Probing Electroweak Top Quark Couplings at Hadron and Lepton Colliders

Although the top quark was discovered almost ten years ago [1,2], many of its properties are still only poorly known [3]. In particular, the couplings of the top quark to the electroweak (EW) gauge bosons have not yet been directly measured. Current data provide only weak constraints on the couplings of the top quark with the EW gauge bosons, except for the ttZ vector and axial vector couplings which are rather tightly but indirectly constrained by LEP data; and the right-handed tbW coupling, which is severely bound by the observed b → sγ rate [4]. At an ee linear collider with √ s = 500 GeV and an integrated luminosity of 100 − 200 fb one can hope to measure the ttV (V = γ, Z) couplings in top pair production with a few-percent precision [5]. However, the process ee → γ/Z → tt̄ is sensitive to both ttγ and ttZ couplings and significant cancellations between the various couplings can occur. At hadron colliders, tt̄ production is so dominated by the QCD processes qq̄ → g → tt̄ and gg → tt̄ that a measurement of the ttγ and ttZ couplings via qq̄ → γ/Z → tt̄ is hopeless. Instead, the ttV couplings can be measured in QCD tt̄γ production, radiative top quark decays in tt̄ events (tt̄ → γWWbb̄), and QCD tt̄Z production [6]. tt̄γ production and radiative top quark decays are sensitive only to the ttγ couplings, whereas tt̄Z production gives information only on the structure of the ttZ vertex. This obviates having to disentangle potential cancellations between the different couplings. Here we briefly discuss the measurement of the ttV couplings at the LHC and compare the expected sensitivities with the bounds which one hopes to achieve at an ee linear collider.