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Register a new userSolving the electron and muon $g-2$ anomalies in $Z$ models
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We consider simultaneous explanations of the electron and muon $g-2$ anomalies through a single $Z$ of a $U(1)$ extension to the Standard Model (SM). We first perform a model-independent analysis of the viable flavour-dependent $Z$ couplings to leptons, which are subject to various strict experimental constraints. We show that only a narrow region of parameter space with an MeV-scale $Z$ can account for the two anomalies. Following the conclusions of this analysis, we then explore the ability of different classes of $Z$ models to realise these couplings, including the SM$+U(1)$, the $N$-Higgs Doublet Model$+U(1)$, and a Froggatt-Nielsen style scenario. In each case, the necessary combination of couplings cannot be obtained, owing to additional relations between the $Z$ couplings to charged leptons and neutrinos induced by the gauge structure, and to the stringency of neutrino scattering bounds. Hence, we conclude that no $U(1)$ extension can resolve both anomalies unless other new fields are also introduced. While most of our study assumes the Caesium $(g-2)_e$ measurement, our findings in fact also hold in the case of the Rubidium measurement, despite the tension between the two.
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In general two Higgs doublet models (2HDMs) without scalar flavour changing neutral couplings (SFCNC) in the lepton sector, the electron, muon and tau interactions can be decoupled in a robust framework, stable under renormalization group evolution. In this framework, the breaking of lepton flavour universality (LFU) goes beyond the mass proportionality, opening the possibility to accommodate a different behaviour among charged leptons. We analyze the electron and muon $(g-2)$ anomalies in the context of these general flavour conserving models in the leptonic sector (g$ell$FC). We consider two different models, I-g$ell$FC and II-g$ell$FC, in which the quark Yukawa couplings coincide, respectively, with the ones in type I and in type II 2HDMs. We find two types of solutions that fully reproduce both $(g-2)$ anomalies, and which are compatible with experimental constraints from LEP and LHC, from LFU, from flavour and electroweak physics, and with theoretical constraints in the scalar sector. In the first type of solution, all the new scalars have masses in the 1--2.5 TeV range, the vacuum expectation values (vevs) of both doublets are quite similar in magnitude, and both anomalies are dominated by two loop Barr-Zee contributions. This solution appears in both models. In a second type of solution, one loop contributions are dominant in the muon anomaly, all new scalars have masses below 1 TeV, and the ratio of vevs is in the range 10--100. The second neutral scalar $H$ is the lighter among the new scalars, with a mass in the 210--390 GeV range while the pseudoscalar $A$ is the heavier, with a mass in the range 400--900 GeV. The new charged scalar $H^pm$ is almost degenerate either with the scalar or with the pseudoscalar. This second type of solution only appears in the I-g$ell$FC model. Both solutions require the soft breaking of the $mathbb{Z}_{2}$ symmetry of the Higgs potential.
According to the FNAL+BNL measurements for the muon $g-2$ and the Berkeley $^{133}$Cs measurement for the electron $g-2$, the SM prediction for the muon (electron) $g-2$ is $4.2sigma$ ($2.4sigma$) below (above) the experimental value. A joint explanation requires a positive contribution to the muon $g-2$ and a negative contribution to the electron $g-2$. In this work we explore the possibility of such a joint explanation in the minimal supersymmetric standard model (MSSM). Assuming no universality between smuon and selectron soft masses, we find out a part of parameter space for a joint explanation of muon and electron $g-2$ anomalies at $2sigma$ level. This part of parameter space can survive the LHC and LEP constraints, but gives an over-abundance for the dark matter if the bino-like lightest neutralino is assumed to be the dark matter candidate. With the assumption that the dark matter candidate is a superWIMP (say a pseudo-goldstino in multi-sector SUSY breaking scenarios, whose mass can be as light as GeV and produced from the late-dacay of the thermally freeze-out lightest neutralino), the dark matter problem can be avoided. So, the MSSM may give a joint explanation for the muon and electron $g-2$ anomalies at $2sigma$ level (the muon $g-2$ anomaly can be ameliorated to $1sigma$).
We propose simple models with a flavor-dependent global $U(1)_ell$ and a discrete $mathbb{Z}_2$ symmetries to explain the anomalies in the measured anomalous magnetic dipole moments of muon and electron, $(g-2)_{mu,e}$, while simultaneously accommodating a dark matter candidate. These new symmetries are introduced not only to avoid the dangerous lepton flavor-violating decays of charged leptons, but also to ensure the stability of the dark matter. Our models can realize the opposite-sign contributions to the muon and electron $g-2$ via one-loop diagrams involving new vector-like leptons. Under the vacuum stability and perturbative unitarity bounds as well as the constraints from the dark matter direct searches and related LHC data, we find suitable parameter space to simultaneously explain $(g-2)_{mu,e}$ and the relic density. In this parameter space, the coupling of the Higgs boson with muons can be enhanced by up to $sim 38%$ from its Standard Model value, which can be tested in future collider experiments.
The stringent experimental bound on $mu rightarrow e gamma$ is compatible with a simultaneous and sizable new physics contribution to the electron and muon anomalous magnetic moments $(g-2)_ell$ ($ell=e,,mu$), only if we assume a non-trivial flavor structure of the dipole operator coefficients. We propose a mechanism in which the realization of the $(g-2)_ell$ correction is manifestly related to the mass generation through a flavor symmetry. A radiative flavon correction to the fermion mass gives a contribution to the anomalous magnetic moment. In this framework, we introduce a chiral enhancement from a non-trivial $mathcal{O}(1)$ quartic coupling of the scalar potential. We show that the muon and electron anomalies can be simultaneously explained in a vast region of the parameter space with predicted vector-like mediators of masses as large as $M_chiin [0.6,2.5]$~TeV.
Two of the most widely studied extensions of the Standard Model (SM) are $a)$ the addition of a new $U(1)$ symmetry to its existing gauge groups, and $b)$ the expansion of its scalar sector to incorporate a second Higgs doublet. We show that when combined, they allow us to understand the electron-like event excess seen in the MiniBooNE (MB) experiment as well as account for the observed anomalous values of the muon magnetic moment. A light $Z$ associated with an additional $U(1)$ coupled to baryons and to the dark sector, with flavor non-universal couplings to leptons, in conjunction with a second Higgs doublet is capable of explaining the MB excess. The $Z$ obtains its mass from a dark singlet scalar, which mixes with the two Higgs doublets. Choosing benchmark parameter values, we show that $U(1)_{B-3L_tau}$, which is anomaly-free, and $U(1)_B$, both provide (phenomenologically) equally good solutions to the excess. We also point out the other (anomaly-free) $U(1)$ choices that may be possible upon fuller exploration of the parameter space. We obtain very good matches to the energy and angular distributions for neutrinos and anti-neutrinos in MB. The extended Higgs sector has two light CP-even scalars, $h$ and $H$, and their masses and couplings are such that in principle, both contribute to help explain the MB excess as well as the present observed values of the muon and electron $g-2$. We discuss the constraints on our model as well as future tests. Our work underlines the role that light scalars may play in understanding present-day low-energy anomalies. It also points to the possible existence of portals to the dark sector, i.e., a light gauge boson field $(Z)$ and a dark neutrino which mixes with the active neutrinos, as well as a dark sector light scalar which mixes with the extended Higgs sector.
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