Light Mediators in Anomaly Free $U(1{)}_X$ Models I - Theoretical Framework

Abstract

We examine theoretical features of $U(1{)}_X$ extensions of the Standard Model whose quantum anomalies are canceled per generation. Similarly to other versions, the theory consists of a Two-Higgs-Doublet Model plus a scalar singlet embedded into the $SM\otimes U(1{)}_X$ gauge group, and introduces small modifications to the $Z$-boson interactions. These changes can be minimized by exclusively charging right-handed fermions under the new Abelian symmetry, and are compensated by the neutral $X$-boson exchange. Non-universality of fermion couplings can also be achieved by requiring one single $X$-charged family. In general, $X$ gauge bosons can be separated into $A^{\prime }$ (dark photons) and $Z^{\prime }$ subsets, distinguished by the presence of axial-vector currents. $A^{\prime }$ physics is commonly simpler to constrain and therefore favored by experimental tests. Finally, the model can be UV completed both by stable $\chi$ fermions or by right-handed neutrinos. The prior case may provide cold WIMPs in the theory.

Comments: 29 pages, 3 figures