no tree-level contribution in Electric Dipole Moment due to non-renormalizability of the dipole term in the Lagrangian. This study mainly focuses on the BSM Higgs sector contribution to the EDM of charged leptons due to phenomenological interest. CP violation is directly connected to the EDM of fundamental particles. Matter-antimatter asymmetry is a mystery of the universe that demands CP symmetry to be violated. The standard model cannot explain the observed CP violation. Standard Model (SM) is so far the best description of nature. Despite having tremendous success, e.g., Electro-weak Symmetry Breaking and Higgs discovery, the Standard model is not complete. Moreover, experimentally, we observed small neutrino mass and neutrino mixing. The Seesaw Mechanism can explain neutrino mass smallness and mixing introducing a large Majorana mass. Left-right symmetric model is the most natural extension of SM which can accommodate the Seesaw mechanism. So EDM contributions from BSM particles in the Left-Right symmetric Model are a good probe of new physics beyond the SM. These contributions then can be used to do phenomenology and test the Model.

Quark level and hadronic contributions to the electric dipole moment of charged leptons in the standard model

We evaluate the electric dipole moment of charged leptons in the standard model, where the complex phase of the Cabibbo-Kobayashi-Maskawa matrix is the only source of CP violation. We first prove that, at the quark-gluon level, it is suppressed by a factor of $m_b^2 m_c^2 m_s^2$ at all orders of perturbation due to the Glashow-Iliopoulos-Maiani mechanism. We then calculate the hadronic long distance contribution generated by vector mesons at one-loop level. The $|\Delta S|=1$ weak hadronic interaction is derived using the factorization, and the strong interaction is modeled by the hidden local symmetry framework. We find that the electric dipole moments of charged leptons obtained from this hadronic mechanism are much larger than the perturbative four-loop level quark-gluon process, by several orders of magnitude.

Large Long-Distance Contributions to the Electric Dipole Moments of Charged Leptons in the Standard Model.

We reevaluate the electric dipole moment (EDM) of charged leptons in the standard model using hadron effective models. We find unexpectedly large EDM generated by the hadron level long-distance effect, d_{e}=5.8×10^{-40}, d_{μ}=1.4×10^{-38}, and d_{τ}=-7.3×10^{-38} e cm, with an error bar of 70%, exceeding the conventionally known four-loop level elementary contribution by several orders of magnitude.

Non-perturbative effects in \u03bc \u2192 e\u03b3

We compute the non-perturbative contribution of semileptonic tensor operators left(overline{q}{sigma}^{mu nu }qright)left(overline{ell}{sigma}_{mu nu}ell right) to the purely leptonic process μ → eγ and to the electric and magnetic dipole moments of charged leptons by matching onto chiral perturbation theory at low energies. This matching procedure has been used extensively to study semileptonic and leptonic weak decays of hadrons. In this paper, we apply it to observables that contain no strongly interacting external particles. The non-perturbative contribution to μ → e processes is used to extract the best current bound on lepton-flavor-violating semileptonic tensor operators, ΛBSM ≳ 450 TeV. We briefly discuss how the same method applies to dark-matter interactions.

Electric dipole moments in the minimal scotogenic model

In this work we consider a minimal version of the scotogenic model capable of accounting for an electron electric dipole moment within experimental sensitivity reach in addition to providing a dark matter candidate and radiatively generating neutrino masses. The Standard Model is minimally extended by two sterile fermions and one inert scalar doublet, both having odd parity, while the Standard Model particles have an even parity, imposed by a ℤ2 symmetry. The neutrino Yukawa couplings provide additional sources of CP violation, and thus a possible impact on electric dipole moments of charged leptons. This model provides two possible dark matter candidates (one bosonic and one fermionic) and our results show that, independently of the ordering of the generated light neutrino spectrum, one can have sizeable electron electric dipole moment within ACME sensitivity reach in the case of fermionic dark matter candidate.

Electric dipole moments of charged leptons at one loop in the presence of massive neutrinos

Violation of $CP$ invariance is a quite relevant phenomenon that is found in the Standard Model (SM), though in small amounts. This has been an incentive to look for high-energy descriptions in which $CP$ violation is increased, thus enhancing effects that are suppressed in the SM, such as the electric dipole moments (EDMs) of elementary particles. In the present investigation, we point out that charged currents in which axial couplings are different from vector couplings are able to produce one-loop contributions to EDMs of charged leptons if neutrinos are massive and if these currents violate $CP$. We develop our discussion around charged currents involving heavy neutrinos and a $W'$ gauge boson coupling to SM charged leptons. Using the most stringent bound on the electron EDM, provided by the ACME Collaboration, we determine that the upper bound on the difference between axial and vector currents lies within $\sim10^{-10}$ and $\sim10^{-7}$ for heavy-neutrino masses between $0.5\,{\rm TeV}$ and $6\,{\rm TeV}$ and if the $W'$ mass is within $0.45\,{\rm TeV}-7\,{\rm TeV}$. This possibility is analyzed altogether with the anomalous magnetic moments of charged leptons, among which we estimate, for the $\tau$ lepton, an anomalous magnetic moment contribution between $\sim10^{-8}$ and $\sim10^{-10}$ for neutrino masses ranging from $0.5\,{\rm TeV}$ to $6\,{\rm TeV}$ and a $W'$ mass between $0.45\,{\rm TeV}$ and $7\,{\rm TeV}$. The general charged currents are also used to calculate the branching ratio for $\mu\to e\gamma$, which gets suppressed if the set of masses of heavy neutrinos is quasidegenerate. In a scenario of nondegenerate neutrino masses, we find that regions of neutrino and $W'$ masses in which the contributions to this flavor changing branching ratio are lower than the current upper bound exist. We show that such regions can be widened if the $W'$ gauge boson mass is larger.

New scalar contributions to the weak dipole moments of charged leptons

We calculate the contributions of new scalar particles to the weak anomalous dipoles moments in alternative electroweak gauge theories.

Electron electric dipole moment in Inverse Seesaw models

We consider the contribution of sterile neutrinos to the electric dipole moment of charged leptons in the most minimal realisation of the Inverse Seesaw mechanism, in which the Standard Model is extended by two right-handed neutrinos and two sterile fermion states. Our study shows that the two pairs of (heavy) pseudo-Dirac mass eigenstates can give significant contributions to the electron electric dipole moment, lying close to future experimental sensitivity if their masses are above the electroweak scale. The major contribution comes from two-loop diagrams with pseudo-Dirac neutrino states running in the loops. In our analysis we further discuss the possibility of having a successful leptogenesis in this framework, compatible with a large electron electric dipole moment.

Dipole moments of charged leptons in the THDM-III with textures

We study the magnetic and weak magnetic dipole moments (WEDMs) of charged leptons in the framework of the two Higgs doublet model type III (THDM-III) with four zero textures. We first analyze the possibility that the discrepancy between the experimental measurements and the theoretical prediction of the muon anomalous magnetic dipole moment () in the standard model can be explained in the context of the model and then find values for the parameters of the THDM-III: neutral and charged Higgs boson masses (mH, ), mixing angles (), CP-violating angles and an additional parameter arising from the mass matrix (), for which a correspondence between theory and experiment is possible. We use and current low energy processes mixing, , , , , , , , and the rare higgs decay in order to determine the allowed parameter space of THDM-III and from these we give a prediction for the magnetic and WEDM of charged leptons, in particular of the τ lepton. The obtained magnetic dipole moment of the τ lepton is of the order of (10−8–10−7) and the weak magnetic is of the order (10−10–10−7).

Electric dipole moments of charged leptons with sterile fermions

We address the impact of sterile fermions on charged lepton electric dipole moments. Any experimental signal of these observables calls for scenarios of physics beyond the Standard Model providing new sources of CP violation. In this work, we consider a minimal extension of the Standard Model via the addition of sterile fermions which mix with active neutrinos and we derive the corresponding analytical expressions for the electric dipole moments of charged leptons at two-loop order. Our study reveals that, in order to have a non-vanishing contribution in this framework, the minimal extension necessitates the addition of at least 2 sterile fermion states to the Standard Model field content. Our conclusion is that sterile neutrinos can give significant contributions to the charged lepton electric dipole moments, some of them lying within present and future experimental sensitivity if the masses of the non-degenerate sterile states are both above the electroweak scale. The Majorana nature of neutrinos is also important in order to allow for significative contributions to the charged lepton electric dipole moments. In our analysis we impose all available experimental and observational constraints on sterile neutrinos and we further discuss the prospect of probing this scenario at low and high energy experiments.

Lepton flavor violation in the Standard Model with general dimension-six operators

We study lepton flavor observables in the Standard Model (SM) extended with all dimension-$6$ operators which are invariant under the SM gauge group. We calculate the complete one-loop predictions to the radiative lepton decays $\mu\to e\gamma$, $\tau \to \mu \gamma$ and $\tau\to e\gamma$ as well as to the closely related anomalous magnetic moments and electric dipole moments of charged leptons, taking into account all dimension-$6$ operators which can generate lepton flavor violation. Also the 3-body flavor violating charged lepton decays $\tau^\pm \to \mu^\pm \mu^+ \mu^-$, $\tau^\pm\to e^\pm e^+ e^- $, $\tau^\pm \to e^\pm \mu^+ \mu^- $, $\tau^\pm \to \mu^\pm e^+ e^- $, $\tau^\pm \to e^\mp \mu^\pm \mu^\pm$, $\tau^\pm \to \mu^\mp e^\pm e^\pm $ and $\mu^\pm \to e^\pm e^+ e^-$ and the $Z^0$ decays $Z^0\to\ell_i^+\ell_j^-$ are considered, taking into account all tree-level contributions.

Lepton dipole moments in supersymmetric low-scale seesaw models

We study the anomalous magnetic and electric dipole moments of charged leptons in supersymmetric low-scale seesaw models with right-handed neutrino superfields. We consider a minimally extended framework of minimal supergravity, by assuming that CP violation originates from complex soft SUSY-breaking bilinear and trilinear couplings associated with the right-handed sneutrino sector. We present numerical estimates of the muon anomalous magnetic moment and the electron electric dipole moment (EDM), as functions of key model parameters, such as the Majorana mass scale mN and tan(\beta). In particular, we find that the contributions of the singlet heavy neutrinos and sneutrinos to the electron EDM are naturally small in this model, of order 10^{-27} - 10^{-28} e cm, and can be probed in the present and future experiments.

CHARGED LEPTON ELECTRIC DIPOLE MOMENT ENHANCEMENT IN THE LORENTZ VIOLATED EXTENSION OF THE STANDARD MODEL

We consider the Lorentz violated extension of the standard model. In this framework, there are terms that explicitly violate CP-symmetry. We examine the CPT-even dμν-term to find the electric dipole moment of charged leptons. We show that the form factors besides the momentum transfer, depend on a new Lorentz-scalar, constructing by dμν and the four momenta of the lepton, as well. Such an energy dependence of the electric dipole form factor leads to an enhancement of the lepton electric dipole moment at high energy, even at the zero momentum transfer. We show that at [Formula: see text] the electric dipole moment of the charged lepton can be as large as 10-14 e cm .

Connecting Dirac and Majorana neutrino mass matrices in the minimal left-right symmetric model.

Probing the origin of neutrino mass by disentangling the seesaw mechanism is one of the central issues of particle physics. We address it in the minimal left-right symmetric model and show how the knowledge of light and heavy neutrino masses and mixings suffices to determine their Dirac Yukawa couplings. This in turn allows one to make predictions for a number of high and low energy phenomena, such as decays of heavy neutrinos, neutrinoless double beta decay, electric dipole moments of charged leptons, and neutrino transition moments. We also discuss a way of reconstructing the neutrino Dirac Yukawa couplings at colliders such as the LHC.

Electric dipole moments of charged leptons from their Majorana-type Yukawa couplings

The electric dipole moments (EDMs) of charged leptons are significantly suppressed in Standard Model. It has been found previously that they are even more severely suppressed in seesaw type models by powers of tiny neutrino masses as far as a leptonic CP source is concerned. We investigate whether a Majorana-type Yukawa coupling between charged leptons and a doubly charged scalar can contribute significantly to their EDMs. An observable EDM would then help unravel the Majorana nature of neutrinos by a lepton number conserving quantity. We find that the EDMs are indeed parametrically large, of the form dα∝emα(mβ2−mγ2)/m4 up to logarithms, where mα and m are respectively the masses of charged leptons and the scalar. And they satisfy a sum rule to good precision, de/me+dμ/mμ+dτ/mτ=0. With the most stringent constraints from lepton flavor violating transitions taken into account, their values are still much larger than the mentioned previous results. Unfortunately, even in the most optimistic case the electron EDM is about three orders of magnitude below the foreseeable experimental sensitivity.

Exploring neutrino models at high energy scale from various processes

We investigate the lepton flavor violation and electric dipole moments of charged leptons in the minimal supersymmetric standard model with right-handed neutrinos. We consider two types of Yukawa structures and discuss the difference in the measurements of lepton flavor violation such as l i → l j γ between these two different models of neutrinos.

Electric dipole moments of charged leptons in the split fermion scenario in the two Higgs doublet model

We predict the charged lepton electric dipole moments in the split fermion scenario in the framework of the two Higgs doublet model. We observe that the numerical value of the muon (tau) electric dipole moment is at the order of the magnitude of $10^{-22} (e-cm)$ ($10^{-20} (e-cm)$) and there is an enhancement in the case of two extra dimensions, especially for the tau lepton electric dipole moment.

Lepton dipole moments in extended technicolor models

We analyze the diagonal and transition magnetic and electric dipole moments of charged leptons in extended technicolor (ETC) models, taking account of the multiscale nature of the ETC gauge symmetry breaking, conformal (walking) behavior of the technicolor theory, and mixing in the charged-lepton mass matrix. We show that mixing effects dominate the ETC contributions to charged lepton electric dipole moments and that these can yield a value of |de| comparable to the current limit. The rate for μ→eγ can also be close to its limit. From these and other processes we derive constraints on the charged lepton mixing angles. The constraints are such that the ETC contribution to the muon anomalous magnetic moment, which includes a significant lepton mixing term, can approach, but does not exceed, the current sensitivity level.

Probing physics beyond the standard model from lepton sector

In this review we discuss physics of the lepton sector, the anomalous dipole moment of muon, the charged lepton-flavor violation, and the electric dipole moments of charged leptons, from viewpoints of the minimal supersymmetric standard model and the extensions.