Light mediators in anomaly free U (1)X models. Part II. Constraints on dark gauge bosons

We consider experimental constraints in the MeV region in order to determine the parameter space for the U(1)X extension of the Standard Model, presented in the first part of our work. In particular, we focus on the model UV-completed by cold WIMPs. We conclude that the electron anomalous magnetic moment and the neutrino trident production provide the most stringent bounds to gX2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {g}_X^2 $$\end{document}∼ 10−6 in the mass interval below the di-muon threshold. By allowing the axial-vector coupling of the dark gauge boson Z′, the interference effect with the SM gauge bosons may reduce the bounds coming from the neutrino trident production. At the same time, such coupling allows a region of the parameter space already favored both by the relic abundance and by the discrepancy between experimental result and theoretical prediction for the muon anomalous magnetic moment. We emphasize that light-Z′ interactions, non-universal for the two first lepton families, necessarily create a difference in the proton charge radius measured in the Lamb shift of the e-hydrogen and μ-hydrogen. Finally, we determine the effects of the new gauge boson on the forward-backward asymmetry in e+e−→f¯f\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{f}f $$\end{document} = μ, τ, and on the leptonic decays M → jνjl+l−, where M = π, K, D, Ds, B and j, l = e, μ.