The mixing of new vectorlike leptons with leptons in the standard model can generate flavor violating couplings of $h$, $W$ and $Z$ between heavy and light leptons. Focusing on the couplings of the muon, the partial decay width of $hto e_4^pm mu^mp$,
where $e_4$ is the new lepton, can be significant when this process is kinematically allowed. Subsequent decays $e_4^pm to Zmu^pm$ and $e_4^pm to W^pm u$ lead to the same final states as $h to ZZ^* to Z mu^+mu^-$ and $h to WW^* to W mu u$, thus possibly affecting measurements of these processes. We calculate $hto e_4 ell_i to Zell_iell_j$, where $ell_{i,j}$ are standard model leptons, including the possibility of off-shell decays, interference with $hto ZZ^* to Z ell_i ell_i$, and the mass effect of $ell_{i,j}$ which are important when the mass of $e_4$ is close to the mass of the Higgs boson. We derive constraints on masses and couplings of the heavy lepton from the measurement of $hto 4ell$. We focus on the couplings of the muon and discuss possible effects on $hto ZZ^*$ from the region of parameters that can explain the anomaly in the measurement of the muon g-2.
The deviation of the measured value of the muon anomalous magnetic moment from the standard model prediction can be completely explained by mixing of the muon with extra vectorlike leptons, L and E, near the electroweak scale. This mixing simultaneou
sly contributes to the muon mass. We show that the correlation between contributions to the muon mass and muon g-2 is controlled by the mass of the neutrino originating from the doublet L. Positive correlation, simultaneously explaining both measured values, requires this mass below 200 GeV. The decay rate of the Higgs boson to muon pairs is modified and, in the region of the parameter space that can explain the muon anomalous magnetic moment within one standard deviation, it ranges from 0.5 to 24 times the standard model prediction. In the same scenario, $h to gamma gamma$ can be enhanced or lowered by ~50% from the standard model prediction. The explanation of the muon g-2 anomaly and predictions for $h to gamma gamma$ are not correlated since these are controlled by independent parameters. This scenario can be embedded in a model with three complete vectorlike families featuring gauge coupling unification, sufficiently stable proton, and the Higgs quartic coupling remaining positive all the way to the grand unification scale.
We present a fit to precision electroweak data in the standard model extended by an additional vector boson, Z, with suppressed couplings to the electron compared to the Z boson, with couplings to the b-quark, and with mass close to the mass of the Z
boson. This scenario provides an excellent fit to forward-backward asymmetry of the b-quark measured on the Z-pole and pm 2 GeV off the Z-pole, and to lepton asymmetry, A_e, obtained from the measurement of left-right asymmetry for hadronic final states, and thus it removes the tension in the determination of the weak mixing angle from these two measurements. It also leads to a significant improvement in the total hadronic cross section on the Z-pole and R_b measured at energies above the Z-pole. We explore in detail properties of the Z needed to explain the data and present a model for Z with required couplings. The model preserves standard model Yukawa couplings, it is anomaly free and can be embedded into grand unified theories. It allows a choice of parameters that does not generate any flavor violating couplings of the Z to standard model fermions. Out of standard model couplings, it only negligibly modifies the left-handed bottom quark coupling to the Z boson and the 3rd column of the CKM matrix. Modifications of standard model couplings in the charged lepton sector are also negligible. It predicts an additional down type quark, D, with mass in a few hundred GeV range, and an extra lepton doublet, L, possibly much heavier than the D quark. We discuss signatures of the Z at the Large Hadron Collider and calculate the Zb production cross section which is the dominant production mechanism for the Z.
