Simultaneous analysis of Z' and new fermion effects: Global constraints in E6 and SO(10) models.

Abstract

In grand unified theories based on extended (rank >4) gauge groups, the new gauge interactions require new fermions to ensure anomaly cancellation. We analyze the two kinds of new physics effects that are naturally present in these models: (i) the effects of the new neutral gauge bosons; (ii) the effects of a mixing of the known fermions with the new ones. Concentrating in particular on ${\mathrm{E}}_{6}$ and SO(10) models, we perform a global analysis of the electroweak data to constrain simultaneously these two new physics effects, and we pay particular attention to their reciprocal interplay. Our set of experimental results includes the data at the CERN ${e}^{+}{e}^{\ensuremath{-}}$ collider LEP on the $Z$ decay widths and fermion asymmetries, low-energy neutral-current experiments (atomic parity violation, $\ensuremath{\nu}$ scattering), the $W$-boson mass ${M}_{W}$, as well as charged-current measurements such as the various tests of the universality of the $W$-lepton couplings and the constraints on unitarity of the Cabibbo-Kobayashi-Maskawa matrix. We derive stringent bounds on the ${Z}_{0}\ensuremath{-}{Z}_{1}$ mixing ($|\ensuremath{\varphi}|\ensuremath{\lesssim}0.02$), on the fermion mixing parameters (${sin}^{2}{\ensuremath{\xi}}_{i}\ensuremath{\lesssim}0.01$ in most cases), and on the mass of the new gauge boson (${M}_{{Z}^{\ensuremath{'}}}>170\ensuremath{-}350$ GeV, depending on the model). In many observables the different sources of new physics induce comparable effects that can compensate each other. We confront the results derived by considering only one effect at a time with the results of a joint analysis, and we point out which of the existing bounds are relaxed and which ones remain unaffected.