Abstract
Top quark production in the process $e^+e^- \to t\bar{t}$ at a future linear electron positron collider with polarised beams is a powerful tool to determine indirectly the scale of new physics. The presented study, based on a detailed simulation of the ILD detector concept, assumes a centre-of-mass energy of $\sqrt{s}=500\,$GeV and a luminosity of $\mathcal{L}=500\,{\rm fb}^{-1}$ equally shared between the incoming beam polarisations of $\mathcal{P}_{e^-}, \mathcal{P}_{e^+} =\pm0.8,\mp0.3$. Events are selected in which the top pair decays semi-leptonically and the cross sections and the forward-backward asymmetries are determined. Based on these results, the vector, axial vector and tensorial $CP$ conserving couplings are extracted separately for the photon and the $Z^0$ component. With the expected precision, a large number of models in which the top quark acts as a messenger to new physics can be distinguished with many standard deviations. This will dramatically improve expectations from e.g. the LHC for electro-weak couplings of the top quark.
Highlights
In contrast to the situation at hadron colliders, the leadingorder pair production process e+e− → ttgoes directly through the ttZ 0 and ttγ vertices
1.3 tight selection reduces the efficiency in case of lefthanded initial electron beams from 55 to 28 %. With this the forward backward asymmetry can be determined to a statistical precision of better than 2 %
It should be pointed out that the considerable precision expected for δ F1ZA benefits strongly from LEP/SLC bounds on the oblique parameters that e.g. render it unlikely that F1ZA flips sign due to New Physics
Summary
In contrast to the situation at hadron colliders, the leadingorder pair production process e+e− → ttgoes directly through the ttZ 0 and ttγ vertices. The corrections of the vector currents lead to the anomalous electro-magnetic and weak-magnetic moments represented by F2XV that correct the gyromagnetic ratio gt of the t quark. Typical values for these corrections are in the range O(10−3− 10−2) [6]. Corrections to the axial-vector current result in the Form Factors F2XA that are related to the dipole moment dtX = (e/2mt )F2XA(0) that in turn violates the combined Charge and Parity symmetry C P. Special emphasis will be put on the selection efficiency and the polar angle of the final state t quarks Both experimental quantities are suited to monitor carefully experimental systematics that may occur in the extraction of form factors and couplings. The results presented in the following are based on the studies described in detail in Refs. [7,8]
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