Probing electroweak top quark couplings at hadron colliders

We consider QCD $t\overline{t}\gamma$ and $t\overline{t}Z$ production at hadron colliders as a tool to measure the $tt\gamma$ and $ttZ$ couplings. At the Tevatron it may be possible to perform a first, albeit not very precise, test of the $tt\gamma$ vector and axial vector couplings in $t\overline{t}\gamma$ production, provided that more than $5{\mathrm{f}\mathrm{b}}^{-1}$ of integrated luminosity are accumulated. The $t\overline{t}Z$ 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 $tt\gamma$ 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 $t\overline{t}V$ ($V=\gamma ,Z$) production has the added advantage that the $tt\gamma$ and $ttZ$ couplings are not entangled. For an integrated luminosity of $300{\mathrm{f}\mathrm{b}}^{-1}$, 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 ($3{\mathrm{a}\mathrm{b}}^{-1}$) LHC.