Generalized top-spin analysis and new physics in $e^{+} e^{-}$ collisions with beam polarization

TL;DR

This paper develops a comprehensive analysis of top-quark spin observables in $e^+e^-$ collisions with polarized beams, exploring new physics couplings and proposing optimal spin bases for detecting deviations from the Standard Model.

Contribution

It introduces a generalized spin analysis framework for $e^+e^- o tar{t}$ with polarized beams, including new physics couplings and optimal spin bases for detection.

Findings

Beamline basis with transverse polarization is highly sensitive to new physics.

Helicity basis is most effective with longitudinal polarization.

Limits on new physics interactions are established at 90% confidence level.

Abstract

A generalized top-spin analysis proposed some time ago in the context of Standard Model and subsequently studied in varying contexts is now applied primarily to the case of $e^+e^-\rightarrow t\bar{t}$ with transversely polarized beams. This extends our recent work with new physics couplings of scalar ($S$) and tensor ($T$) types. We carry out a comprehensive analysis assuming only the electron beam to be transversely polarized, which is sufficient to probe these interactions, and also eliminates any azimuthal angular dependence due to standard model or new physics of vector ($V$) and axial-vector ($A$) type interactions. We then consider new physics of general four-Fermi type of $V$ and $A$ type with both beams transversely polarized and discuss implications with longitudinal polarization as well. The generalized spin bases are all investigated in the presence of either longitudinal or transverse beam polarization to look for appreciable deviation from the SM prediction in case of the new physics. 90% confidence level limits are obtained on the interactions for the generalized spin bases with realistic integrated luminosity. In order to achieve this we present a general discussion based on helicity amplitudes and derive a general transformation matrix that enables us to treat the spin basis. We find that beamline basis combined with transverse polarization provides an excellent window of opportunity both for $S$, $T$ and $V$, $A$ new physics, followed by the off diagonal basis. The helicity basis is shown to be the best in case of longitudinal polarization to look for new physics effectsdue to $V$ and $A$.