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Register a new userCharged lepton flavor violation in light of Muon $g-2$
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The recent confirmation of the muon $g-2$ anomaly by the Fermilab g-2 experiment may harbinger a new era in $mu$ and $tau$ physics. As is known, the effect can arise from one-loop exchange of sub-TeV exotic scalar and pseudoscalars, namely $H$ and $A$, that have flavor changing neutral couplings $rho_{taumu}$ and $rho_{mutau}$ at $sim 20$ times the usual tau Weinberg coupling, $lambda_tau$. A similar diagram induces $mu to egamma$, where $rho_{etau}= rho_{tau e} = {cal O}(lambda_e)$ brings the rate right into the sensitivity of the MEG II experiment, and the $mu egamma$ dipole can be probed further by $mu to 3e$ and $mu N to eN$. With its promised sensitivity range and ability to use different nuclei, the $mu N to e N$ conversion experiments can not only make discovery, but access the extra diagonal quark Weinberg couplings $rho_{qq}$. For the $tau$ lepton, $tau to mugamma$ would probe $rho_{tautau}$ down to $lambda_tau$ or lower, while $tau to 3mu$ would probe $rho_{mumu}$ to ${cal O}(lambda_{mu})$.
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A systematic investigation on muon anomalous magnetic moment and related lepton flavor-violating process such as $mto eg$, $tto eg$ and $tto mg$ is made at two loop level in the models with flavor-changing scalar interactions. The two loop diagrams with double scalar exchanges are studied and their contributions are found to be compatible with the ones from Barr-Zee diagram. By comparing with the latest data, the allowed ranges for the relevant Yukawa couplings $Y_{ij}$ in lepton sector are obtained. The results show a hierarchical structure of $Y_{m e, t e} ll Y_{m t} simeq Y_{mm}$ in the physical basis if $Delta a_{mu}$ is found to be $>50times 10^{-11}$. It deviates from the widely used ansatz in which the off diagonal elements are proportional to the square root of the products of related fermion masses. An alternative Yukawa coupling matrix in the lepton sector is suggested to understand the current data. With such a reasonable Yukawa coupling ansatz, the decay rate of $tto mg$ is found to be near the current experiment upper bound.
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.
In the minimal Standard Model (SM) with four generations (the so called SM4) and in standard two Higgs doublets model (2HDM) setups, e.g., the type II 2HDM with four fermion generations, the contribution of the 4th family heavy leptons to the muon magnetic moment is suppressed and cannot accommodate the measured $ sim 3 sigma$ access with respect to the SM prediction. We show that in a 2HDM for the 4th generation (the 4G2HDM), which we view as a low energy effective theory for dynamical electroweak symmetry breaking, with one of the Higgs doublets coupling only to the 4th family leptons and quarks (thus effectively addressing their large masses), the loop exchanges of the heavy 4th generation neutrino can account for the measured value of the muon anomalous magnetic moment. We also discuss the sensitivity of the lepton flavor violating decays $mu to e gamma$ and $tau to mu gamma$ and of the decay $B_s to mu mu$ to the new couplings which control the muon g-2 in our model.
We present a comprehensive analysis of the potential sensitivity of the Electron-Ion Collider (EIC) to charged lepton flavor violation (CLFV) in the channel $epto tau X$, within the model-independent framework of the Standard Model Effective Field Theory (SMEFT). We compute the relevant cross sections to leading order in QCD and electroweak corrections and perform simulations of signal and SM background events in various $tau$ decay channels, suggesting simple cuts to enhance the associated estimated efficiencies. To assess the discovery potential of the EIC in $tau$-$e$ transitions, we study the sensitivity of other probes of this physics across a broad range of energy scales, from $pp to e tau X$ at the Large Hadron Collider to decays of $B$ mesons and $tau$ leptons, such as $tau to e gamma$, $tau to e ell^+ ell^-$, and crucially the hadronic modes $tau to e Y$ with $Y in { pi, K, pi pi, K pi, ...}$. We find that electroweak dipole and four-fermion semi-leptonic operators involving light quarks are already strongly constrained by $tau$ decays, while operators involving the $c$ and $b$ quarks present more promising discovery potential for the EIC. An analysis of three models of leptoquarks confirms the expectations based on the SMEFT results. We also identify future directions needed to maximize the reach of the EIC in CLFV searches: these include an optimization of the $tau$ tagger in hadronic channels, an exploration of background suppression through tagging $b$ and $c$ jets in the final state, and a global fit by turning on all SMEFT couplings, which will likely reveal new discovery windows for the EIC.
The recent experimental status, including the confirmation of the muon $g-2$ anomaly at Fermilab, indicates a Beyond Standard Model (BSM) satisfying the following properties: 1) it enhances the $g-2$ 2) suppresses flavor violations, such as $mu to e gamma$, 3) suppresses CP violations, such as the electron electric dipole moment (EDM). In this letter, I show that if the masses of heavy leptons are generated radiatively, the eigenbasis of the mass matrix and higher dimensional photon operators can be automatically aligned. As a result, the muon $g-2$ is enhanced but the EDM of the electron and $mu to e gamma$ rate are suppressed. Phenomenology and applications of the mechanism to the B-physics anomalies are argued.
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