Diagnostic power of future colliders for Z' couplings to quarks and leptons: e+e- versus pp colliders.

Abstract

We study the diagnostic power of future $e^+e^-$ colliders with ${\sqrt {s}} = 500$ GeV (the New Large Collider ($NLC$)) for a model independent determination of the $Z'$ gauge couplings to quarks and leptons. The interference of the $Z'$ propagator with the photon and the $Z$ propagator in the two-fermion final state probes are sensitive to the magnitude as well as relative signs of quark and lepton charges. For $Z'$ with $M_{Z'} \sim 1 $ TeV {\it all } the quark and lepton charges can be determined to around $10-20 \%$, provided heavy flavor tagging and longitudinal polarization of the electron beam is available. The errors are $2-10$ times larger without polarization, and very little information can be obtained about quark charges without heavy flavor tagging. We point out the complementarity of future hadron colliders. At the CERN Large Hadron Collider ($LHC$) primarily the magnitude of three out of four corresponding couplings can be measured; however, their error-bars are typically by a factor of $\sim 2$ smaller than those at the $NLC$.