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

Abstract

Violation of CP invariance is a quite relevant phenomenon that is found in the Standard Model, 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 Standard Model, such as the electric dipole moments 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 electric dipole moments 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 Standard Model charged leptons. Using the most stringent bound on the electron electric dipole moment, provided by the ACME Collaboration, we determine that the upper bound on the difference between axial and vector currents lies within ∼ 10 −10 and ∼ 10 −7 for heavy-neutrino masses between 0.5 TeV and 6 TeV and if the W ′ mass is within 0.45 TeV − 7 TeV. This possibility is analyzed altogether with the anomalous magnetic moments of charged leptons, among which we estimate, for the τ lepton, an anomalous magnetic moment contribution between ∼ 10 −8 and ∼ 10 −10 for neutrino masses ranging from 0.5 TeV to 6 TeV and a W ′ mass between 0.45 TeV and 7 TeV. The general charged currents are also used to calculate the branching ratio for µ → eγ, 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 estimate of the chromomagnetic dipole moment of quarks in the standard model

“…The anomalous magnetic dipole moment (MDM) of fermions has been a fertile field of study, giving rise to a plethora of theoretical work within the context of the standard model (SM) [1,2,3,4], as well as beyond the SM (BSM) theories [5]. Furthermore, the fermion electric dipole moment (EDM) has also been analyzed in several models [6,7,8,9,10,11]. More recently, the calculation of radiative corrections to the gluon-quark-quark qqg vertex has also become a topic of considerable interest.…”

On the role of leptonic CPV phases in cLFV observables

“…In addition to the masses of the new states and their mixings to the active neutrinos, constructions relying on heavy sterile states also open the door to new sources of CP violation: other than new Dirac CP violating (CPV) phases, should the massive states be of Majorana nature, further phases can be present. The role of these new CPV phases has been explored in analyses dedicated to CP violating observables, as is the case of electric dipole moments (EDM) of charged leptons [45][46][47][48]. New CPV phases have been also recently shown to play a crucial role in what concerns interference effects in LNV (and cLFV) semileptonic meson and tau decays [49], when more than one HNL is involved.…”

About heavy neutrinos: lepton-flavor violation in decays of charged leptons

“…It turns out that the sum of the subdiagrams l α l β γ * (γ * denotes a virtual photon) that are part of the reducible diagrams of Fig. 1 (with light neutrinos and the Standard-Model W boson replacing the corresponding heavy particles in this figure) is finite if they are calculated on shell [69], γ * → γ. But if the photon is off shell instead this sum is ultraviolet divergent.…”

“…This neutrino-mass spectrum was considered in Ref. [69] to investigate one-loop contributions to electric dipole moments and anomalous magnetic moments of Standard Model leptons, and the lepton decay µ → eγ as well. We consider, for practical purposes, a mass m N , such that m N ≈ m 2 and m N ≈ m 3 , to characterize the pair of quasi-degenerate neutrino masses.…”

“…The contributions to µ → eγ from charged currents involving heavy neutrinos, heavy charged gauge bosons and Standard-Model charged leptons, in a model-independent context, were recently calculated, analyzed and discussed in Ref. [69]. The authors of Ref.…”