Multi-parameter fits to the t-tbar threshold observables at a future e+e- linear collider

TL;DR

This paper investigates the precision with which a future e+e− linear collider can extract the top-quark mass, strong coupling αs(MZ), top width, and top Yukawa coupling from a t tbar threshold scan. It extends previous two-parameter analyses by employing a full four-parameter fit to three observables (cross section, top momentum peak, and forward-backward asymmetry), using a fast interpolation-based approach to manage computational demands. With 300 fb−1 and realistic detector/beam conditions, it finds m_t can be measured at ~16–19 MeV, αs at ~0.0012, and Γ_t at ~32 MeV (about 2%), while λ_t remains difficult to constrain, typically at the 30–60% level depending on systematics and parameter constraints. The study demonstrates substantial parameter correlations, justifying a comprehensive multi-parameter strategy, and confirms that theoretical cross-section uncertainties have limited immediate impact, though more sophisticated theory error models could further affect the results.

Abstract

A realistic study of the physics reach of a t-tbar threshold scan at a future e+e- linear collider is presented. The results obtained take into account experimental and, to a large extent, theoretical systematic errors, as well as beam effects. Because of the large correlations between the physical parameters that can be extracted from the threshold scan, a multi-parameter fit is seen as mandatory. It is shown that the top mass, the top width and alpha_s(M_Z) can be extracted simultaneously with uncertainties around 20 MeV, 30 MeV and 0.0012, respectively, while the top Yukawa coupling can be measured, with the previous three parameters, to an uncertainty of about 35%, after assuming an external prior on alpha_s of +/-0.001.

Figures (7)

• Figure 1: The expected scan results for the cross section, the peak of the top momentum distribution and the forward-backward charge asymmetry in the conditions described in the text.
• Figure 2: Sensitivity of the observables to the top mass. The different markers correspond to $\Delta m_t = 200$ MeV intervals.
• Figure 3: Sensitivity of the observables to the strong coupling constant. The different markers correspond to $\Delta \alpha_s = 0.004$ intervals.
• Figure 4: $\Delta\chi^2=1$ contour as a function of $m_t(1S)$ and $\alpha_s(M_Z)$.
• Figure 5: Sensitivity of the observables to the top width. The different markers correspond to $\Delta \Gamma_t = 400$ MeV intervals.