Probing electroweak top quark couplings at hadron colliders

We consider QCD tt̄γ and tt̄Z production at hadron colliders as a tool to measure the ttγ and $ttZ$ couplings. At the Tevatron it may be possible to perform a first, albeit not very precise, test of the ttγ vector and axial vector couplings in tt̄γ production, provided that more than 5 fb^-1 of integrated luminosity are accumulated. The tt̄Z cross section at the Tevatron is too small to be observable. At the CERN Large Hadron Collider (LHC) it will be possible to probe the ttγ couplings at the few-percent level, which approaches the precision which one hopes to achieve with a next-generation e⁺e^- linear collider. The LHC's capability of associated QCD tt̄V (V=γ,Z) production has the added advantage that the ttγ and $ttZ$ couplings are not entangled. For an integrated luminosity of 300 fb^-1, the $ttZ$ vector (axial vector) coupling can be determined with an uncertainty of 45-85% (15-20%), whereas the dimension-five dipole form factors can be measured with a precision of 50-55%. The achievable limits improve typically by a factor of 2-3 for the luminosity-upgraded (3 ab^-1) LHC.