Probing electroweak top quark couplings at hadron colliders

Abstract

We consider QCD $t\overline{t}\ensuremath{\gamma}$ and $t\overline{t}Z$ production at hadron colliders as a tool to measure the $tt\ensuremath{\gamma}$ and $ttZ$ couplings. At the Tevatron it may be possible to perform a first, albeit not very precise, test of the $tt\ensuremath{\gamma}$ vector and axial vector couplings in $t\overline{t}\ensuremath{\gamma}$ production, provided that more than $5\text{ }\text{ }{\mathrm{f}\mathrm{b}}^{\ensuremath{-}1}$ 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 $tt\ensuremath{\gamma}$ couplings at the few-percent level, which approaches the precision which one hopes to achieve with a next-generation ${e}^{+}{e}^{\ensuremath{-}}$ linear collider. The LHC's capability of associated QCD $t\overline{t}V$ ($V=\ensuremath{\gamma},Z$) production has the added advantage that the $tt\ensuremath{\gamma}$ and $ttZ$ couplings are not entangled. For an integrated luminosity of $300\text{ }\text{ }{\mathrm{f}\mathrm{b}}^{\ensuremath{-}1}$, the $ttZ$ vector (axial vector) coupling can be determined with an uncertainty of $45\ensuremath{-}85%$ ($15\ensuremath{-}20%$), whereas the dimension-five dipole form factors can be measured with a precision of $50\ensuremath{-}55%$. The achievable limits improve typically by a factor of $2\ensuremath{-}3$ for the luminosity-upgraded ($3\text{ }\text{ }{\mathrm{a}\mathrm{b}}^{\ensuremath{-}1}$) LHC.